The Original FAQ

***NOTE that these pages have not been updated since 15 January 2000
since there is now a completely revised introduction to the FAQ
However, there is much of interest and still very useful information in these pages.

Last Date that this page was updated: 15th Jan 2000
Issue: 16

Use these navigation bars to move around the FAQ:

0. Index 1. Intro. 2A. Q/A's[The basics] 2B. Q/A's[Background] 3. Topics 4. Sites 5A. Books 5B. Mags. 6. Obs. 7. Suppliers 8. Glossary
..... to go directly to the 'frames' version, with additional links to other meteorology-related FAQ's, click HERE

Summary of changes since last version:
* against the index number indicates a slight textual change, or addition.
** against the index number indicates the section has either been completely rewritten, or is new.

Section 1: (General information and main index)
Section 2A:( Basic information and definitions. )
(added) 2A.27: How do I convert millibars to inches etc?
Section 2B:( Background and special topics. )
Section 3: Questions relating to sources of information.
Section 4: Pointers to other sources of information.
Section 5A: Some books that are worth reading.
Section 5B: Some magazines/periodicals etc.
Section 6A: Recording/reporting a weather event
Section 6B: A weather event ...what to look out for
Section 6C: Some thoughts on 'non-standard' instrument siting
Section 7: Some suppliers of meteorological equipment and services.
(now split between equipment and software suppliers)
Section7A: General suppliers of equipment & services.
Section7B: Suppliers of software and associated support services.
(added): Richard H. Brockmeier
Blizzard, Bomb (revised definition), Cold-front wave, Ensemble (expanded definition), Ensemble mean, gusts, IPV, mean wind, pressure units, various abbreviations, vorticity (expanded definitions, including relative, absolute & potential), wind force definitions, Warm-front wave.

This article is copyright (c)2000 by Martin Rowley. It may be freely distributed for non-commercial purposes only, provided that this copyright notice is not removed.

Use these navigation bars to move around the FAQ:

0. Index 1. Intro. 2A. Q/A's[The basics] 2B. Q/A's[Background] 3. Topics 4. Sites 5A. Books 5B. Mags. 6. Obs. 7. Suppliers 8. Glossary

1. General introduction.

1.1 About this FAQ
1.2 About
1.3 About the author, and his relationship to this FAQ.
1.4 About the units used in this FAQ.

2. Questions relating to general meteorology.

2A. Basic information and definitions.

2A.1 What are 'jetstreams'?
2A.2 Where are the principal jetstreams in the atmosphere, and what are their characteristics?
2A.3 Why is the Polar Front Jet so called, and why is it important to us in NW Europe?
2A.4 Stable and unstable airmasses - what does all this mean?
2A.5 Thickness: what is it?
2A.6 What are the names for the various levels of the atmosphere and of what significance are they?
2A.7 What are the heights corresponding to the pressure levels in the atmosphere?
2A.8 What are the various types of satellite imagery available?
2A.9 What height are the clouds?
2A.10 Why do some high flying aircraft leave white trails in their wake?
2A.11 There are other 'trails' visible from aircraft - what are they?
2A.12 What is an 'inversion'?
2A.13 What's the difference between a 'shower' and an 'outbreak of rain'?
2A.14 What's the difference between an air frost and a ground frost?
2A.15 When do maximum and minimum temperatures occur?
2A.16 What is the dew point?
2A.17 What's the difference between Humidity and Relative Humidity?
2A.18 Does the dew point temperature have to be above a certain value for a thunderstorm?
2A.19 Why does the weather sometimes get 'stuck in a rut'?
2A.20 What is a trough?
2A.21 How do I use a geostrophic wind scale?
2A.22 What are some typical and extreme values of thickness?
2A.23 How does a single-cell shower differ from a multi-cell thunderstorm, or even a 'supercell'?
2A.24 What is 'helicity'?
2A.25 Snow situations at lowland locations are often marginal in maritime NW Europe. Why is this so, and why do forecasters find it so difficult to get it right?
2A.26 So, what are the factors that can modify the temperature structure in the lowest few hundred metres in marginal snow situations?
**2A.27 How do I convert millibars to inches etc?

2B. Background and special topics.

2B.1 The October 1987 storm - a 'hurricane', or not?
2B.2 What does the terminology in the Shipping Forecast mean?
2B.3 Can I go anywhere to look at archived weather data, monthly summaries etc?
2B.4 When was the concept of an "air mass" proposed?
2B.5 So, how is an 'air mass' defined?
2B.6 How will 'global warming' affect rainfall patterns over north-western regions of Europe?
2B.7 Is there a system of classifying synoptic weather types over the British Isles?
2B.8 How can I obtain details about periodicals/magazines that are published in the British Isles which deal with meteorology?
2B.9 Would anyone be interested in my weather observations?
2B.10 What is the North Atlantic Oscillation?
2B.11 What is the Central England Temperature series?
2B.12 What impact does 'El Nino' have on the weather over Europe?
2B.13 What are 'sferics', and how are they obtained?
2B.14 How do I set my barometer?
2B.15 Why does some rainfall leave a coloured dust on my car?
2B.16 Why are there letters near some fronts/centres on Bracknell charts ?
2B.17 And what about some of the other things on these charts?
2B.18 What is an "Indian Summer", and why is it so-named?
2B.19 What is a 'Polar low'?
2B.20 I want to learn more about 'the weather' - how do I go about it?

3. Questions relating to sources of information.

Where can I find ......
3.1 ... a map of the BBC Shipping Forecast areas?
3.2 ... details on meteorological codes and coding?
3.3 ... information on Climate change?
3.4 ... information on Noctilucent cloud?
3.5 ... information about Satellite systems?
3.6 ... sites relating to the use of computers in meteorology?
3.7 ... more information on dew point, relative humidity etc?
3.8 ... information on using Beaufort wind force estimates, and use of Beaufort letters for weather reports?
3.9... more information relating to ozone concerns, both at stratospheric and near-surface altitudes?
3.10... a site to decode a METAR?
3.11 ... sites detailing extremes, notable past events etc?

4. Pointers to other sources of information.

4.2 ... TORRO

5A. Some books that are worth reading.

5A.1 ... Handbook of Aviation Meteorology
5A.2 ... Pilots' Weather
5A.3 ... A course in elementary meteorology
5A.4 ... Guinness Book of Weather Facts and Feats
5A.5 ... Weatherwise
5A.6 ... Observer's Handbook
5A.7 ... Essentials of Meteorology
5A.8 ... Climate, history and the modern world
5A.9 ... Teach yourself weather
5A.10 ... Regional Climates of the British Isles
5A.11 ... Aviation Weather
5A.12 ... Climate and the British Scene
5A.13 ... Images in weather forecasting

5B. Some magazines/periodicals etc.

5B.1 ... The Journal of Meteorology
5B.2 ... Weather
5B.3 ... Meteorological Applications
5B.4 ... Quarterly Journal of the Royal Meteorological Society
5B.5 ... COL - Monthly Bulletin
5B.6 ... Weather eye

6. Some notes on observing and reporting weather events to the newsgroup.

6A ... Recording/reporting the event
6B ... The event itself...what to look out for
6C ... Some thoughts on 'non-standard' instrument siting

**7. Some suppliers of meteorological equipment and associated services.

**7A. General suppliers of equipment & services.
... Campbell Scientific Ltd
... Casella Limited
... Diplex Ltd
... ICS Electronics Ltd
... Prodata Associates Ltd
... Sales and Service Company

**7B: Suppliers of software and associated support services.
... Richard H. Brockmeier
... University of Liverpool/software
... Colin Tandy

Use these navigation bars to move around the FAQ:

0. Index 1. Intro. 2A. Q/A's[The basics] 2B. Q/A's[Background] 3. Topics 4. Sites 5A. Books 5B. Mags. 6. Obs. 7. Suppliers 8. Glossary

SECTION 1: General introduction.

1.1 About this FAQ: It is important to note that this FAQ is not a 'do-it-yourself' course in meteorology. It simply aims to answer some common puzzling questions that might be posed by the non-professional whilst browsing met-related web pages, or lurking in one of the weather newsgroups.

Update/posting frequency: every month (to the newsgroup; every two months for the html versions):

held at:
and via the UK Weather Information Site at:
There is also now a simple 'frames' version of this FAQ, with easier access to other meteorology related FAQ's held at:
Queries, comments, suggested corrections etc., relating to the content, to the author:
Martin Rowley:

and notification of problems with the structure of the web page, html errors etc., to :

1.2 About The following is an extract from the Charter for the newsgroup: "This group is essentially for the discussion of daily weather events, chiefly affecting the UK and adjacent parts of Europe, both past and predicted. The discussion is open to all, but contributions on a practical scientific level are encouraged. It may also contain postings of observations during interesting weather episodes. The group is expected to be patronised by both amateurs and professionals (including academics), but it is primarily for weather enthusiasts rather than research scientists. Any discussion of climate issues should be from a scientific standpoint and not a political (or environmental-activist) one."

PLEASE NOTE: Binary files, (such as interesting satellite images, or weather charts) should not be posted into the newsgroup. To do so is a great annoyance to users, and contravenes the Acceptable Use Policy of some ISP's and associated peering carriers. Post such files into newsgroups specifically set up to carry binary-encoded information, and then post to the newsgroup the location. Alternatively put on your own web site.

1.3 About the author, and his relationship to this FAQ: The author is an employee of the UK Meteorological Office. Any views expressed herein are his own and do not represent the views of the Met.Office. In particular, because of restrictions that UK Civil Servants, and Met.Office employees in particular are obliged to follow, no comment or information on policy matters, availability of data etc., will be found in this FAQ. I have obviously tried to eliminate any errors, but no doubt some have crept in. Please advise me via e:mail and I will correct as necessary.

1.4 About the units used in this FAQ: There is always a problem with units in meteorology, because the 'operational' community use, and are used to, different units to those of the academic/theoretical persuasion, and so our trade is littered with anomalies: the most bizarre can be heard 4 times per day on the BBC shipping forecast, where low values of visibility are given in metres, and higher values in nautical miles! In this FAQ, I have used degrees Celsius for temperature as this will be familiar to most, but for height/altitude, both feet (used by the aviation world), and metres/km equivalents are given - mostly approximations. Wind speeds, where given, will be in knots (used in practical observing/aviation forecasting) and metres/second. The relationship between the two units is assumed to be knots=2*m/s, as only approximations are quoted. Note also that other approximations are often used, for example the Dry Adiabatic Lapse Rate is quoted as 10 degC per 1 km, whereas it is calculated to be 9.8 degC/km. (Q/A 2A.4)

Use these navigation bars to move around the FAQ:

0. Index 1. Intro. 2A. Q/A's[The basics] 2B. Q/A's[Background] 3. Topics 4. Sites 5A. Books 5B. Mags. 6. Obs. 7. Suppliers 8. Glossary

SECTION 2A: Basic information and definitions.

Q. What are 'jetstreams'?
A. At high altitudes, very strong winds are found, conventionally (for aviation forecasts) with speeds of 80 knots (40 m/s) or more, but most often with speeds in the range 120 to 160 knots (60-80 m/s), and in extreme cases, over 200 knots (100 m/s). These very strong winds are found in relatively narrow horizontal, and even narrower vertical space, and are known to meteorologists as jetstreams: named by Carl-Gustav Rossby in 1947, following research in the USA. However, the existence of jetstreams had been suspected theoretically for many years before, and, though not recognised as such, had been picked up by Zeppelin flights in the Great War (1914-1918) flying at 20000 ft/6 km on return to Germany.

Q. Where are the principal jetstreams in the atmosphere, and what are their characteristics?
1. The Polar Front Jet: As its name implies, this jet stream is associated with the marked discontinuity found at the boundary of well defined air masses - polar to the north/sub tropical to the south (in the northern hemisphere), conventionally found at the polar front. It meanders markedly in response to global/regional scale atmospheric changes but has a latitudinal 'home' roughly from 45 to 65 deg N/S. Its altitude is somewhere between 28000 ft to 34000 ft (8.5 - 10.5 km), with its own distinctive tropopause level. Speeds are of the order 80-130 knots (40-65 m/s), but may be as high as 180 knots (90 m/s), and downstream of main continental land masses in late winter/early spring, in excess of 200 knots (100 m/s). Although it is regarded as a 'single' ribbon of strength encircling the hemispheres, in reality the jet is broken and in developmental situations, can become very distorted with new jets re-forming at different levels from the 'main' baroclinic jet.
2. The Sub Tropical Jet: The average level of the core of this westerly jet lies at an altitude of about 40 000 ft/12 km, just below the tropical tropopause. It occurs in the latitude range 25-40 deg N/S, and is most marked during the winter and early spring of each hemisphere, but is not associated with any surface frontal structure. Wind speeds are generally 80-150 knots (40-75 m/s), but can be much greater over eastern seaboards of large land masses, e.g. speeds of 400 kt (200 m/s) have been reported over east Asia/NW Pacific.
3. Stratospheric Night Jet: This occurs at times during the winter and early spring when the stratosphere near the poles is much colder than it is further south due to the absence of insolation at these times of the year. Its direction is westerly overall, with high variability, and has speeds in the range 100-200 kt (50-100 m/s) at altitudes around 70 000 ft/21 km and occurs on the poleward side of latitude 70 degrees.
4. Equatorial Easterly Jet: This jet occurs in the northern summer between 10 and 20 deg N, chiefly over or just to the south of high land masses such as in Asia and Africa. Its occurrence is due to a temperature gradient with colder air to the south which produces sufficient temperature differential above 50 000 ft/15 km to give wind speeds of over 100 kt (50 m/s). Because colder temperatures at height are to the south, it is an easterly jet. (This jet is now more usually known as the Tropical Easterly Jet(TEJ) ... perhaps more correctly as it lies some distance from the Equator.)

Q. Why is the Polar Front Jet so called, and why is it important to us in NW Europe?
A. In NW Europe, when meteorologists refer to a jetstream, it's the Polar Front Jet (PFJ), that is usually meant. As its name implies, it is associated with the classical 'Norwegian model' polar front - the surface discontinuity between cold/ex-polar latitude air, and the warm, relatively moist air originating in the sub tropical anticyclone belt. When air masses (see 2B.4 and 2B.5) lie adjacent to one another, the temperature difference isn't just found at mean sea level, but throughout the troposphere. Because atmospheric pressure decreases more quickly with height in cold/polar air than warm/sub-tropical air, there arises a pressure differential, which gives rise to intense pressure gradients at altitude, and hence the very strong winds observed. Because of the high wind speeds involved with jetstreams, any slight changes, in either velocity or direction, or both, leads to vertical motion in the air below the jet, and is a major player in the processes of atmospheric development.
(For more on upper air meteorology, jetstreams etc., visit:

Q. Stable and unstable airmasses - what does all this mean?
A. First, to visualise what 'stable' and 'unstable' states mean in a physical sense, stand a round pencil on end on a level surface. From Newton's First Law of Motion, it will remain upright until a force is applied. Once displaced, the pencil falls over, failing to pass through its original (upright) position. This is the UNSTABLE state.
Now lay the pencil on its side, at the bottom of an incline. Displace the pencil slightly up the incline, then remove the force of displacement. The pencil will return to its original position. This is the STABLE state.

In the atmosphere, whether air that is displaced does so in an unstable or stable environment depends upon the vertical temperature profile of the air -- its lapse rate -- and upon the moisture content of the parcel. These differences are fundamental to understanding why clouds take up the form they do.

In the atmosphere, when a 'parcel' of air moves vertically upwards (or downwards), it cools (upward motion), or warms (downward motion), in accordance with thermodynamic rules ... if the air is unsaturated (air temperature > dew point temperature), the cooling/warming will be at a rate of 3 degC per 1000 ft (or 10 degC per 1 km): This is known as the Dry Adiabatic Lapse Rate/DALR; If the air is/becomes saturated (air temperature=dew point temperature), this rate is roughly halved in the lower troposphere, due to the release of latent heat upon condensation. This rate is known as the Saturated Adiabatic Lapse Rate/SALR.

Such ascent/descent is said to be adiabatic, which means that the energy/heat changes are confined to that particular parcel.
Provided the parcel is warmer (less dense) than the environmental air through which it is passing, it is buoyant, and rises. If the parcel is colder (denser) than ambient air, then it will descend, or try to descend.
Because the rates of cooling (ascent), and warming (descent) of individual parcels are fixed, the important variable is the overall lapse rate (i.e. the rate of change of temperature with height) of the atmosphere. On average, this is 1.98 (call it 2 degC) per 1000 ft, or 6.5 degC per 1 km in the troposphere, but this average conceals a wide variety of cases which are important in meteorology.

Where the temperature falls off slowly with height, or indeed rises, e.g. in a slow moving anticyclone, or a tropical maritime airmass, then an air parcel subject to lifting/adiabatic cooling will readily find itself colder than its surroundings ... denser ... and try to return to its original position: The air is ABSOLUTELY STABLE.
Where the temperature falls off quickly with height, e.g. in a cold/polar air mass over NW Europe in late winter/spring, then an air parcel subject to ascent, although cooling, may still find itself warmer/less dense than its surrounding air ... it will be buoyant, and tend to rise further: the air is ABSOLUTELY UNSTABLE.

Problems arise when, on ascent, the dew point of the air is reached, and the rate of cooling is therefore less - it follows the SALR figure. If, however, the parcel is still warmer/less dense, then it will continue to rise, and the condition of the air is said to be CONDITIONALLY UNSTABLE .. i.e. conditional upon whether the parcel is saturated or not. This is by far the most common situation in the 'real' atmosphere, accounting for some 65-70% of situations taking the troposphere as a whole.

Stable airmasses generally imply the absence of 'free' vertical motion, and any ascent that does occur must be forced, i.e. frontal (dynamic) or orographic (mechanical) ascent, and the cloud structure is essentially layered.
(NB: Forced ascent comes about in several ways: frontal ascent due to large-scale air motion within frontal systems, with of course adjacent descent; convergence into an area of low pressure - the converging air can't go down near the surface - it has to go up; and topographical forcing, that is, air is forced to rise over major upland ranges. )

Unstable airmasses imply free vertical motion (given an initial trigger action), and the cloud structure is 'heaped' or cumuliform. If the vertical motion is vigorous and deep enough, and there is sufficient moisture, then heavy showers/thunderstorms are likely.
(NB: Trigger action: method of causing air to rise initially, which in the lower troposphere include not only the 'wide-area' triggers noted above under stable conditions, but also smaller/mesoscale mechanisms such as differential heat response between land and sea, coastal convergence, etc.)

For more information on these subjects, see a good textbook on meteorology, for example, Essentials of Meteorology:(Taylor and Francis/D.H.McIntosh and A.S.Thom).

Q. Thickness: what is it?
A. 'Thickness' is a measure of how warm or cold a layer of the atmosphere is, usually a layer in the lowest 5 km of the troposphere; high values mean warm air, and low values mean cold air. It would be perfectly feasible to define the average temperature of a layer in the atmosphere by calculating its mean value in degrees C (or Kelvin) between two vertical points, but an easier, practical way to measure this same mean temperature between two levels can be gained by subtracting the lower height value of the appropriate isobaric surface from the upper.

Thus one measure of thickness commonly quoted is=height (500 hPa surface) - height (1000 hPa surface)

The 500-1000 hPa value is used to define 'bulk' airmass mean temperature, and can be seen on several products available on the Web.
For more information see:

(see also Q/A 2A.22 for typical figures, extremes etc.)

Q. What are the names for the various levels of the atmosphere and of what significance are they?
A. The atmosphere is divided up into layers with names which describe the dynamic or thermal structure of that particular layer. The two layers which are of most interest to us are the troposphere and the stratosphere.

>>Troposphere: (overturning or changing sphere) - The lowest layer of the atmosphere. Positive lapse of temperature (temperature overall decrease with height). It is the most important for operational meteorology, as this layer contains almost all the water vapour, and by far the greatest part of the mass of the atmosphere. Because of its mean thermal structure, it is the region of greatest vertical motion (up and down) in the atmosphere, even without the help of vigorous thunderstorm complexes, which in themselves may occupy the entire depth of the troposphere. At some level, there is usually an abrupt change in the lapse rate from positive (decrease with height), to isothermal (no change), or a slight rise. This level is the tropopause.
Typical heights of the tropopause, and therefore thickness of the troposphere, are:

High arctic/antarctic latitudes: 6 to 8 km (20000-25000 ft)
On/near the equator: 16 to 18 km (50000-60000 ft)

In mid-latitudes, the temperate zone, which is of most interest to us in NW Europe, the tropopause is highly variable, from cold to warm season, and from cold to warm air mass. For example, it is lower in winter, and in cold/polar air masses (typically 8 to 10 km/25000 to 30000 ft), than in high summer, and in warm/sub tropical air masses (typically 12 to 14 km/35000 to 45000 ft)

>>Stratosphere: (the 'layered' sphere) - the next layer ascending through the atmosphere. Isothermal or small lapse rate of temperature(positive or negative). Because of the temperature structure, very little overturning of air takes place, either within the stratosphere, or with the troposphere (except in the special case of rapidly developing storms of various kinds), and once gases, particulates etc. penetrate to this layer, they remain there for very long periods, hence the concern regarding such substances due to both the actions of mankind (e.g. CFC's) and those of natural processes (e.g. volcanic ash).
As with the troposphere, the stratosphere varies in thickness, but as an average figure the top of this layer, the stratopause, occurs around 45-48 km (148000-158000 ft).

Q. What are the heights corresponding to the pressure levels in the atmosphere?
A. These equivalents are based on the International Standard Atmosphere and promulgated by ICAO:

mbar(or hPa) nominal altitude (ft to nearest 1000 ft; metres to nearest 100 m
100 53 000 ft/16 200 m
200 39 000 ft/11 800 m
250 34 000 ft/10 400 m
300 30 000 ft/ 9 200 m
400 24 000 ft/ 7 200 m
500 18 000 ft/ 5 600 m
600 14 000 ft/ 4 200 m
700 10 000 ft/ 3 000 m
850 5 000 ft/ 1 500 m

For full details, see: This site also has the definitions of QFE, QNH, QFF and QNE.

Q. What are the various types of satellite imagery available?
A. There are four principal types of satellite imagery used in operational and research meteorology. Each has its advantages and disadvantages. Many examples of each type can be found at meteorology related web-sites.

1. Visible Imagery (VIS)
Images obtained using reflected sunlight at visible wavelengths, in the range 0.4 to 1.1 micrometres. Visible imagery is displayed in such a way that high reflectance objects, e.g. dense cirrus from CB clusters, fresh snow, nimbostratus etc., are displayed as white, and low reflectance objects, e.g. much of the earth's surface, is dark grey or black. There are grey shades to indicate different levels of albedo (or reflectivity). Very dependent upon angle of incident sunshine, and of course, not available at night, though some military/research satellite sensors can utilise reflected moonlight to detect cloud.

2. InfraRed(IR)
These images are obtained by sensing the intensity of the 'heat' emissions of the earth, and the atmosphere/atmospheric constituents, at IR wavelengths in the range 10-12 micrometres. The earth, and its components, radiate across a wide spectrum of wavelengths, but for many of these, the atmospheric gases, of which water vapour is an important constituent, absorb a significant proportion of such radiation. Thus so-called 'windows' need to be chosen to allow the satellite sensors to detect such radiation unhindered, and the 10-12 micrometre band is one such. IR imagery is so presented that warm/high intensity emissions are dark grey or even black, and low intensity/cold emissions are white. This convention was chosen so that the output would correspond with that from the VIS channels, but there is no need to follow this scheme - indeed in operational meteorology, colour slicing is frequently used whereby different colours are assigned to various temperature ranges, thus rendering the cooling/warming of cloud tops (and thus the development/decay) easy to appreciate: warming/darkening of the imagery with time indicates descent and decay; cooling/whitening images imply ascent and development.

3. Water Vapour(WV)
This imagery is derived from emissions in the atmosphere clustered around a wavelength of 6.7 micrometre. In contrast to the IR channel, this wavelength undergoes strong absorption by WV in the atmosphere (i.e. this is not a 'window'), and so can be used to infer vertical distribution and concentration of WV - an important atmospheric constituent. WV imagery uses the radiation absorbed and re-emitted by water vapour in the troposphere. If the upper troposphere is moist, WV emissions will be dominated by radiance from these higher levels, swamping emissions from warmer/lower layers; this radiation is conventionally shown white. If the upper troposphere is dry, then the sum of the radiation is biased towards lower altitude WV bands: it is warmer/less intense radiation, and this is displayed as a shade of grey, or even black. WV imagery is very important in the study of cyclogenesis, often being displayed as a time-sequence.

4. 'Channel 3'(CH3)
Imagery from a specific wavelength of 3.7 micrometre, lies in the overlap region of the electro-magnetic spectrum between solar and earth-based/terrestrial radiation. It is sometimes referred to as 'near infrared' (NIR). CH3 images use a mixture of back-scattered solar radiation plus radiation emitted by the earth and atmosphere. It is used in fog/very low cloud studies. Interpretation is sometimes complex, especially in the presence of other tropospheric clouds.

Q. What height are the clouds?
A. In the troposphere (the 'weather' zone ... see Q/A 2A.6), the layers are divided up into three broad levels: (approx.heights only)

Polar latitudes Temperate regions Tropics
High 10 000 - 25 000 ft
/3 - 8 km
16 500 - 45 000 ft
/5 - 14 km
20 000 - 60 000 ft
/6 - 8 km
Medium 6 500 - 13 000 ft
/2 - 4 km
6 500 - 23 000 ft
/2 - 7 km
6 500 - 25 000 ft
/2 - 8 km
Low Surface -- 6 500 ft
/up to 2 km
Surface -- 6 500 ft
/up to 2 km
Surface -- 6 500 ft
/up to 2 km

The heights assigned to the 'divisions' between levels should not be followed slavishly, and assignment of clouds to the various 'groups' should be made with the appearance and composition in mind.

High clouds are primarily composed of ice crystals; Medium clouds are a mixture of water droplets (usually super-cooled) and ice crystals, in varying proportion, and low clouds primarily water droplets, but in individual cases these descriptions are probably simplistic.
(NB: Super-cooled: means that although the temperature of the droplet is below 0 deg.C, it remains liquid - this is a common state in the middle part of the troposphere.)

In the 'Low' cloud classification come: Stratus (St); Stratocumulus (Sc); Cumulus (Cu) and Cumulonimbus (Cb). However, note that both Cumulus and Cumulonimbus clouds often extend well into 'medium' levels, and towering Cu, and Cb extend to 'high' levels.
In the 'Medium' cloud class come: Altostratus (As); Altocumulus (Ac) and Nimbostratus (Ns). Nimbostratus often has a base within the 'low' cloud category.
In the 'High' cloud group are: Cirrus (Ci); Cirrocumulus (Cc) and Cirrostratus (Cs).

Use these navigation bars to move around the FAQ:

0. Index 1. Intro. 2A. Q/A's[The basics] 2B. Q/A's[Background] 3. Topics 4. Sites 5A. Books 5B. Mags. 6. Obs. 7. Suppliers 8. Glossary

Q. Why do some high flying aircraft leave white trails in their wake?
A. The white trails are ribbons of ice crystals. As a by-product of the exhaust of aircraft engines, water vapour is trailed from the engine exhaust which adds to the local humidity of the air the aircraft is flying through, and which tends to super-saturation of the air. However, the exhaust gases are of course hot, and so these hot gases help to raise the temperature of the air and thus is can hold more vapour before saturation is reached. There are therefore two opposing mechanisms at work: the water vapour in the exhaust trying to saturate the air; the hot gases of the exhaust trying to decrease relative humidity. When the balance between outside air temperature (OAT) and local humidity is just right, then condensation trails will occur: usually abbreviated to CONTRAILS, and sometimes referred to, from old coding conventions, as COTRA. Sometimes, such trails persist for a considerable time, gradually spreading out to form large, sometimes dense areas of cirriform cloud - for this reason, the production of aircraft condensation trails form part of the debate on the overall global radiation balance.

Q. There are other 'trails' visible from aircraft - what are they?
>> Wake trails: As an aircraft passes through a lower troposphere having a high relative humidity, on landing or take-off, very short, non-persistent 'trails' can sometimes be seen coming from the wing tips, or from the trailing edges of the main wing, control surfaces etc. As an aircraft moves forward, air accumulates (pressure builds), at leading edges, with a compensating depletion of air (fall of pressure) at trailing surfaces. The reduction of temperature in the near-saturated environment, consequent upon the slight lowering of pressure, can be enough to cool the air to the dew point, and trails of water droplets are observed, which evaporate quickly again due to mixing with the non-saturated environment in the wake of the aircraft.

>> Dissipation trails (DISTRAILS): In contrast to the formation of CONTRAILS (see Q/A 2A.10), aircraft on passage at high levels can cause the dissipation of pre-existing cirriform cloud, due to the local increase in temperature consequent upon the ejection of hot exhaust gases from the aircraft engine. The passage of the aircraft will be marked by a clear lane in the cloud. However, it will be obvious from the description (above) relating to condensation trails, that the heat outflow must markedly outweigh the injection of water vapour from the spent fuel, and the phenomenon is rare. The effect may also be caused by turbulent mixing with dry air just above the cloud layer, caused by the aircraft motion, and this mechanism can lead to temporary clear lanes in other cloud forms, e.g. thin stratocumulus or altocumulus. However, beware of a similar phenomenon, whereby the shadow of a 'normal' condensation trail is cast on thin cirriform cloud below - leading to a visibly dark band in the cloud. This is not a dissipation trail.

Incidentally, whilst on the subject of 'trails', if you are looking at visible satellite imagery over the region of a slow-moving anticyclone, and notice lots of thin, white lines criss-crossing the region, which don't appear on the corresponding InfraRed image, these are ships' trails, caused by exhaust particles from the vessel acting as condensation nuclei, and 'seeding' the humid, near sea surface environment, and betraying the presence of the ship by a thin band of water droplets which are not dispersed due to the very light winds and minimal mixing in the anticyclone.

Q. What is an 'inversion'?
A. The average condition of temperature change in the Troposphere is for there to be an overall decrease of temperature with increasing height: a positive lapse rate (see Q/A 2A.6). However, in the 'real' Troposphere, frequent reversals of this 'normal' lapse are observed, particularly in the lower layers - these zones of increasing temperature with height are inversions (i.e. the inverse of the average state), and are very important for both synoptic/mesoscale meteorology (e.g. fog/stratus formation/dispersal), and pollution dispersion studies, as they cap layers of markedly stable and potentially stagnant air masses. Examples of inversions include those due to anticyclonic subsidence; cooling land by night (nocturnal inversions); and sea-breeze inversions, where cooler sea air under-cuts warmer land air. Where the inversion is associated with an abrupt lowering of the moisture content (sharp fall of dew point), at the altitude of the temperature rise, then interesting radio-refraction conditions occur, familiar to viewers of terrestrial television in stagnant anticyclonic episodes.

Q. What's the difference between a 'shower' and an 'outbreak of rain'?
A. In fact, if you are caught out in one, there is no difference. You can still get wet! Meteorologists however distinguish between precipitation (rain, snow, hail etc.) falling from cumuliform cloud in an unstable environment - a shower, from that falling from layer clouds in a generally stable environment which are just called rain, snow, sleet etc. However, rain from layer cloud in a frontal situation for example can be rather hit-and-miss, especially in a weakening situation, and so forecasters will try and get around such problems by talking about 'patchy rain', 'outbreaks of rain', 'splashes of rain' etc. The opposite problem comes when a well defined trough sweeps across an area, in which the cloud structure is most likely of an unstable type: cumulus, cumulonimbus and altocumulus. Given the definition above, the short, very sharp falls of rain might be called 'showers' (and probably coded as such by observers), but this would be misleading to members of the public caught out in such precipitation: hence the 'showery outbreaks of rain', 'showery bursts of rain', 'localised downpours' etc.

Q. What's the difference between a ground frost and an air frost?
A. In day-to-day meteorology, the temperature of the lowest layer of the atmosphere is measured at a height of 1.25 m (about 4 feet) above local ground level. Usually, though not always, this is achieved by placing thermometers in a double-louvered screen with the bulbs of the thermometers, or the sensor heads (for distant reading thermometers), placed so that they cluster around the 1.25 m standard. The temperature so read is usually called 'the air temperature' and it is these values that appear, for example, in the World Cities reports in newspapers/teletext, or plotted on standard synoptic charts, and also it is at this level that the forecast temperatures seen on tv weather maps are based. When the temperature as measured in this way falls below 0.0 deg C, then an AIR FROST is recorded. For other purposes though, e.g. horticulture, road gritting operations etc., we need to know what the temperature is at the surface of the ground, and most weather stations set at least two thermometers to record these values: a grass minimum thermometer, set just above/in contact with short grass, and a concrete minimum thermometer, set so that its sensor/bulb is in contact with a concrete slab of standard dimensions/composition. When the temperature as measured by the thermometer set over grass falls below 0.0 deg C, then a GROUND FROST is recorded. The difference between the two levels can be considerable: On still, clear nights, with air of a low humidity content, 5 degC or more is not uncommon.

Q. When do maximum and minimum temperatures occur?
A. A common misconception, is that it must be coldest in the middle of the night, and warmest around midday. On some occasions, mainly due to air mass changes, this may be correct, but not usually. The lowest (minimum) temperature usually occurs a little while after sunrise, and the highest (maximum) temperature usually occurs after midday --- sometimes as late as 3 or 4 hours after midday.
To understand why, it is necessary to consider that thermal energy during the 24 hours is radiating continually from the surface of the earth (at long wavelengths), and incoming solar (relatively short wave) radiation obviously only when the sun is above the horizon. With the sun below the horizon (night), outgoing radiation allows the surface to cool, and the temperature drops. After sunrise, incoming solar radiation counteracts this loss of heat, but only after a lag - which can be up to an hour or so in winter with a low solar elevation. The minimum temperature occurs when there is a balance between outgoing and incoming radiation. As the sun rides higher in the sky, increasing amounts of short-wave radiation are available to heat the ground, and therefore available to heat the overlying air. Although outgoing land-based radiation is also increasing, solar heating is dominant. The temperature rises, until, past noon, incoming solar radiation starts to decline again. The highest(maximum) temperature occurs when heat gain due to incoming solar radiation, and heat loss due to outgoing terrestrial radiation balance: this occurs some time after midday.

Q. What is the 'dew point'?
A. For any particular sample of air, which is cooled at constant pressure, there will be a temperature below which water vapour condenses to form liquid water drops, assuming sufficient hygroscopic nuclei present. That temperature is known as the Dew Point and is a measure of the Absolute Humidity (see Q/A 2A.17).

Q. What's the difference between Humidity and Relative Humidity?
(1) Absolute Humidity, often just referred to as 'the humidity', is a measure of the actual amount of water vapour in a particular sample of air: measured as a partial pressure (vapour pressure/hPa or millibars); a mixing ratio (gm water vapour/kg of dry air), dew point etc.
(2) Relative Humidity - expressed commonly as a percentage value, is the ratio of the actual amount of water vapour present in a sample (the Absolute Humidity) to that amount that would be needed to saturate that particular sample.

The two terms are not interchangeable and can lead to confusion; e.g. on a cold, raw winter's day close to the east coast of England, the dew point might be 1 degC and an air temperature of just 2 degC...this would give a RH of=93%; a 'high' Relative Humidity, yet few would refer to such conditions as 'humid'. Conversely, on a hot summer's day, with a dew point of 18 degC, and an afternoon temperature of 30 degC, that's a RH=49%; a 'low' Relative Humidity, but high Absolute Humidity.

Q. Does the dew point temperature have to be above a certain value for a thunderstorm?
(thanks to Will Hand for this answer....)
A. Only for the special case of thunderstorms coming up from the south in summer. I have seen many thunderstorms (real crackers as well) in April with air temperatures of 8 degC and a dew point of 4 degC. What is really important is that the air must be unstable (see Q/A 2A.4), usually achieved by warming at the bottom or by cooling high up or both. Then you need a trigger to release the instability, usually heating and input of moist air (high dew point), but if the air is unstable enough just the heating will do. Other triggers are forced lifting of air over hills or forced lifting by convergence (e.g. sea breezes).

Q. Why does the weather sometimes get 'stuck in a rut'?
A. At mid to high latitudes in the upper part of the troposphere (above roughly 5 km ), the mean wind flow exhibits a broadly west-to-east motion - this applies in both hemispheres. On many occasions, particularly in mid-latitude/temperate zone regions, the flow is directed more or less directly from west to east, crossing few latitude zones within the same longitude range: this is a 'highly zonal' type - any short-wave disturbances embedded in the flow will be carried quickly along and the weather is ever-changing as a succession of frontal systems, interspersed with transient ridge conditions cross any one point. However, on both average (e.g. monthly) pressure maps and on individual days, long-wave trough/ridge patterns can be found - some having large amplitude, i.e. the airflow meanders a long way north and south around the loops of the pattern, crossing many parallels of latitude in a relatively limited longitudinal range: a 'meridional' type; Usually, some west-to-east progression of the looped pattern can be seen over a 24 hr period, and the associated surface weather type changes, albeit more slowly than the zonal type described earlier. However, if the 'loops' in the pattern become locked in one geographical area, then depending where you are in relation to the upper flow, the associated surface patterns are often little changed from one day to another, and in extreme cases, from one week to another - the pattern is said to be 'blocked'. In, and just to the east of a slow-moving trough in the upper flow, the surface weather will tend to be of a low pressure/convective/showery type, and perhaps cool for the time of year (but not necessarily); In, and just to the east of a static ridge in the flow, the surface pressure will tend to be high, with settled conditions lasting until the block is destroyed. This latter case is responsible for prolonged dry/hot weather in summer, but cold/sometimes grey conditions in winter, and considerable pollution build-up can occur at all seasons due to the stagnation of the lower level air and high air-mass stability encountered.

For a personal view of some aspects of upper air meteorology, and some further explanation of the terminology used go to:

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Q. What is a trough?
A. A trough on a mean sea level pressure chart, (or an upper air contour chart) can be picked out by an arrangement of isobars (contours) which are concave towards an area of low pressure (low contour height) along a particular axis, and that axis is defined so as to lie along the points of maximum curvature on the individual isobars (contours). If this sounds complicated, it isn't really: the feature is analogous to the 'valley' on an OS map and defined in the same way - pressure, or contour heights 'fall' into the trough line.
A front may have troughing along its length, but not all troughs are frontal! Indeed, not all troughs have 'weather' associated with them in the cloud/rainfall sense. Lee troughs found downwind of a major range of hills/mountains are often cloudless, and thermal troughs forming over land during the day due to mesoscale heating may only be found by careful drawing of isobars: if the air is dry and/or stable, little significant cloud will be associated with this feature. A modern complication on charts used on the GTS is that plumes of high humidity...e.g. in the case of very humid/warm air coming northward out of France/Iberia, are also shown as 'trough' lines for want of any other identifier. Although with development pressure may become lower along this 'plume' than surrounding areas, and therefore qualify as a trough by the above definition, often the difference is small or initially non-existent.

Q. How do I use a geostrophic wind scale?
A. Find the distance between adjacent isobars in the area that you are interested in - making sure that the isobaric interval is the same as that for which the scale was constructed - often 4 mbar. (Dividers can be used, but a strip of paper suitably marked is just as good.) Using the geostrophic scale for the *correct latitude*, put one end of your marked distance on the left-hand end of the scale, and read off at the right-hand end the geostrophic wind speed for that isobaric spacing at that latitude. Remember though that many corrections are needed to find an approximation to the 'real' wind .. see the Glossary and any good book relating to meteorology. .. see the Glossary and any good book relating to meteorology.

Q. What are some typical and extreme values of thickness?
(see also Q/A 2A.5 above: thanks to Jon O'Rourke for looking up the extreme values we hold in the NMC at Bracknell)
A. As already noted elsewhere, the values of the (total) thickness between levels at 500hPa and 1000hPa give a useful measure of the mean temperature of that layer. In summer, values might range from 546dam (cool, showery northwesterly) to 560dam (warm, settled anticyclonic spell); in winter from 530dam( brisk, chilly, showery flow, with inland night frosts) to 550dam (mild, open-warm sector type). (The values are given for comparitive analysis only, and the weather types of course don't necessarily follow from the values); Values below 528dam in winter would herald the arrival of potentially wintry conditions, and in summer, thickness values above 564dam might be a precursor to some notably high temperatures.

As to extremes, for the UK mainland land-mass only, the highest Jon could find came out around 576dam in July over southeast Kent (SE England), and the lowest around 495dam on the extreme tip of NE Scotland in January. Bear in mind that the figures have to be interpolated from charts.
For a graphical representation of maximum and minimum thickness values for 6 points around the NW of Europe, see:...

Q. How does a single-cell shower differ from a multi-cell thunderstorm, or even a 'supercell'?
A. All are formed within an unstable environment (see Q/A 2A.4), and all require the following to be in place: (i) Instability through a reasonable depth of the troposphere; preferably (but NOT necessarily) extending above the freezing level; (ii) sufficient moisture to sustain the cloud-building process - medium level dryness will often kill shower formation unless low-level inflow of moisture is substantial; (iii) a trigger action - i.e. something to kick the whole process into life by lifting the parcel that goes on to grow into a moderate depth cumulus cloud, or a well-developed 'supercell' complex.

Once these conditions are met, then consideration of things like shear, CAPE, helicity, etc., are needed as follows:- (for definitions, see the Glossary, and in particular for helicity, see Q/A 2A.24 (below))

>> Single-cell showers: the 'classic' growth/decay model of a Cumulus cloud , whereby a single moist convective cell develops in an airmass that is moderately unstable (CAPE values ~ 100 J/kg), provided of course that there is sufficient depth of moisture and there is an initial trigger action. When the updraught and the precipitation downdraught occupy virtually the same atmospheric column (there is little or no vertical relative wind shear to tilt the cloud), the downdraught quickly swamps the updraught - the shower soon decays (perhaps lasting only a matter of minutes - the cloud would last longer though), yielding small amounts of rain/snow. However, when there is a change of wind speed with height (but little directional change), the updraught column is tilted forward, and the resultant precipitation downdraught is held clear of the downdraught, allowing greater development and moderate intensity showers occur. The cold downdraught though soon swamps the inflow of surface air, cutting off the updraught and the shower decays after about 20 to 30 minutes. These events would be typical of Polar Maritime airmasses.

>> Multi-cell thunderstorms: Whenever wind shear is present in an unstable atmosphere, the developing convective clouds will be tilted to a greater or lesser extent. As seen above (single-cell showers), when only the wind speed changes, then short-lived, non-propagating showers are produced. However, given *both* change of wind speed and direction with height (relative to the storm motion), and sufficiently high CAPE (> ~ 250 J/kg), then the precipitation downdraught is skewed well to the side of the storm updraught, and does not interfere with it - allowing that storm cell to develop its full potential - other necessary factors (e.g. sufficient moisture) being in place. In addition, the downdraught will hit the surface and spread horizontally as a cold density current (gust front). At some point, this will meet the low-level inflow, and a new 'daughter' cell (see the Glossary) may be initiated which may grow into a full-scale storm cell in its own right. This usually (but not always) occurs to the right of the cloud motion, and the whole storm complex appears then to move to the right .. in fact the daughter cells take over from each successive parent to produce this effect. Large Cumulonimbus (Cb) clouds are produced with these processes; each cell lasting at least half-an-hour, and depending upon external forcing agents (e.g. coastal convergence, synoptic troughs, orographic lifting), the storm complexes may last for several hours.

>> Supercell thunderstorms: In some ways, this can be regarded as a special case of the multi-cell storm, with some additional factors. The environment is still sheared in the vertical, indeed markedly so in the lower layers, and daughter cells are produced. However, the storm motion is minimal and as the spawned cells form close to the base of the parent cloud - often several daughter cells coinciding - these form an almost self-perpetuating 'supercell' system lasting several hours. This mechanism produces the most severe late spring/summertime thunderstorms with local intense rainfall leading to flooding, plus occurrence of hail, possible tornadoes etc. CAPE values for such events will typically be ~1000 J/kg or more, and helicity will also tend to be high - hence the tendency to rotation of the storm complex, and its individual elements. Potential instability at medium levels (circa 500 hPa/5 to 6km) is also required, as is an initial inhibiting factor (warm/dry air capping surface based instability) to allow the 'loaded gun' effect to build up.

Q. What is 'helicity'?
(thanks to Will Hand for providing this answer)
A. This is a derived parameter which quantifies the tendency for airflow in the lower levels of the troposphere to 'corkscrew' and thus encourage the formation of storms with strong mesoscale circulations, possibly leading to tornadic activity.

Helicity has units of energy and can therefore be interpreted as a measure of wind shear energy that includes the directional shear. If there is no directional shear then the helicity is zero: if the wind backs with height then the helicity is negative; if it veers with height (more normal in storms in maritime NW Europe) then the helicity is positive.

Helicity is usually derived in a storm frame of reference, the 'storm relative' helicity, [ Hr ] between the surface and a height, [ h ] and is calculated as an integral between those limits thus: (Vh - C) x Wh x dh [units=m**2/s**2 ] Where [ Vh ] is the environmental horizontal wind velocity , [ C ] is the storm velocity and [ Wh ] is the local relative vorticity. Often [ Hr ] is calculated between expected cloud base and cloud top.

Studies in North America looked at the use of helicity (ignoring sign) for forecasting the risk of tornadoes. They found the following:
Helicity 150-299 ... weak tornadoes
Helicity 300-499 ... strong tornadoes
Helicity > 450 ... violent tornadoes these figures remain to be tested in the UK where helicity will normally lie between -200 and +200 m**2/s**2

Q. Snow situations at lowland locations are often marginal in maritime NW Europe. Why is this so, and why do forecasters find it so difficult to get it right?
(with thanks to Rodney Blackall for advice & suggestions with this and the following entry.)
A. Whether snow penetrates to the surface as snow, or melts to rain or sleet on the way down depends upon the height of the 0 degC level (ZDL) above local terrain. It should be easy over relatively flat ground: forecast yes/no for precipitation and use a good forecast model (or dense network of boundary-layer radio-sonde ascents) to find the ZDL. If you are above this level, then expect snow, if below expect rain or sleet.

The 'air-mass' zero degree level is relatively straightforward to forecast. The problem is that snow situations in our part of the world often occur with surface temperatures 'around zero', and minor deviations from the air-mass (or synoptic-scale) ZDL are important, but difficult to predict. There are several factors that must be taken into account when assessing these potential variations in the ZDL. Among these are modification of the temperature profile in the lowest layers of the troposphere due to passage over warm or cold surfaces; cooling due to evaporation of the precipitation elements as they fall through the air and cooling due to latent heat exchanges when snow begins to melt in situations that are 'marginal'. These, and other modifying effects, are discussed in Q/A 2A.26, but they will alter, sometimes dramatically, what type of precipitation actually reaches the surface.

In many of the countries of 'maritime' NW Europe, the major conurbations and the principal highways lie below the 200 m (circa 650 ft) contour. Variations in the low-level temperature structure, often involving changes in intensity from 'light' to 'moderate' or heavier precipitation can cause chaos, yet be difficult to predict and protect against except with very vague generalisations within forecasts. They are also difficult for road, rail & airport authorities, as it can be raining quite happily for several hours (when no precautionary measures can be taken), then all of a sudden, several cm of snow will accumulate as the precipitation intensity changes - or perhaps freezing rain is the result with obvious consequences. A ground height change of more than 30 m (around 100 ft) is quite normal within a town, so it is not uncommon for sleet to fall in one part of the town causing few problems, but snow in another spot nearby.

Q. So, what are the factors that can modify the temperature structure in the lowest few hundred metres in marginal snow situations?
(1): SYNOPTIC-SCALE MODIFICATION of the temperature structure of the lower troposphere. If the air passes over the sea (or similarly warm surface), then the sensible flux of heat to the air above will raise the ZDL, perhaps tipping the balance towards rain or sleet, rather than snow - windward coastal plains may miss out on the worst of the snow. ( However, these same areas may be the only places to experience moist convection in winter and provided the air is cold enough, and the sea is close and upwind, then snow showers can be frequent. ) Heat from major urban areas (provided areally extensive) can also tip the balance in highly marginal situations. If the air passes over an ice or snow-covered surface, then a flux of heat from the air to the surface occurs, modifying the ZDL structure, usually resulting in a sharp, shallow inversion. The air-mass (highest altitude) ZDL is unlikely to be affected but a secondary pair of ZDL's may form as the thermal structure of the lowest 300m is distorted and either freezing rain or ice pellets, rather than 'proper' snow is the result. This is often a difficult situation to get right after a long cold period is trying to break down.

(2): EVAPORATIVE COOLING of the air through which the snowflakes are falling. Even with the most intense precipitation, there is always lots of air around the falling raindrops or snowflakes and evaporation of the precipitation elements will occur. This will lead to a microscale cooling (due to latent heat exchanges as the liquid/ice evaporates), which multiplied by the huge number of precipitation elements leads to a net cooling of the environment through which the droplets/crystals are falling. This in turn leads to a lowering of the ZDL. The effect is proportional to the precipitation intensity and is greater when the ambient relative humidity is well under 95%.

(3): PHASE-CHANGE COOLING of the air through which the snow is falling. Rain/snow situations are often marginal at low altitudes. This means that more often than not, snow is melting in the lowest 200m or so and thus the environment is cooled due to heat exchanges consequent upon the melting of the ice crystals into liquid water. Again, intensity of precipitation is a major factor - greater intensity means that there are more precipitation elements involved which means greater overall cooling. The effect compounds that at (2) above, the net effect of evaporative and phase-change cooling is quite significant - lowering the ZDL by some hundreds of metres in prolonged precipitation. This is especially pronounced in stable air and catastrophic in near-isothermal conditions in a frontal zone. Some of the worst low-level icing conditions for aircraft occur in these situations, and of course, the ground isn't very far away!

(4): BULK (DOWNWARDS) ADVECTION of cold air due to drag by precipitation elements and by downdraughts in a markedly convective environment. Another effect that is related to precipitation intensity is the cold air that is dragged down by the falling elements and the associated downdraughts. Descending air warms adiabatically so this introduction of colder air from upper levels is offset somewhat and is the least effective modulator of those considered above. [ However, in such situations, the relative humidity will fall (greater separation between air and dew point temperature) so evaporative cooling will become more effective - see (2) above. ]

(5): OROGRAPHIC UPLIFT COOLING. As air in a thermally stable environment is forced to rise over a range of hills or mountains then the adiabatic cooling will cause the temperature to fall with height more rapidly than in the undisturbed environment. This will lower the ZDL allowing a greater downward penetration of the snow that might otherwise be expected. (This is important in a few of our major towns and cities that rise into the 'foothills' of major hill ranges, e.g. Manchester, Sheffield & Bradford.)

(6): FALLING OR SETTLING PROBLEM. Apart from the factors mentioned at the end of (1) above, snow is pretty well guaranteed to fall and settle if the surface temperature is at or below 0 degC. Rain is almost guaranteed if the surface temperature is above 4 degC. In between there is a degree of uncertainty and quite small changes in intensity can switch between sleet and snow, and between snow thawing faster than it falls or vice-versa.

Q. How do I convert millibars (or hectopascals) to inches (or millimetres) of mercury?
A. In the 17th century, when the concept of the barometer was first developed and refined by Torricelli & Pascal (amongst others), atmospheric pressure values were noted in terms of the height of the mercury column supported within a tube which had one end closed and the other end immersed in a bath of the liquid exposed to the atmosphere. Barometers continued to be marked in units of length (inches or mm of mercury) long after aneroid barometers became a common instrument - hence even today it is not unusual to see barometers marked in either inches (British / Imperial and US sources) or millimetres (European / Continental sources).

1 millibar (or hectopascal/hPa), is equivalent to 0.02953 inches of mercury (Hg). It is therefore only necessary to multiply a reading in millibars by the latter figure, to achieve the required conversion. E.g. for 1023 mbar, multiply by 0.02953=30.21 inches. To go the other way, the relationship is 1 inch of mercury=33.8639 mbar; again as an example, to convert 29.45 inches, multiply by 33.8639=997 mbar. (For those of you reading this on the continent, you are more likely to be dealing with millimetres, and the appropriate conversions are: 1 mbar=0.750062 mm Hg and 1 mm Hg=1.333224 mbar. )

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SECTION 2B: Background and special topics.

Q. The October 1987 storm - a 'hurricane', or not?
A. Well, it all depends who you talk to! Meteorologists make a clear distinction between hurricanes, a regional name given to tropical cyclones occurring in the tropical Atlantic and east Pacific basins; and intense mid-latitude storms. Reference to the following will show why:-

>> Hurricanes, or tropical cyclones, form in an environment of little or no vertical wind shear. Vertical shear (which doesn't have to be throughout the troposphere, but can also be over a very shallow layer) destroys the convection around the centre of the tropical cyclone (the 'eye'). For a tropical cyclone to continue to develop, there must be inflow of warm air at lower levels, and upper level outflow: the convection provides the 'pathway' for the necessary rising air. They form over very warm waters - sea surface temperatures (SST) values above about 27 degC, south of the sub-tropical anticyclone belt, with the capture/inflow of water vapour and sensible heat from the tropical oceans being essential to the physics of the storm. The storm is warm core, especially in the lower troposphere, with little overall (synoptic scale) 500-1000 hPa thickness gradient, and therefore hurricanes have no 'Norwegian model' fronts associated with them. Low pressure is primarily due to the contrast between the warm core of the storm, and the unperturbed tropical environment, with some contribution from compressional warming of descending air within the 'eye' of the storm, and the speed of movement of the storm is generally less than 15 knots.

>> Mid-latitude severe storms form in a strongly sheared environment, such as is found at high levels around the parent Polar Front jet core - jetstreams play no (direct) part in the formation of a hurricane. The formation of a mid-latitude storm is triggered by a short wave eastward moving disturbance embedded in the upper flow, with consequent distortion of the pre-existing baroclinic (i.e. frontal) zone. There is a strong 500-1000 hPa gradient involved. There is appreciable disturbance of the tropopause in the vicinity of the storm, particularly to the rear in the 'dry slot', and this is thought to be important in that it indicates the intrusion of dry stratospheric air - a key ingredient in the 'explosive cyclogenisis' aspect of these storms. (At the present time, it is not known for certain whether stratospheric air is involved with tropical cyclones: studies are underway to investigate this). The low (or lowering) pressure at the surface is due to an excess of divergence of mass aloft over convergence below, coupled to strong warm advection. The speed of movement of such storms are often in excess of 30 knots.

So far, so good - meteorologists are not going to get the two phenomena mixed up, but when looking at the October 1987 storm that hit the southeast of England, these clear scientific differences must be balanced against the reality of the event. For example, some very warm/moist air was entrained in the storm, possibly the remnants of a former tropical cyclone. Although the 10 minute mean winds in most cases failed to reach the threshold of 64 knots for a hurricane, two reports within the circulation over/adjacent to the English Channel did exceed this threshold, and although not analysed, 1 minute means, which the Miami NHC uses to classify hurricanes, almost certainly would have reach or exceeded this level, particularly when set against observed gusts of 70-90 knots or more, which are easily attained in mature tropical cyclones. There was widespread damage and disruption, with millions of trees damaged or felled, several people dead, ferries stranded on windward shores and given these facts, it easily matched the OED definition of a hurricane.

Prior to dawn on the 16th October, 1987, the image most members of the general public had of the damage wrought by hurricanes came from television pictures from the US or the Far East. The folk of the south east of England then are surely to be forgiven if venturing out and finding the car under a substantial tree, or whole communities cut off from electrical power, they refer to this event as "... the 'hurricane' of 1987".

(help with information relating to tropical cyclones was supplied by: Sim Aberson, a meteorologist with NOAA's Hurricane Research Division in Miami, Florida.) For more detail, visit the Tropical Cyclone FAQ at:


Q. What does the terminology in the Shipping Forecast mean?
A. The BBC Shipping Forecast, which is provided by the Met.Office, and broadcast four times daily on BBC Radio 4, is highly structured to maximise the use of the available time. The basic order of the forecast is:


(*) From April 6th, 1998, certain bulletins no longer carry coastal weather reports.

Most of the forecast is self-explanatory, but in the synoptic preamble, and in the weather reports which follows, some terms are used which may not be familiar.

Movement of pressure centres: (in forecast preamble/general situation)

Slowly up to 15 knots
(approx: up to 8 m/s or 28 km/hr)
Steadily 15 - 25 knots
(approx: 8 - 13 m/s or 28 - 46 km/hr)
Rather quickly 25 - 35 knots
(approx: 13 - 18 m/s or 46 - 65 km/hr)
Rapidly 35 - 45 knots
(approx: 18 - 23 m/s or 65 - 83 km/hr)
Very rapidly over 45 knots
(approx: over 23 m/s or 83 km/hr)

Pressure changes:(in coastal station reports/3 hours is a 'standard' time period used in synoptic meteorology in mid/high latitudes.)

Steady Change less than 0.1 mbar in past 3 hours
Rising/Falling slowly Change 0.1 to 1.5 mbar in past 3 hours
Rising/Falling Change 1.6 to 3.5 mbar in past 3 hours
Rising/Falling quickly Change 3.6 to 6.0 mbar in past 3 hours
Rising/Falling very rapidly Change more than 6.0 mbar in past 3 hours

Veering/Backing of wind: When a wind direction changes such that it moves with the clock, e.g. from east to south through south-east, that is a veering wind; A wind therefore that changes against the normal clock motion is a backing wind.

...and for the visibility categories the following apply:

FOG < 1 km < 1100 yds
POOR 1 to 3.9 km 1100 yds to 2 nautical miles
MODERATE 4 to 9 km 2 to 5 nautical miles
GOOD >=10 km > 5 nautical miles

For more details, visit the BBC Weather Centre site at:

and click here for the LATEST FORECAST.

Q. Is there anywhere I can go to look up past weather information and refer to specialist meteorological literature?
A. As part of the Public Met. Service, the Met.Office maintains the National Meteorological Library and Archive, which are open to all, particularly those with an interest in meteorology, both amateur and professional. The facilities are located at Bracknell, Berkshire, with the Library being an integral part of the Met.Office HQ building on London Road. The Library houses weather summaries extending back well into the last century, and has an excellent collection of literature, covering most of the earth sciences. It also holds most of the specialist scientific journals on the subject - several from volume 1 number 1, e.g. Weather, Meteorological Magazine, Weatherwise, Journal of Meteorology etc. The Archive holds weather charts from 1867 and observation registers for many sites at home and abroad. (For Met.code buffs, the Library also holds copies of the WMO international coding manuals.)
The library and archive are open to the public from 0830 to 1630, Monday to Friday, (Archive closed for lunch 1300-1400 and staff training takes place early on Tuesday). It is a very pleasant place to study, and can be reached easily by rail and road (although parking can be a problem at times) It would be advisable, before making a lengthy journey, to contact the Information desk on 01344 854841 to discuss your requirement and confirm opening times etc.

To go direct to the Met.Office Library WWW site, use:

Q. When was the concept of an "air mass" proposed?
A. Not long after the electric telegraph made simultaneous (i.e. 'synoptic') observations possible in near 'real time', it was realised that in regions of 'disturbed' weather, (i.e. close to what we now call a depression), two different 'streams' of air could often be found converging into the disturbed zone - each having markedly different properties. In the British Isles, Robert FitzRoy, the first director of the Meteorological Office is usually credited with highlighting this fact in 1863, though other workers, particularly in France, Germany, Holland and the United States were thinking along the same lines at the same time. Upon the death of FitzRoy, the concept tended to falter, until later workers took up the theme and elaborated upon it: Abercromby in 1887, Napier Shaw and Lempfert in 1911 and of course by the 'Bergen school': V and J Bjerknes and H. Solberg and others during and just after the Great War. These latter workers proposed the now familiar 'Norwegian model' of the life-cycle of a mid-latitude depression, whereby a minor wave develops along the boundary between two well defined air masses, amplifies (develops) and is carried forward in the general flow. The poleward air mass has an east-to-west component of air motion at low levels, is relatively cold (ex. Polar), and therefore dense, and has a relatively lower humidity value (lower dewpoint) than the 'opposing' air mass. This latter has a generally west-to-east component of motion (at all levels in the troposphere), is warmer (ex. sub-Tropical) and therefore lighter, and has a higher humidity/dew point value. The colder air mass was designated Polar Maritime, and the warmer air mass Tropical Maritime. The boundary between the two air masses came to be known as the Polar Front (see also Q/A 2A.3 and Q/A 2B.5).

Q. So, how is an 'air mass' defined?
A. An air mass is classically defined as a large body of air (many hundreds to a few thousands of km in extent),having quasi-uniform horizontal temperature and humidity characteristics. Indeed, once upper-air soundings became available on a regular basis, it could be seen that this uniformity extended vertically, such that each air mass has a distinct vertical profile of temperature and humidity. To attain these uniform (or nearly so - nothing is that clean-cut in meteorology!) signatures, a large body of air has to remain over one area for a considerable time - measured in weeks rather than days. This requires a pressure pattern which allows stagnation of the air - and this usually means a slow-moving anticyclone such as is found in the great sub-tropical high pressure belts, the polar high pressure regions or the Asiatic (or other great continental) winter anticyclones. These are said to be the 'source' regions of an air mass. Once an air mass leaves its source region, it is modified, depending largely upon the type and temperature of the underlying surface over which it moves. For example, air that moves polewards from the sub-tropical high pressure belts encircling the earth will be cooled from below as it passes over progressively colder seas, and this will in turn affect the relative humidity (increasing it leading to formation of cloud/precipitation), and although these processes may slightly lower the absolute humidity, it will still have a higher humidity value than air coming from polar latitudes, which will be warmed from below and will become increasingly buoyant as heat is input to the lower layers. Air Masses can be classified as 'polar' (having originated in cold/high latitude regions), or 'tropical' (having come out of the stagnant regions around the sub-tropical high. They are further sub-classified as either 'maritime': having passed over a sea surface, or 'continental' having moved over a land mass. This then gives rise to the four principal types of interest to us in north-west Europe:- tropical maritime (Tm or mT), polar maritime (Pm or mP), tropical continental (Tc or cT) and polar continental (Pc or cP). There are of course many modifications , and a full treatment of air masses is outside the scope of this FAQ. See the list of recommended reading at Section 5.

Q. How will 'global warming' affect rainfall patterns over north-western regions of Europe?
(thanks to Keith Dancey for this answer....which is his reply to a question in the newsgroup.....)
A. If more heat is pumped into the system (system=earth) then more water vapour will be put into the atmosphere. How that would effect our (local) weather depends upon how it would effect the world's climate, and how the world's climate effects our (local) weather. Precise answers to these questions are not known. There are climate models which can be run, and they are improving, but they are not 100% accurate, and they may never be! Such models require knowledge of the atmosphere and oceans that are beyond us at the moment, and computing power that can represent all the processes that are going on all the time. A rather tall order. You might be interested to learn that the Gulf Stream (a natural phenomenon that defines, to a large extent, the UK's mild climate for it's latitude) might even become disrupted under certain conditions in some ocean models. So whether global warming is happening, and how far it might go, is really very important, even to us. Global warming, per se, can be tested by measuring the average temperature of the surface of the sea, and keeping records for a long time. We have historical records, of varying accuracy and varying coverage. We now have instruments orbiting the globe that can measure the sea-surface temperature to breathtaking accuracy. The data indicates warming. The period is rather short. But we don't know (for certain) that this is because of us (human economic activity) or some natural phenomenon that we have yet to discover. Most scientists working in the field believe the former. Increased rainfall (and other local climate change) for the UK and Europe can indeed be an outcome of global warming. When a possibly chaotic system such as the world's climate is perturbed, it might be impossible to predict the outcome, other than there is going to be change. Global warming does not necessarily mean "drier". It certainly does not mean "drier everywhere". And it also does not mean "warmer everywhere".

Q. Is there a system of classifying synoptic weather types over the British Isles?
(NB: 'synoptic' in meteorology is used in the sense that the weather is analysed over a wide area at approximately the same time.)
A. In the early 1950's, Hubert Lamb expanded upon a classification system originally proposed in an article in 'Weather', into the now widely used Lamb's circulation types. The late Professor Lamb(*) was responsible in the UK for much work involved with deciphering the climatological changes that have undoubtedly occurred, and will continue to occur. Indeed, in the early days, the work was rather unfashionable, but is now required study given current concerns. Professor Lamb consolidated his distinguished career by taking a professorship, and the post of first director, at the University of East Anglia's Climatic Research Unit. (see 3.3 below) The system is based on the analysis of the direction of the overall isobaric pattern (not the individual wind direction at any one place) over the region 50-60N, 10W-02E. Once one of the 8 compass point directions, from which the wind blows is allocated, the curvature of the flow is considered, and the directional letters are prefixed by either: A, anticyclonic or C, cyclonic or it is left unclassified (neutral or irregular), when a qualifying letter is not used. Three other categories are recognised: A=anticyclonic (i.e. a notable high pressure itself over the region), C=cyclonic (i.e. a notable low pressure over the region), or U=unclassifiable. This gives rise to 27 classes.

Visit the UAE site to find out more, and to view the catalogue maintained by them.
(*) Professor Lamb died on Friday, 27th June, 1997

Q. How can I obtain details about periodicals/magazines that are published in the British Isles which deal with meteorology?
A. The British Isles are well served by English language magazines, periodicals etc., that cover a wide range of interest in the subject, from the keen amateur to the 'cutting-edge' academic end of the spectrum. More details are set out in Section 5B of this FAQ. Also, several sites listed elsewhere have good links/information on such matters: for example:
The Royal Meteorological Society:
and Roger Brugge's site:

Q. Would anyone be interested in my weather observations?
A. Indeed YES! For a start, the newsgroup itself is always a good place to post if you've seen something that might be of interest, particularly in the 'unusual' or 'severe' category. The best way to approach this is to 'lurk' for a short while to get an idea of what interests us, then dive in when you feel happy. If you are interested in the 'weather', this is the place for you. To try and help, there is a complete section in this FAQ (Section 6), which deals exclusively with weather observing...not intended to be exhaustive, but might give you some ideas, tips etc.

>>>With notable 'convective' weather, e.g. a decent thunderstorm, whirlwinds, tornadoes, etc., then TORRO (see 4.2) would be interested in a report. Visit their site at: for general information on the work of TORRO, and follow the appropriate link from their home page for advice on how to submit reports.
(Listed currently as: "Appeal for Information and Questionnaires").

>>>And, how about becoming a member of the Climatological Observers Link? This organisation has been in existence since 1970 and is open to anyone with an interest in meteorology. Both amateurs and professionals are registered observers. Visit: for more details.

also, a UK Weather Diary can be viewed and added to, which can be viewed at:
[ Although the national domain is 'uk', don't be put off reporting/joining up to the above outside the United Kingdom. The weather knows no national boundaries! ]

Use these navigation bars to move around the FAQ:

0. Index 1. Intro. 2A. Q/A's[The basics] 2B. Q/A's[Background] 3. Topics 4. Sites 5A. Books 5B. Mags. 6. Obs. 7. Suppliers 8. Glossary

Q. What is the North Atlantic Oscillation?
(This note prepared with the help of: Dr. Rob Wilby, Department of Geography, University of Derby.)
A. In very crude terms, it is possible to visualise the mean sea level pressure patterns affecting the north-east Atlantic as varying (or 'oscillating') between two extremes: At one extreme is a minimally perturbed westerly type, with disturbances rattling swiftly across the Atlantic, hurried along by very strong winds. At the other extreme lies a weak, sometimes ill-defined pressure pattern, but with a strong tendency for stagnation of weather types over and downwind of the north-east Atlantic. In climatological studies, some attempt must be made to quantify such variations, frequency, periodicities etc., to judge, for example, whether patterns are changing, and to find correlations to particular rainfall or temperature regimes, not only over maritime NW Europe, but also further afield.

One method is to use a measure of the 500 mbar strength between defined latitudes: By taking the difference between mean 500 mbar contour heights between latitude 35 and 60 North, this simple method yields high numbers ( a high zonal index ), for strong westerly types, and low numbers ( a low zonal index ), for weak westerly, or blocked types. Another method would be to categorise circulation types using, for example, Lamb's Weather Classification.

However, a simple measure, using observed msl pressure differences from long-term 'normals', can be employed, and can of course be extended back to times well before upper air information became routinely available. Upwind of the British Isles/NW Europe, stations in Iceland and the Azores are used, by convention, to define the North Atlantic Oscillation Index (NAOI).

The method of calculation means that lower than normal mslp over Iceland and/or higher than normal mslp over the Azores gives rise to a +ve NAOI. The converse situation gives rise to -ve NAOI values.

It should be noted that the NAOI has been strongly positive since the 1980s resulting in unusually wet and mild winter conditions over most of NW Europe and Scandinavia during this period, with concomitant changes in regional runoff.

to see data relating to the NAOI go to:

Q. What is the Central England Temperature series?
A. In studies of the climate for any region, locality etc., it is important to have a homogeneous record to describe such atmospheric variables as rainfall, sunshine, temperature which eliminate as far as possible changes in site (both location and characteristics), observing practice and so on. Professor Gordon Manley (1902-1980) developed one such series which dealt with the temperature of 'central England', defined as the area stretching from the Lancashire Plain southwards across the Midlands, and constructed using stations in the Lancashire (including the modern-day Merseyside/Gtr.Manchester) area, and the east and west Midlands. The series has been maintained since his death, although there have been changes in stations used, as some have closed/altered. Corrections also have to be applied, particularly to latter-day observations to take account of urbanisation, however the 'CET' series remains one of the longest and most widely used of its kind in the world. The record now extends, on a monthly basis, back to 1659, and on a daily record back to 1772. However, the values prior to 1721 are regarded as less reliable than later data, a fact acknowledged by Manley amongst others. Nothwithstanding this caveat, useful clues to changes in climate can be gleaned from this work. It has also been shown that the CET series is a statistically useful indicator of changes of mean temperature for a somewhat wider area than just the 'English Midlands'.

to see the monthly series maintained by the University of East Anglia go to:

Q. What impact does 'El Nino' have on the weather over Europe?
A. The 'El Nino' phenomenon, or more strictly the warm El Nino -Southern Oscillation (ENSO) event is coupled closely to remarkable shifts in weather patterns in the immediate Pacific basin, and adjacent areas: e.g. parts of North America. For example, it is clear that the altered distribution of warm/cold water across the equatorial Pacific is the primary reason why excessive rain can fall in places like Peru, and a general deficit of rainfall is experienced in Indonesia, parts of Australia and the Philippines. There is also a generally accepted link between a less-than-'normally' active Atlantic hurricane season and the notably warm event that characterises what has come to be called, THE El Nino.

What is not clear, and has not been satisfactorily proven (or indeed disproved), is whether there is any season-by-season link between the ENSO variations (warm-normal-cold), and regional climates well away from the Pacific/Equatorial region, particularly at latitudes well to the north and south of the area of SST variability. The possible scenarios seem to boil down to these:

(a) There is NO effect.
(b) There IS an effect, but it is on a scale that is dwarfed by regional variations closer to home, e.g. long-term thermal inertia in SST distribution in the N. Atlantic, or continental/oceanic temperature differences across the North America - North Atlantic - Eurasian 'super-region'.
(c) There is a direct, and marked effect that leads to verifiable modification of the weather types across the NE Atlantic/European - Mediterranean region.

(a), as Sir Humphrey Appleby might have said to Jim Hacker, is a "... brave statement, Minister!". The fact that, for example, a strong warm-ENSO event can significantly decrease Atlantic hurricane activity, and thus reduce the chance of extra-tropical elements (humidity - heat - momentum) being swept up in the mid-latitude flow, means that we must consider at the least, an indirect effect.

(b) seems to be the favourite solution least for the present, and certainly seems to be the one favoured by the majority of subscribers to Indeed, even in studies published which set out to prove the link between warm/cold ENSO regimes, and impacts over Europe, caution is always advised relating to local/regional scale modification.

(c) for the media is the one that is most attractive, and there have been studies published that purport to show a direct link between a warm ENSO season, and, for example, altered rainfall/temperature anomalies across west/central Europe. No lesser person than J.Bjerknes postulated in 1966 that altered activity in the equatorial Pacific appeared to significantly alter the strength/orientation of the PFJ over and downwind of the NE Pacific, which in turn must have at least some effect on the long-wave structure downstream. This appears to have been accepted in later studies.

However, this topic will be kept under review, as will this Q/A, and our ideas may change...for the moment though, for more on El Nino/ENSO etc., see the following sites:

[WMO home page]

[TORRO statement re:El Nino and Severe Weather]

[El Nino theme page sponsored by NOAA/TOGA-TAO]

... and of course, a search of the WWW will throw up many active sites dealing with El Nino.

In addition, on my own web site, I have set out in summary format some of the arguments/references that subscribers to asked for
... to go directly to this see

Q. What are 'sferics', and how are they obtained?
A. When a lightning discharge occurs, radio waves are emitted over a broad spectrum of frequencies. For the vast majority of people, such 'atmospherics' (or 'sferics') are simply a nuisance, leading to the familiar 'crackle' that can be detected on a home radio set, particularly in the 'AM' medium or long wavebands.(@see note 1 below)

However, in the 1920's and 1930's, Robert Watson-Watt, a British scientist (and sometime employee of the UK Meteorological Office), developed a method of displaying the discharge information on a crude cathode ray tube, and by taking simultaneous observations on the same flash, the source could be located with reasonable accuracy. (@see note 2 below) This triangulation method continued in use in this country until 1988.

The system now employed in the UK is the Arrival Time Difference (ATD) system. The origin of the lightning flash is computed from the time difference of an atmospheric arriving at several widely-spaced 'listening' stations - 5 in the UK, and 2 in the Mediterranean. Each lightning stroke has an individual signal, or wave-form, and by using accurate (atomic) clocks, and synchronisation between the detector stations, and the 'master' station at Beaufort Park, near Bracknell, an accuracy of some 5 km (soon to be less) can be achieved, although in GTS SFUK bulletins (known within the Met.Office as SFLOC's - SFeric+LOCation), the accuracy is limited by the code form to 0.5 deg lat/long.

One of the listening locations acts as a selector, and reports the time at which it detects a 'sferic' event to the master station at Bracknell. This master station then 'asks' the remainder of the outstations for detailed wave forms of sferic events close to this time and calculates the time differences and so computes a set of possible locations. Provided three or more outstations are active in acquiring that event, a unique location can be determined for that particular return.

The system is fully automatic, and theoretically can detect lightning over a large portion of the globe - up to 400 flashes an hour can be handled by the system - this too will be upgraded in the future.

For further information regarding decode of the GTS reports, and more background to 'sferics' and the like, go to the TORRO site where a specific page dealing with such matters is maintained at:..

(@1:This means that an ordinary home radio set can be used as a crude lightning detector, by tuning to a portion of the waveband -- try the LW section -- that is not used by a broadcast station. During lightning activity, irregular crackles will be heard, and with a little experience it will soon be possible to pick out 'close' from 'distant' discharges by this method - a good reason to hang on to your old portable radios after the 'digital revolution'! )
(@2:This use of triangulation of signals was later adapted in his method of aircraft detection used during the early part of the second World War.)

Q. How do I set my barometer?
A. Unless you are situated at some considerable altitude ... say above about 3000 ft (about 1000 m), then it is best for *home* use to have your barometer indicate the pressure at mean sea level. You can then relate your reading to those in newspapers, television charts etc. However, you should not expect a high degree of accuracy when using many barometers bought for 'decorative' use, and if you intend making weather reports for the synoptic network using a precision aneroid barometer (or similar), then the appropriate professional authority that collects and checks weather data should be consulted - the procedure is very different and involves careful periodic checking against a reference barometer and the use of correction tables/algorithms. (See also the Observer's Handbook... 5A.6)

For most people though the following will suffice:.... Choose a day when the atmospheric pressure is not changing association with a slow-moving anticyclone is best (but see also below re: checking over a range). Log onto a site that gives out hourly METAR reports (see the UK Weather Information url for some good sites: ), and pick a station/airfield nearest to your location. If there is no such location, then you may have to plot out several reports for the same time...draw a few simple isobars...then interpolate to find a value. With most home barometers, the nearest whole millibar is about the most you can expect in accuracy. Adjust the barometer by (usually) turning a recessed screw to the rear of the unit until the reading is correct. Keep tapping (gently!) the barometer to overcome friction within the mechanical linkage. Replace the barometer in a shaded/indoor location free from the possibility of accidental damage etc.

You should try and maintain, for say a month, a check against an adjacent site over a wide range of pressure values. By logging your values against those of this nearby site, you will be able to see if there is a systematic or random error in your reading. The former can be allowed for by slight re-adjustment or 'on-the-day' correction; the latter means you have a faulty unit, or its sited poorly -- in direct sunshine for example.

Incidentally, please take NO NOTICE of the absurd descriptive terms often placed around the dial of a barometer. When these originated is not known for sure, but it is known that Robert Hooke, the inventor of the 'wheel' barometer used such terms from about 1670: 'Change' was set at 29.5 inches; then 'Rain', 'Much Rain' and 'Stormy' at each half-inch on the lower side, and 'Fair', 'Set Fair' and 'Very Dry' on the high side. The regular spacing gives the clue to the lack of scientific credibility of such a scheme, and in my opinion they have no practical value.

Q. Why does some rainfall leave a coloured dust on my car?
A. Through the action of widespread and vigorous duststorms over places such as the Sahara, huge quantities of very fine desert sand can be carried to high enough levels (around 12000 ft/4000 m), where it can be dispersed for considerable distances downwind of the source. On many occasions, such dust is so diffused vertically and horizontally that there is little or no effect observed at ground level. However, sometimes the dust remains in sufficiently high concentrations, and can become involved with a medium level weather system, which results in the dust being transported towards such places as France, Britain and Ireland. If rain falls, the dust falls as well. This is primarily due to washing out of the dust by large raindrops. This leaves a dusty residue on car windscreens, rain gauges etc., with the most common colour being similar to old mortar: i.e. light beige, but deeper brown, orange and red hues have been observed. The effect is usually noted after light, showery rainfall, often involving medium level instability - heavier rainfall tends to wash the evidence away. A warm, southerly (Tropical continental) low-level airstream, together with a strong southerly middle level flow (circa 700 mbar), originating from the North African area are the conditions required for such events in the northwest of Europe.

Such reports are always of interest...some guidelines are contained in section 2 of this FAQ, part 6B (Notes on observing.)

Q. Why are there letters near some fronts/centres on Bracknell charts?
(thanks to Martin Stubbs for this and the following answer.)
A. To identify the more important pressure centres, and their associated fronts on actual (ASXX) and forecast (FSXX) output from Bracknell (EGRR) synoptic charts, a system of letters is used (see below). This is useful for internal Met. Office use, as the Chief Forecaster in the National Meteorological Centre (NMC) can identify such features within the written guidance issued to Weather Centres. These identification letters also enable other users of charts to follow features from chart to chart, hence the inclusion of these letters on the charts published in some of our newspapers.

The letters are normally assigned in alphabetical order as the pressure feature in question either appears from the west (or elsewhere), or develops in situ. A pressure centre (high or low) will usually carry the same letter throughout its 'life' within the area of primary interest (roughly between 50W and 30E, and 70N and 35N), although when a new centre develops and deepens rapidly and absorbs the parent depression the letter of the apparently quasi-stationary area of low pressure takes the letter of the new development. There are some exceptions to this general rule of working through the alphabet. For example, it has been the custom to give the quasi-stationary thermal depression that forms over Spain the letter 'S' and the anticyclone that forms over Greenland in the winter the letter 'G'. The letters 'X', 'Y' and 'Z' are reserved for those features that are expected to be short-lived, for example, a polar low forming to the North-west of Scotland in a North-westerly airstream may be given such a letter, or a feature that may only become significant on re-analysis when more data becomes available.

It should be remembered that the analysis (ASXX) is the first analysis carried out on the data available at quite an early cut-off time following the data time. This has to be so since the analysis has to be ready for coding and lettering just over two and a half hours after data time (the actual chart analysed covers the whole of North America, the North Atlantic, Europe and into western parts of Asia), the ASXX only being a small section of that area. Later, when later data are plotted on the charts and all the upper-air information is available, the main Atlantic chart is finalised and during this process it may well be that additional fronts or even a new developing centre maybe evident and has to be allocated a new letter which again upsets the lettering sequence.

Fronts are also identified: a 'classical' depression appearing in the western North Atlantic, with an identifier 'A', will have its warm front labelled 'A', and its cold front 'B'. The subsequent occlusion will usually carry the letter previously ascribed to the warm front (in this case 'A'). If a wave depression develops on cold 'B', its low centre will carry the letter 'B', and its warm front will be warm 'B', and its cold front 'C', and so on. Situations are never simple, and the sequence outlined here is often broken. The aim is to identify the features clearly, not to be a slave to a system of lettering! The letters on the forecast chart (FSXX) should follow on logically from the analysis on which the forecast is based, but occasionally for good meteorological reasons, but sometimes due to pressure of work, the letters change. Of special note are the occasions when tropical storms and/or hurricanes that did have names allotted, enter the area of interest. It is usual in these cases to allot the initial letter of the name to that depression, thus the extra-tropical depression that was Hurricane 'Juno', would carry the letter 'J'.

As far as is known the practice in the UK Met Office has always been to allocate letters to the features on the charts although it is thought that this may have been a numbering system which was started in about 1944. In fact the WMO International Analysis Code (still in existence) actually caters for the identification of fronts or systems using a number defined by the code 'NN'. The actual code form is 99NNSS where NN is defined in the WMO Manual on Codes as the identity number of the system or front. Thus analyses prepared at the Central Forecasting Office in Dunstable in the 1940s and early 1950s for example, may well have had identification letters, but when coded a depression with the letter 'A' would be coded as 990100, and possibly referred to on the outstations as Depression '1'. The UK actually coded its analyses and forecasts in three different ways after the Second World War. The coded analysis/prebaratic that went out on the international circuits carried the identifier group 99NNSS (for example, a depression labelled 'A' would have been coded 990100 81297 59346 . . . etc.), the analyses that went out internally within the Met Office converted this to plain language (for example, LA 81297 59346 . . . etc.) and for the marine bulletin to the ships on the North Atlantic the identifier was dropped altogether (81297 59346 . . . etc.).

Q. And what about some of the other things on these charts ?
A. In addition to the labelling described at 2B.16, much more information is carried on the Bracknell output. To begin with the legend, 'ASXX' or 'FSXX', identifies the product as either an analysis (A), or a forecast (F) relating to the surface (S) level - strictly mean sea level - for an undefined region (XX). The letters EGRR are the ICAO identifying letters which are assigned to the Bracknell Telecommunications Centre. Other abbreviations include MSLP (mean sea level pressure), DT (data time, that is the time at which the observations were made or adjusted to), VT (verifying time, i.e. the time of the expected developments indicated on the chart), and UTC - universal co-ordinated time (an acronym chosen to satisfy both the English and French speaking communities since it does not have a direct equivalent in either language - in French UTC is read as temps universel coordonne). UTC is based on an atomic standard, but for all practical purposes is equivalent to GMT.

The use of ASXX and FSXX dates from the time when the bulletins were coded using the International Analysis Code when ASXX/FSXX were the headers in much the same way as SMUK is the header for a bulletin of synoptic reports made at a main hour from the UK.

For international transfer of pictorial information the bulletins/files containing that information carry headings such as PPVA89, PPVI89 and so on. The letter P (or Q) indicates pictorial information, the second letter P indicates the information refers to pressure, the V defines the area for which the information is provided and the fourth letter indicates whether an analysis (A), or a forecast where E,G,I,J,K,M and O are for 24/36/48/60/72/96/120-hour forecasts respectively). The figure 89 refers to any parameter at sea level.

In the opposing corner of the charts are two scales; a Geostrophic wind scale (see the Glossary elsewhere and Q/A 2A.21 below), and a scale for finding distances. A tip here ... remember that one degree of LATITUDE is equivalent to 60 nautical miles (n miles), (thus 1.5 degrees is equivalent to 90 n miles and so on) Therefore, if you want to measure off how far a depression has travelled over the period of six hours between analyses, step off the distance with a ruler or pair of dividers, then lay this distance along a line of LONGITUDE in the same area of the chart, and count the number of degrees latitude that this represents. For example, if the distance measured off is five degrees of latitude then this is equivalent to 300 n miles (i.e. 5 times 60, which is 300 n miles in that 6 hours; The overall speed of movement of the feature is even simpler to define for that 6 hours: one has only to remember that 1 degree latitude in 6 hours (i.e. 60 n miles/6hr)=10 knots; therefore if a feature 'steps-off' 3 degrees of latitude in that 6 hours, it must be moving at 30 knots.

These simple calculations can also be used to forecast the expected movement of fronts using the simple methods described in text books (for example, active cold fronts can be advected (moved) at a speed of four-fifths the measured geostrophic wind measured just ahead of the front, the vector being in the direction of the warm sector isobars. A factor of two-thirds can be applied to the measured geostrophic wind to give the expected movement of the warm front. Now to the chart itself:...The manner of allocating letters to the high's and low's (points of highest and lowest pressure respectively with respect to the surrounding isobaric pattern) has already been described at Q/A 2B.16. In addition, on the ASXX, the past 12 hour track of the major low and high pressure systems are shown by short-dash lines with the time/pressure value of the past location annotated alongside its former position. On the 24 hour FSXX, the past track from the previous analysis position (24 hours ago) is sometimes shown, again with a date/time and pressure value. These past tracks are used sparingly, as the chart can become cluttered.

Isobars are drawn every 4 millibars on charts originating in the UK, the USA and Canada but note that a 5-millibar spacing is more common on charts originating in countries in continental Europe. Isobars are labelled with values in whole millibars (or hecto-Pascals/hPa), starting at 1000 hPa.

Fronts are drawn with heavy solid lines, distinguished by solid 'triangles' for cold fronts, solid 'bobbles' for warm fronts, and a mixture of the two for an occlusion. The triangles/bobbles point in the direction that the front is heading/thought to be heading and placed on alternate sides of the line when the front is quasi-stationary. Heavy lines with no such additions indicate troughs (the word 'TROUGH' may or may not be indicated beside the line). There are occasionally variations in the graphical representation of fronts. If the front is considered to be a feature more significant in the upper atmosphere than at the surface then the 'bobbles'/'triangles' are left unshaded. If the front is significantly weakening (frontolysis) then a cross hatch is placed across the frontal line between the triangles/bobbles, and if a front is considered to be forming (frontogenesis) then the solid line appears broken.

On the 'medium-range' charts (e.g. T+48, 72 etc.), there are additional long-dash lines. These are the 500-1000 hPa total thickness lines at 18 decametre intervals... see Q/A 2A.5 earlier in this FAQ.
(The medium-range charts are currently listed as Additional Products within the context of the WMO Resolution 40 (WMO Twelfth Congress 1995) and may not always be available on the Web.)

Most of the output is now produced using on-screen analysis and field modification tools. The days of hand-drawing charts for both actual and forecast purposes are coming to an end.

Q. What is an "Indian Summer", and why is it so-named?
A. (this quoted directly from the Meteorological Glossary, HMSO): " A warm, calm spell of weather occurring in the autumn, especially in October and November. The earliest record of the use of this term is at the end of the 18th century, in America, and it was introduced into the British Isles at the beginning of the nineteenth century. There is no statistical evidence to show that such a warm spell tends to recur each year. "

C.E.P. Brooks, in his 'Climate in everyday life', notes that it is the counterpart of our 'Old Wives Summer', here in Europe, and tends to follow the first severe frost and to persist for several days.

It is thought that the phrase was coined by european settlers on the Atlantic coast of North America. Paul Marriott, in his 'Red Sky at Night, Shepherd's Delight', says..." strictly an Indian Summer is a lengthy dry sunny spell from late September into November. The name probably derived from the N. American Indians who relied on a similar fine spell in late autumn for harvesting. " Philip Eden, in his 'Weatherwise' (see entry 5A.5 of this FAQ) also ascribes this reasoning to the term.

Such spells of fine, warm dry weather may be 'reliable' in the Atlantic states of the USA; this is not so for our own climate, and Marriott (amongst others) found expectation of a period of such weather in the U.K. to be misplaced.

Q. What is a 'Polar low'?
A. When arctic-origin air in winter flows southward (northward in the southern hemisphere) across (relatively) warmer seas, strong surface heating acts both to enhance the degree of instability, and trigger vigorous moist convective towers. This is sufficient alone to give rise to heavy, wintry showers/cumulonimbus clusters etc., but often marked troughing, or even a closed circulation in the isobaric flow is found; the resultant low-level convergence/positive vorticity enhancement, plus the localised concentration of the latent heat energy released, enhances development within the system, and an intense (but synoptically small) area of rain, hail, sleet or snow & squally winds can result - a polar low (or polar depression in some texts).

The dynamics of such systems are not fully understood, and it is only with the (recent) arrival of very high-resolution satellite imagery & sensors in a wide variety of spectral bands that the detail within such systems can be studied. Even so, for operational meteorologists, careful monitoring of all available data is required; Geostationary satellite imagery has a rather course resolution at high latitudes, and the visible channels are of little use in the winter season. Polar orbiter passes (which give much higher resolution imagery) may not be frequent enough to maintain a continuous watch on developments.

Numerical models also have difficulty with such events; they are born in data-sparse regions, and most schemes 'paramaterize' convection i.e. models don't explicitly forecast each individual convective event, but rather indicate the degree of instability expected, its areal extent etc., and thus have problems going one step further and turning an area of disorganised (model) convection into an organised self-sustaining polar low/trough, where upper troughs are not the primary forcing mechanism. The one remaining Norwegian weather ship, and a handful of research and fishing vessels may be the only clues to developments taking place in, for example, the Norwegian Sea.

Polar Lows can develop, and move (in the prevailing flow) with surprising speed, and lead to considerable dislocation of normal life in regions directly affected. Preferred locations for genesis are to the west of large, slow-moving occluded depressions - i.e. those with a pre- existing rear-flank arctic flow. It may be that the geography of the regions in question play a significant part in genesis of polar lows - Dave Wheeler, who I am grateful to for checking much of the above, suggests that vortices shed by high-arctic island groups (e.g. Svalbaard) are enhanced by the land mass of Scandinavia (Norway) to the east and Greenland and its ice shelf to the west.

Use these navigation bars to move around the FAQ:

0. Index 1. Intro. 2A. Q/A's[The basics] 2B. Q/A's[Background] 3. Topics 4. Sites 5A. Books 5B. Mags. 6. Obs. 7. Suppliers 8. Glossary

Q. I want to learn more about 'the weather' - how do I go about it?
A. A big question, and it all depends on the area that interests you most and whether you want to extend a 'hobby' interest, or study as part of a career choice.

Many are interested in the 'weather' as an absorbing hobby, perhaps introduced to the subject via school/college (often part of the geography syllabus), or because of a sporting/recreational activity e.g. yachting, surfing or gliding. A good number of books have been written over the years and the first port of call I suggest is to go to your local lending library and see what is on the shelves. Don't be put off because a book has been written for 'sailors' or 'pilots'. These are often very well written by professional meteorologists who have a keen interest in the sport/activity involved, and will cover the elementary facts you need to know in a clear way, without the use of complex mathematics.

At some stage you will want to purchase one or more books that you can refer to at leisure. Some ideas are given in Part 5A of this FAQ. Not all the books are currently in print, although as an addicted browser of second-hand bookshops, I have found it surprisingly easy to pick up good quality books relating to meteorology in this way. It is also worth asking about availability of titles from a good bookseller.

If you are involved in sports/activities such as gliding, sailing, ballooning etc., the associations or clubs that you will probably belong to may offer courses, ad-hoc instruction etc. Contact them for details. The Royal Yachting Association (RYA) in particular encourage its members to be aware of weather processes, availability and interpretation of forecasts etc: visit their web site at:
Residential study courses (e.g. Field Study Courses, Met. Office College courses), are available, which are an excellent way to get to grips with meteorology in a somewhat deeper way, as well as enjoying some congenial company and the benefit of an experienced instructor. Find out about these from 'activity' magazines, good travel agents, newspaper weekend supplements etc. For the Met.Office College courses, visit their web site at: -
Subscribe to one of the magazines listed in Section 5B: 'Weather' or 'The Journal of Meteorology' will provide much of interest. Don't be put off if you are 'new' to the subject; there is much to appreciate about the daily changes in the atmosphere which surround us for which you don't need a degree in Maths! These magazines will also publicise new books coming on to the market, and 'Weather' in particular often carries articles that deal with elementary meteorology.

And of course the Internet itself is increasingly a help with self-education. Many of us are trying to work up information pages on basic meteorology ... I have a few items on my web site ( , and this FAQ and its Glossary attempt to cover some topics that frequently appear in the newsgroup. Use a search engine to do a bit of hunting: many North American sites carry some elementary instruction - one site that I particularly like is:
and for a general listing of sites, (with a heavy North American bias) go to:-

Now, moving on to study of the subject on a professional level, then you need to decide in which area your interests lie. My job is part of what loosely can be regarded as 'operational meteorology', i.e. forecast services for the general public, aviation, maritime and commercial customers. But this is one small area of what we might regard as the atmospheric sciences discipline. The 'flavour of the moment' is of course the study of climatology, both in historical terms, ( trying to reconstruct the climate of centuries past to detect long-term trends ), and for the future - for example predicting how atmospheric gas composition will change due to industrial and other processes, and how these changes will affect the weather in the decades and centuries to come. Atmospheric chemistry (for example the study of ozone in the high atmosphere) has an important part to play in the protection of human (and other) life on the planet from harmful solar radiation, as well as being important in understanding the heat budget of the atmosphere. Another specialism is the study of 'micro-climates' around mature woodlands, or in urban situations for example. Studies also increasingly cross formerly rigid disciplinary boundaries, such as into the realms of oceanography and vulcanology.

To pursue a career in meteorology, a solid grounding in mathematics and science subjects is required, particularly physics. The Royal Meteorological Society (see Q/A 4.4) have some excellent advice on their web go directly to this page, go to:.....
From this page, there is a link to a list of Universities that provide courses, with sites not only within the United Kingdom, but in other parts of Europe and across North America. To go straight to this page use:.....
and to find the latest information about University courses within the UK, and a host of other information about tertiary level education in the atmospheric sciences field, visit the Universities and Colleges Admissions Service (UCAS) site at:...
..... finally, a word of warning. If like me you decide to go into 'front-line' forecasting, be prepared for some sleepless nights! Not only do we work shifts of course, but you must be prepared for the disappointment of being woken at 4am to the sound of rain lashing against your window, that you confidently expected to hold off until lunchtime at least: a thick skin, and a sense of humour are a requirement!

Use these navigation bars to move around the FAQ:

0. Index 1. Intro. 2A. Q/A's[The basics] 2B. Q/A's[Background] 3. Topics 4. Sites 5A. Books 5B. Mags. 6. Obs. 7. Suppliers 8. Glossary

3. Questions relating to sources of information.
[ NB: The list below is rather 'hit and miss' in that it only contains the sites that I am aware of. If I've missed you out -- please let me know, and I'll add it to future issues...its not a deliberate snub! ]

Q. Where can I find a map of the BBC Shipping Forecast areas?
A. There is a map of the areas, rather small, at:

Q. Where can I find information on codes and coding?
A. Try Dave Wheeler's site - a wealth of information on the SYNOP code - at:
>>>> also hosts a display of the 'ww' symbols for present weather.<<<<

Q. Where can I find information on Climate change?
A. There are many sources of information on the current debate relating to our changing climate, both its natural variation, and anthropogenically forced change. A useful FAQ to download is the Climate Change FAQ, held at: and which is also posted about every 2 months to sci.geo.meteorology and sci.answers, amongst others. It can also be retrieved via anonymous ftp from: Try also visiting the following web sites:
The Hadley Centre:
University of East Anglia:

and the US Global Change Research Information Office (GCRIO) makes available, on-line, a booklet which sets out to answer common questions relating to current concerns about climate change. This is a joint publication of the UN Environment Programme, and WMO. It would be particularly suitable as a first source reference for senior school level studies.

For an introduction, and some data, dealing with the part that ocean circulations play in climate change, go to:
(thanks to Rodney Blackall for the site suggestion)

Q. Where can I find information on Noctilucent cloud?
A. The Noctilucent cloud (NLC) FAQ can be viewed at:

Q. Where can I find information on Satellite systems?
A. There is a multi-part FAQ relating to satellite imagery which will tell you almost everything you wanted to know about satellite systems and the imagery available. It is posted every month to sci.answers and news.answers and is also available from:

Q. Where can I find information on the use of computers in meteorology?
For the ECMWF suite of models go to:
For the UK Met Office go to:

For the NCEP models go to:
For the US Navy, Fleet Numerical Meteorology and Oceanography system (NOGAPS) go to:
For the DWD suite of models go to:

Q. Where can I find more information on dew point, relative humidity etc?
A. The _Temp, Humidity & Dew Point_ ONA (Often Needed Answers), FAQ is posted on an irregular basis to the following newsgroups: sci.geo.meteorology, sci.answers, news.answers and tells you all you would want to know about the subject.

Best seen at the following www site:

Q. Where can I find information relating to the estimation of Beaufort wind force, and the use of Beaufort letters?

Q. Where can I find more information relating to ozone concerns, both at stratospheric and near-surface altitudes?
A. For information on the chemistry, and current distribution, of low-altitude ozone (and other pollutants) in the UK, visit the DETR site at:

For the FAQ relating to stratospheric (high-altitude) ozone concerns, go to:

... and for current and recent stratospheric ozone measurements over the British Isles go to:

Q. Where can I find a site to decode a METAR?
A. Amongst other useful information, and links to other sites, data sources etc., this site has a decode page relating to the METAR and TAF code: ... then follow the route from the current weather button.

Q. Where can I find sites detailing extremes, notable past events etc?
A. With the increased availability of web-space for home use provided by many ISP's, many contributors to usw have provided data on past weather events for anyone to view. The list here is begging to be added to, so if you have such a source of data (for european events), then let me know. TORRO host a site detailing extremes for such as temperature, wind speed etc., at:
Dr. Trevor Harley, Dundee University has worked up the following interesting site of 'notable' events: (via)
The BBC Weather Centre maintain summaries of recent months/years weather at:
The Monthly Summaries held on the University of Reading server are also a good first point of call for figures over the past few months/years:

Use these navigation bars to move around the FAQ:

0. Index 1. Intro. 2A. Q/A's[The basics] 2B. Q/A's[Background] 3. Topics 4. Sites 5A. Books 5B. Mags. 6. Obs. 7. Suppliers 8. Glossary

4. Pointers to other sources of information.

sci.geo.meteorology, has a series of FAQ's for data sources. The multi-part FAQ is posted regularly to sci.geo.meteorology. You can obtain current copies of this FAQ series by anonymous FTP at:
or in hypertext form at

4.2 TORRO:
For the latest news on severe thunderstorm events, tornadoes, waterspouts etc., a visit to the web site of the Tornado and Storm Research Organisation is essential. TORRO have produced a FAQ relating to matters that they are particularly interested in - a 'short' version is now available. In addition to the information on "severe" convective weather events, there are a host of other useful data and www links. Also visit this site for details of the TORRO Membership Scheme:

This is one of the best maintained and most comprehensive of sites with links to 'weather data', i.e. both actual(real-time), forecast and climatological, not only for Europe, but around the world. There are also a multitude of links to other sources of information e.g.
> information on magazines/journals
> information on jobs available
> Climatological Observers Link (see Q/A 2B.9)
> Sources of monthly summaries
> Meteorological definitions
... and so on: far too many to list here.
Find it at:
... and for the NW european area:

Useful site with information about the activities of the Society, publications, services and links to other sites. The site is maintained by Roger Brugge:

Well known for its work on climate research/change etc:

The Central England Temperature (CET), England and Wales Precipitation (EWP) and other data sets are available via this site. To go directly to these data use:(courtesy of Mike Hulme)

And, for graphs showing such as Global temperatures, Southern Oscillation Index and North Atlantic Oscillation Index go to:

Lots of information on the presentation of weather forecasts, the team at the BBC Weather Centre, and background to the forecasts. Also, a fuller description of the Shipping Forecast terminology, and monthly summary details including provisional CET and EWR values.

A primary source for weather related information of interest to all contributors. maintained by: David Reynolds and David Wheeler:

a comprehensive site of links, information and data. maintained by: Ian Waddell:

links, services and articles e.g. El Nino, Long-range forecasting etc. maintained by: Peter Wright:

local - Berkshire - weather, forecasts for winter, links etc. maintained by: Will Hand:

links to forecasts, information etc., relating to Ireland:

Lots of local data, plus useful links to many other weather sites:

"This is a site for weather enthusiasts, hosted by Prodata Associates. It is intended as a resource especially for those with a fascination for setting up and using their own automatic weather station". [ Although this site carries some promotional material, its contents and advice will prove useful enough to justify inclusion in this FAQ.]

Many useful links to, for example, satellite imagery, current & past weather, forecasts, weather reports/summaries for Birmingham & a useful list of meteorological abbreviations, decoding information etc. maintained by Richard Adams: or

Various links and weather services/information etc., and details of books written by Ian Currie, the author of this web site. Of particular interest is the information relating to the magazine "Weather Eye" (see also 5B.6):

The main benefit of this site for many will be the very comprehensive 'Education' pages. Some fine work explaining Geopotential, Vorticity and Skew-T/Log(P) diagrams, plus associated methods of use and how to work out things like CAPE, Lifted Indices etc.

Use these navigation bars to move around the FAQ:

0. Index 1. Intro. 2A. Q/A's[The basics] 2B. Q/A's[Background] 3. Topics 4. Sites 5A. Books 5B. Mags. 6. Obs. 7. Suppliers 8. Glossary

5A. Some books that are worth reading.

Don't forget books! In fact, whilst there is a wealth of information on meteorology available via 'on-line' or other electronic methods, a good book on meteorology on your shelf to refer to is worth many hours of idle browsing. The list below is not intended to be 'the final word' but are ones that I have found useful. Some are no longer in print, so you will need to hunt them out at second-hand bookstores, or request them from your local library. The title is given first, followed by (publisher/author) on the next line.

5A.1 Handbook of Aviation Meteorology:
(HMSO/Meteorological Office)
This book was first published in 1960, and has recently (1994) been completely revised to its third edition. It is a weighty tome, and expensive, but worth the outlay, as much of the basic meteorology contained therein doesn't change radically. Note that it also includes useful information on decoding SYNOP, METAR and TAF data, and a guide to interpretation of Aviation SIGWX charts.

5A.2 Pilots' Weather:
(John Murray/Ann Welch)
Probably only available in libraries, but well worth hunting down, whether you are a pilot or not! Some fine case studies.

5A.3 A course in elementary meteorology:
(HMSO/Meteorological Office)
A 'back to basics' book that does perfectly what the title suggests - it is 'elementary' in the sense of being a thorough grounding, not superficial.

5A.4 Guinness Book of Weather Facts and Feats:
(Guinness Superlatives/Ingrid Holford)
Plenty of useful information on 'the weather', though you need to be wary of the 'extremes' of course, as they tend to change! Worth obtaining a copy for some of the photographs, and is a useful 'first-stop' for information.

5A.5 Weatherwise:
(Macmillan-for the Sunday Telegraph/Philip Eden)
As well as writing for the Daily and Sunday Telegraph, presenting weather forecasts on BBC Radio 5/'live', and acting as a consultant for the PA WeatherCentre, Philip Eden is a regular contributor to the newsgroup, and indeed was responsible for its inception! To quote from the back cover: 'Weatherwise takes the reader through the year month by month, looking at both the typical and the freakish.....'. The book is a wealth of information on the British weather and written in an easy-to-read style, without losing any of its authority.

5A.6 Observer's Handbook:
(HMSO/Meteorological Office)
A fine source of information relating to observational procedure and standards of observing, with individual chapters relating to the principal elements to be observed, e.g. clouds, visibility, weather, wind etc. A useful section on observing and recording special optical phenomena, e.g. halo, noctilucent clouds etc.

5A.7 Essentials of Meteorology:
(Taylor and Francis/D.H.McIntosh and A.S.Thom)
A good 'first read' when trying to get to grips with the finer points of, for example, the gradient wind equation. Even tells you how to construct your own tephigram! Don't be put off by the mathematics early on in the book though - there is plenty of good general meteorology that will interest all.

5A.8 Climate, history and the modern world:
(Methuen/H.H. Lamb)
Any book written by the late Professor Lamb (see also Q/A 2B.7 above) is well worth reading, and this book in particular is an ideal introduction to the subject of the study of past climates, and the impact that climate change can have on mankind.

5A.9 Teach yourself weather:
(Hodder & Stoughton/Ralph Hardy)
Good for beginners and provides further references for those who want to get into the mathematical side of things. It provides a good grounding on models, forecasting, observing and lots of other useful stuff. (thanks to Ian Waddell for this write-up)

5A.10 Regional Climates of the British Isles:
(Routledge/ed: Dennis Wheeler and Julian Mayes)
This recently published (1997) volume is packed with information that all reading this newsgroup will find fascinating. After an introductory chapter dealing with the global circulation, and its relevance to regional/local weather systems in the British Isles, the following chapters focus on the regions which make up these islands. Each is packed with data and accompanied by illustrations, photographs, maps, synoptic charts etc. The final chapter deals with climate change and its impact on our region. In both hard and soft-back.

5A.11 Aviation Weather:
(Jeppesen Sanderson Training Products/Peter F. Lester)
The title is self-explanatory and its content should be of interest to many regulars and irregulars (!) of this forum. The publication comes highly recommended by Norman Lynagh, a stalwart of this newsgroup, and an individual with long experience in meteorology and its applications. He gives the contact point as:- Jeppesen UK Ltd., Three Bridges, Crawley, Sussex.

5A.12 Climate and the British Scene:
(Collins/Gordon Manley)
Originally written nearly half-a-century ago; the most recent revision I can trace is dated 1962. Therefore it will not be found except via libraries, second-hand bookshops and personal collections. It is worth tracking down for its authorship alone: Professor Manley is acknowledged to have had a deep understanding of the climatology of the British Isles, and his lasting legacy to climate studies (amongst a wealth of papers, books etc.), must be the CET series (see Q/A 2B.11). And the book is a "good read" - what better recommendation can there be? (thanks to John Hall for the suggested entry)

5A.13 Images in weather forecasting:
(Cambridge Univ.Press/Bader
First published in 1995, this is probably the most comprehensive guide to interpreting satellite and radar imagery that you can possibly imagine. Packed full of images, diagrams, conceptual models and explanatory notes, this volume is rapidly becoming *the* standard reference for use by 'practical' meteorologists .. rather expensive in hardback, but recently issued in paperback format.

Use these navigation bars to move around the FAQ:

0. Index 1. Intro. 2A. Q/A's[The basics] 2B. Q/A's[Background] 3. Topics 4. Sites 5A. Books 5B. Mags. 6. Obs. 7. Suppliers 8. Glossary

5B. Some magazines/periodicals etc.

Some magazines/journals which deal with meteorology, atmospheric sciences etc., on various levels are listed below: (N.B: If I've missed any, particularly for Ireland and regional areas in the UK, please forgive, but let me know the details so I can publicise!)

5B.1 THE JOURNAL OF METEOROLOGY: published monthly (sometimes every 2 months), in association with TORRO (see also entry 4.2: below). write to:- The Journal of Meteorology, 54, Frome Road, Bradford-on-Avon, Wiltshire BA15 1LD. [ This magazine is an excellent read for all interested in meteorology, and although it has a core specialisation in thunderstorms and allied phenomena, it also carries well-written articles relating to a wide range of general meteorological topics. Also contains a statistical summary of the UK weather for a selection of sites and detailed summaries of thundery/tornadic activity. Suitable for anyone with a keen interest in the subject, both amateur and professional. ] follow the appropriate link from:

5B.2 WEATHER: published monthly by the Royal Meteorological Society (see also entry 4.4: below.) write to: -Royal Meteorological Society (Weather Subs), 104, Oxford Road, Reading, Berkshire. RG1 7LL. [ This well produced magazine contains articles covering the full range of the meteorological science, from 'case-studies' of single events, through reviews of notable months/seasons in recent and long-past history, to long-term climatological and atmospheric physics surveys. You also get the monthly 'Weather Log' (currently edited by Philip Eden), which is an invaluable record of the weather, as it contains a series of miniature daily weather maps covering Europe and the North Atlantic, and a written summary of the weather, statistics etc. Suitable for all levels of expertise, both amateur and professional. ] follow the appropriate link from:

5B.3 METEOROLOGICAL APPLICATIONS: published quarterly in association with the Royal Meteorological Society (see also entry 4.4: below) Write to:- Journals Marketing Dept., Cambridge University Press, The Edinburgh Building, Shaftesbury Road, Cambridge. CB2 2RU. [ This magazine, (which replaced in large measure the long-running 'Meteorological Magazine' series in the UK), is aimed primarily at professional meteorologists, and users of meteorological services - hence the 'applications', and some of the subjects are given a rigorous mathematical/physical treatment. Despite this, keen amateurs would find something of interest in most issues, but unfortunately the annual subscription may be rather steep for many. ] follow the appropriate link from:

5B.4 QUARTERLY JOURNAL OF THE ROYAL METEOROLOGICAL SOCIETY: published eight times per annum by the Royal Meteorological Society (see also entry 4.4: below). Write to:- Royal Meteorological Society (QJRMetSoc), 104, Oxford Road, Reading, Berkshire. RG1 7LL. [ This publication is one of the leading meteorological journals in the science. It contains the results of ground-breaking research in the atmospheric sciences and applied meteorology. Its deep treatment of the subject, plus the annual subscription level mean that this publication is aimed at the academic and professional end of the market. ] follow the appropriate link from:

(NB: the Royal Met.Soc web site also contains useful links relating to other, more specialised magazines that are available.)

5B.5 CLIMATOLOGICAL OBSERVER'S LINK - BULLETIN: published monthly. Details (and a specimen/free copy) are available from: Roger Brugge, 16 Wootton Way, Maidenhead, Berkshire. SL6 4QU. [ Roger Brugge is the secretary of COL which aims to publish the bulletin by about the 24th of the following month --- whilst the previous month's weather is still relatively fresh in the mind. They claim that the bulletin is one of the fastest sources of monthly meteorological data to be published. In addition to the monthly station summaries (currently around 300), there is a synopsis of the month's weather, letters page, mean surface pressure maps etc.] for more details, go to:

5B.6 WEATHER EYE: Three issues per year. Details via the web site: or from: Frosted Earth, 77, Rickman Hill, Coulsdon, Surrey. CR5 3DT [The following is taken from the description on Ian Currie's web page..."If you have ever marvelled at a spectacular sunset, 'Weather eye' is for you. If you have set up a thermometer or rain gauge in the garden or regularly tune into the TV or radio for a Weather Forecast then 'Weather eye' will keep you absorbed in what is a fascinating subject"]

Use these navigation bars to move around the FAQ:

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6: Some notes on observing and reporting weather events to the newsgroup.

It has been suggested that some advice regarding the reporting of weather events in posts within the newsgroup might be made available. This section attempts to fulfil that requirement.
I do not claim any special expertise in this matter, so please regard this as an attempt to stimulate thought and discussion. Also, this should not replace a good work of reference regarding observational standards: see item 5A.6 (The Observer's Handbook) above. Where possible, I have added pointers to web sites that give more information. If you know of others, please advise me and I'll add them in on the next revision.

The note is divided into the 'mechanics' of the report, i.e. time-keeping, recording location etc (6A); the actual report, i.e. what to look our for; some notes relating to units used, standards etc.(6B), this latter with the help of Paul Bartlett, and a section dealing with requirements for instrument exposure (6C).

6A. Recording/reporting the event:
An important thing to get right is the time of your observation, and even more important the time standard. In the UK, during the summer, we add 1 hour to GMT (or UTC, or 'Z' time...they are all interchangeable for our purposes), and call it British Summer Time. However, international meteorology runs effectively to GMT. So, when you report a phenomenon, use GMT. When BST is in force (in the uk), this means taking one hour off your watch time, so a waterspout seen at 7 pm on the south coast in mid-July should be reported as occurring at 1800 GMT. If you suspect your watch/clock might be in error, check against a time-signal (or similar e.g. teletext clock), and adjust accordingly.

Double-check the date as well! This might not seem an obvious point, but particularly when you are reporting something a day or so later than the event, its easy to get the days mixed up. Watch particularly the time around midnight when we are on BST. Something happening at 15 minutes past midnight by your watch on the 15th, should actually be reported as happening at 2315 GMT on the 14th.

Locations are most important. Its easy to report an exciting event as having happened, and forget to tell us where you are! Have a 'sig file' made up that includes your location, height amsl and other important facts, and use it as appropriate. Obviously if you are out and about, then include as much information about the observing point as possible, or if on a car/train journey, the area where the observation was made. This includes not only the lat/long and/or grid ref. (or town/village if that's easier), but a description of the terrain, location of adjacent water surfaces, direction/height of hill/mountain ranges etc., if they would not be universally known. These latter points of information might help diagnose phenomena in difficult situations, and allow others to relate their observations to yours.

Get into the habit of having a scrap of a pencil and some paper with you to note down important details for later transcription to the newsgroup. Even better of course would be a dictating machine, or a palmtop to record the events. Also, keep a copy of what you post, if your newsreader doesn't already allow this. Someone may want to follow up your report weeks, or even months in the future. Don't rely on your plays tricks! Always note down the important features ASAP after the event, preferably as it is occurring.

When photographing events, as well as the usual location, time, date etc., note down the readings from the camera...f-stop, film speed/type, shutter speed etc. What state the sun (or other illumination) was in..cloudy/part cloudy/clear; behind/in-front of observer etc. The information may not be needed, but then again, it might! Camcorders are becoming more popular for recording weather events. However, even if you think you have the world's finest footage, make a brief note of the event in longhand just in case what you saw doesn't quite live up to expectations when the tape is re-played. Try to get objects of known dimensions and distance in your shot so that some comparative assessment of the tornado/waterspout etc., can be made. In the specific case of photographs of hailstones, include some form of measure, such as a centimetre rule.

As well as your own report, try to gather other eye-witness accounts, particularly in the case of 'severe' weather events. Newspaper cuttings are invaluable, even if they turn out to be rather sensationalist. If you do actually 'cut' out the report, make sure you annotate with the date of publication, publisher's address etc., in case anyone wants to follow up the news item. Make a note of any local radio and tv reports, and in the latter case try to record the news report when any action shots are broadcast.
(see also Q/A 2B.9)

6B. The event itself...what to look out for:
There are four broad areas you might want to consider for detailed weather reports:
1. Convective activity.
2. Wintry weather.
3. Severe winds.
4. Other 'interesting' phenomena.

1. Convective activity:
Note particularly the development of the cloud/s giving rise to the thunderstorm/tornado/waterspout/etc. What we are looking for is the rapidity of build of the cloud; its vertical extent in a noted time. Is it building directly upwards, or sheared to one side? Is it possible to determine whether you are observing a single or multi-cell complex? Is any rotation observed in the cloud elements etc? What was the wind regime, before, during and after the event? Note particularly the onset of notable gustiness, changes of wind in direction/speed as compared with onset of precipitation etc. Was a sea breeze front involved? This can sometimes be inferred by a change in humidity (it should 'feel' more humid), and there may be a line of precursor cumulus development, with a change in visibility and wind direction perhaps.
The type, duration and intensity of precipitation (abbr: ppn) should be noted; the Beaufort shorthand notation can be usefully employed for this (see for example::
If possible, amounts of liquid and/or solid ppn should be recorded, particularly accumulations of hail and snow, the diameter of hail, the appearance of hail (i.e. cloudy, translucent, mixed layers - these will define the various cloud regimes that the hail-stone has spent its life amongst.); the type of snow/pellets etc., and if possible, determine which portion of the cloud the ppn appeared to originate from. for advice on types of solid precipitation, see::
In very severe rainfall events, its worth noting how water-butts and other containers fill up and to what extent. Whilst not being used to give a definitive rainfall amount, they can sometimes help to verify the order of magnitude of adjacent reports when the final report is written. Note the winds aloft prior to the storm onset. Severe convective storms form in an environment of marked vertical wind shear, both in direction and speed and this can sometimes be inferred from visual observations. Be careful to judge such motion against a static object though. Its easy to be deceived by relative motion of other clouds. You can usually position yourself in such a way as to have a tree, or corner of a building, or pylon or similar in the eyeline to provide the 'fixed' point.
Note the cloud features preceding the event, in particular any Altocumulus castellatus, floccus or towering cumulus development. Note the damage caused, flooding experienced etc. In particular, the period over which flood waters both rise, and subside, and the horizontal extent of the flood waters. Hail and wind gust damage reports are most useful, and the TORRO event reporting form is the best place to refer to for guidance on these aspects. see:

2. Wintry weather:
Although winter doesn't necessarily mean snow, that's the element that interests a great many people dipping in to the newsgroup. As an aside, although the UK radarnetwork can detect areas of precipitation very well, there are problems with detecting areas of rain versus snow, due to the narrow temperature/humidity band within which the phase-change occurs. The more reports of snow (or not-snow) are made, the better is the overall picture. Also, the more reports of snow lying are made the better as the 'synoptic/climatological' network doesn't always pick up the variability of nation-wide snow cover. A record of the times of onset/cessation of 'wintry precipitation', intensity, types etc. should be made, together with accumulations. This means not only noting the total depth of snow, but also the accumulation of fresh snow on top of old snow cover.
Is the snow drifting or blowing? Some types of snow will drift more that others - note whether snow is drifting after having earlier fallen, or is blowing around as it falls. In blizzard conditions the distinction will not always be clear. Note how old snow settles..its continuity (complete or patchy cover) and regularity (an even depth or irregular depth, with drifts)..its persistence from day to day.. try to note the depth/extent etc., at 0900 GMT as this is the reference time for snow cover for climatological reports, and your report can be integrated with other observations. Simply coming on line just after 0900GMT and reporting ....'I've got a snow cover of 2 cm', or ' all yesterdays snow has gone', can be a great help in gauging the overall picture.
Remember that snow can evaporate (sublimate) and settle, as well as melt, so get into the habit during a prolonged spell of snow-lying of always measuring the snow depth in the morning - don't assume its still the same depth just because its still there. Take three samples of undrifted snow that represent the 'general' picture, and average out, but also report notable departures from these readings. Garden canes etc., can sometimes be used as semi-permanent depth markers, provided of course the underlying soil isn't so frozen that you can't push the cane in the ground! In particular, dimensions of notable drifts will always be of interest. Note cases where heavy rain aids snow melt etc. Glazed ice - measure thickness - rapidity of accumulation - temperature fluctuations before and during event. The weather immediately preceding the event. Length of the event and consequences on traffic etc. Photographs of these events are always interesting.

3. Severe winds:
In particular, damage reports are most useful. Is the damage observed in a narrow/focused swathe, or widespread and indiscriminate. What is the scale of damage - i.e. a few minor branches, whole trees, chimney stacks down etc. The period during which the most damage was caused. Any special features .. twisting of branches, tree trunks etc. Other weather changes as damaging gusts occurred .. i.e. cold front/trough passage/squall line/cloud changes/ppn changes etc.
Wind observations are very important, especially when noting severe convective events. Even without expensive anemometers etc., just noting the direction and Beaufort Force of the wind can be interesting. (for advice on using the Beaufort wind scale, see: ... and indeed this is the url for a complete introduction to many of the finer points of coding/de-coding etc.

4. Other 'interesting' phenomena.
These reports include such as optical phenomena, coloured precipitation, unusual objects falling in rain showers, unusual (or rarely seen) cloud types etc. As much detail as possible should be included, and if you are not sure what it is you are looking at, someone will do for sure! I have deliberately not gone into details here, as the subject is vast.

A general note on standards etc.: (thanks to Paul Bartlett for additional suggestions here) Some people contributing to the newsgroup will have access to some pretty sophisticated observing equipment which provide valuable record of weather events. However, if all you have is a plastic gauge from the garden centre, or a Six's max. and min on a north wall, don't be put off providing the information.
For reports of wind speed and direction: try using the 16 point compass (and remember that the wind comes from a direction, so a SSW wind comes from the SSW), and the Beaufort scale, as already mentioned above. So, a wind observation might be SSE/4 gusts 5; or, upon a cold frontal passage, SSE/3-4 becoming SW ocnl W 3 gusting 4 or 5.
Reports of temperature should be in degrees C. Don't try for 1/10's degree accuracy unless the thermometer allows it. Try for the nearest half of a degree, but say that that is the standard to which you are observing.
Atmospheric pressure (reduced/corrected to msl) in millibars/mb (or hecto Pascals if you prefer/hPa). Similarly with pressure change: the usual period to note a pressure change over in temperate latitudes is 3 hours, but hourly changes are valuable where possible. [ In subtropical/tropical regions, 24 hour changes are usually used, to eliminate the influence of diurnal changes. ]
Reports of rainfall (or melted snowfall) should be in mm. Dimensions of solid precipitation (e.g. hail), should be cm, as should snow depth.
Visibility (broadly how far you can see, though the strict definition is considerably longer and more complex), should be in metres up to and including 5000 m and km beyond that.
Cloud amounts can be made in oktas (eighths of sky covered) ...see for some advice on this, or more generally, the aviation cloud amount classification could be used, as under:

SKC: no cloud at all
FEW: 1 or 2 eighths/oktas of cloud
SCT: 3 or 4 eighths/oktas of cloud
BKN: 5 to 7 eighths/oktas of cloud
OVC: 8 eighths/oktas of cloud - i.e. complete overcast.

Cloud base will be difficult, and I won't go into detail on estimation of cloud bases.The Observers Handbook (see 5A.6) is your best guide. At present, the foot is still the 'standard' for cloud height observations, but many countries use metres so I suppose either is acceptable.

As to standards, as long as you tell people how you are measuring the variable you report, they can make their own judgement of relative accuracy. It would be a pity to lose data because you might feel its not of sufficient quality. Quite frankly, there is never enough data on current weather, and with the situation in many areas now where official 'eyeball' observations are too expensive to maintain, amateur observations, carefully made and reported, will once again come into their own.

And a final thought...don't be shy of saying you don't know what it is you are seeing. Either someone will pop up with the right answer, and we'll all learn by it, or it might genuinely be something 'new', and stimulate a discussion... which after all is what is all about.

Here are some specific notes regarding the observing and reporting of dust deposits after rainfall. I am grateful to Stephen Burt for this:.....

" Observing a fall of dust rain is not difficult, but it helps to have some ideas of what to look out for. A daily routine also helps (such as, in my case, the morning inspection of the raingauges) but even a few seconds careful examination of the car windscreen before driving to work can be worthwhile. If you own a raingauge, check the funnel daily for a ring of dust, often pale orange in colour and very fine in texture. The larger the funnel, the better: the largest falls even show muddy streaking. Most dustfalls are considerably less obvious than this, often with only the pale ring at the neck of the funnel visible. Surprisingly perhaps, the actual rainwater sample collected may not be very obviously different from normal (at least visually). In my experience, dust rain is most obvious with small amounts of rain, certainly less than 1-2 mm, for otherwise much of the evidence is either washed away or so dilute as to be unrecognisable. Perhaps this is the fate of many falls of dust. Many of my observations of dust rain have been in overnight rainfall, possibly because my raingauge is normally checked only once per day at the morning observation. Another excellent instrument for observing falls of dust is a car, preferably a clean one. Even on a car that is fairly dirty a heavy fall of dust will be very obvious as muddy runs on surfaces that are cleaned regularly (generally the windscreen). A clean car will collect a fall of dust extremely well; the aggravation of having just washed it should be balanced by the thrill of having recorded a fairly rare event! Of course, dust from considerably closer than the Sahara can build up on a car or in the raingauge funnel, particularly after a long dry spell. A regular wipe with a clean damp cloth solves that problem in the raingauge funnel. Locally-deposited wind-borne dust (especially prevalent during hot, dry summers) is often coarse in texture. Beware of pollen in spring and early summer; light showers can bring down considerable amounts and can deposit a surprisingly yellow ring in the raingauge. Harvesting operations during late summer can stir up a lot of fairly fine dust, but common sense (not to mention a check on the synoptic situation) will usually enable an astute observer to ratify a possible sighting." (Extracted from the paper: Falls of dust rain within the British Isles,Weather 1991, 46, pp 347-353, by SHFJ Burt...text of above also published in newsgroup)

6C. Some thoughts on 'non-standard' instrument siting:
The notes below have been written using replies to a survey in the newsgroup and using information extracted from the UK Met.Office handbooks dealing with instruments, and their siting, which in turn are based on internationally agreed standards published by the World Meteorological Organisation (WMO). The notes are only concerned with siting & exposure, not the instruments themselves, or the construction of the screen or a shield in the case of temperature measurements. For advice on these, contact a reputable supplier (Section 7 of the FAQ), or refer to the Observer's Handbook (Section 5A.6). , and the Met Office has some basic information relating to automatic weather measurements on its web site at: -

Standards are set for a reason: data from many different sites around the world need to be compared one with another, in the knowledge that, as far as possible, the instruments used are exposed in the same way and subject to the same errors. This requirement is particularly important when trying to determine long-term trends in meteorological parameters, both using mean values, and with regard to extremes.

However, even when the WMO recommended standards cannot be met, it is natural that people will want to install weather monitoring kit to enhance their interest in the subject. This note therefore attempts to advise on what is, and what is not possible. At the end of the day it is for the individual user to determine whether the expense involved is worth the outcome. One thing must be made clear though: throwing money into expensive equipment will not improve the exposure!

>> Temperature: Basic requirement: For synoptic and climatological meteorology, the temperature required is a representative one of the 'free air' conditions over as wide an area surrounding the observing point as possible, with an internationally agreed height (for the thermometer bulbs, sensors etc.) of 1.25 m above local ground level. A fixed height must be specified, because vertical temperature gradients can be intense: for example on a clear, calm night or around the middle of the day with strong solar heating.

The best site for a screen, or thermometer shield for a land station is therefore over level ground, freely exposed to the sun and wind, but not sheltered by buildings, trees, bushes etc. The temperature sensor must be shielded from direct sunshine (hence a screen or shield) and precipitation (or a dry bulb becomes a wet bulb), and there must be a good circulation of air around the bulb/sensor head. If you have a garden, then the 1.25m above ground level can usually be met with ease. What it usually problematic is gaining sufficient clearance from adjacent buildings, trees etc.

The screen/shield should be positioned over grass (or less preferably, but still acceptable, loose soil), but not compacted soil, tarmac or concrete, as these media absorb and radiate solar energy strongly, and affect the readings quite significantly.

If the garden is not suitable, or you have no garden, then consideration may be given to mounting a screen on a north facing wall. There is a problem in this case with possible contamination from heat energy emitted by the building itself. A practical compromise would be to use such a wall, but carry the screen/shield away from the wall on a bracket - this would allow a free airflow around the equipment. A distance of 20-25 cm for an unshielded sensor has been suggested, and this would certainly minimise any contamination from the walls. For a shielded (or screened) sensor, then 10 cm or so has been suggested as a useful distance. Even a north wall mounting needs watching around the summer solstice, particularly at more northern latitudes, as care needs to be taken to shield the thermometers/sensors from early morning and late evening sunshine with an unobstructed horizon to the northeast or northwest.

The roof is not considered suitable. Not only is the construction of such similar to a solid surface (e.g. tarmac or concrete), and therefore subject to the errors noted above, but a roof is obviously more than 1.25 m above ground level. However, it is worth noting that many of our current crop of weather centres, with London being a notable example, have for many years mounted thermometer screens at a considerable elevation above local ground/street level. If a roof location is all you have, use it, but bear in mind the limitations.

>> Rainfall: Basic requirement: Rainfall amounts are quoted as a depth of water that would result in any one location on a flat surface after a fall of rain, if there were no run-off, evaporation or percolation. The depth measured in a gauge is assumed to be representative over an area around the gauge, so it is necessary to eliminate as far as possible any local sources of error.

There are many sources of error in rainfall assessment: evaporation, adhesion (sticking of the droplets to the side of the gauge), splash etc., but by far the greatest source of error is due to inadequate exposure. The 'ideal' location is one where objects which might disturb the airflow are some considerable distance away from the gauge, i.e. they are so far away that any perturbations of the wind-flow are so small as to be part of the general 'ground-effect' turbulent flow always present as air passes over the earth's surface.

The recommended standard is that the distance from surrounding objects should be not less than twice the height of such objects, and ideally at least four times. In most suburban gardens, even if the fences are low enough to just about site a gauge to these standards, surrounding trees, neighbour's bushes, and of course, the house (and adjacent buildings) usually are the largest objects, and cannot be realistically circumvented. Most hobby observers cannot meet the latter (4 times) requirement, but something approaching the twice-times-height standard is often attainable.

Official texts completely rule out mounting on a wall or roof (apex or flat), as these features cause marked eddies of wind which grossly distort the passage of falling rain across the mouth of the gauge. However, a flat roof might be a best approximation, if there are no adjacent buildings within the '2 x object height' footprint mentioned above.

The middle of a lawn is all that most of us have .. and provided that it is not grossly shaded, would present a reasonable guide, but unless the wind is very light, under-reading of rainfall (with respect to the 'standard' sites) must be expected. With time, it is usually possible to judge where in a garden is unduly sheltered, and careful note made to avoid these locations. Remember that the 'shadow' changes with wind direction. Try setting up identical collecting receptacles (e.g. the bottom half of plastic lemonade bottles) and note the variation in catch over a period of several months. Another method is to note at the start of any rainfall event which areas become wet first and which stay dry longest. Or perhaps a matrix of collectors, and take an average!

At the end of the day of course, you are measuring rainfall that is of significance to you. Indeed, in extreme rainfall events (such as notable local storms), any measurements are better than none; adjustments and allowances can be made for exposure, and even an 'official' gauge under extreme conditions has difficulty in capturing a 'true' measure of the event. The rain/snow that falls is the amount of rain (to a reasonable approximation) that has fallen in your garden, on your roof or whatever, and as such it is a meaningful record. For this reason alone, it is worth attempting such measurements - the only suggestion is that you don't spend huge amounts of money doing it!

>> Wind: Basic requirement: The wind speed and direction in the first 30 metres or so of the atmosphere varies rapidly with height, due to the varying frictional effect of the general 'surface roughness'. It is greatly affected by undulating ground, and by adjacent obstacles such as trees, bushes, buildings etc. This is a common experience - noted for example within built up areas, major shopping centres etc.

For synoptic and climatological work therefore, a 'standard' exposure is required. That standard is for the wind speed and direction over a level surface to be measured at a height of 10 m above ground level (agl). When these conditions cannot be met, it is permissible to raise the anemometer to give an effective height of 10 m, provided the obstructions are not large, and are distributed uniformly around the instrument site.

It will be immediately apparent, that in the common 'back-garden'/urban development situation, a considerable mast is needed to carry the anemometer clear of these 'ground effect' generating obstacles. For example, consider an outer-suburban garden with houses/trees of approx. height 6m in height, the recommended exposure height would be 6 m (obstruction) + 10 m (standard height)=16 metres. (That's around 50 feet!) This provides problems in maintenance of the sensor, and also there would possibly be planning and structural constraints. To be stable, such a structure would need to be well braced which will not be easy. When considering larger obstructions, such as large blocks of flats, or office blocks, then the sensors would need to be raised even more. An example: For an obstruction of some 15 m in height (a typical large building), which is about 75 m from the site of the intended anemometer site, then the wind vane/cups would need to be about 25 m above ground level.

If such conditions are beyond the scope of your pocket or what the neighbours will allow, then the best compromise would be a fitting a short height above the ridge of a house, provided always that adjacent buildings do not unduly affect the airflow at the sensor level.

As will be appreciated from the above, the best advice we can give when 'standard' conditions cannot be met is to think seriously whether its worth the cost and effort. By all means mount a relatively inexpensive anemometer just above the roof level of your house etc., but treat this simply as a monitor of the conditions for your site. The reading you get will not be of use to compare with adjacent 'standard' instrumentation, or even with someone a few streets away with similar problems. However, it is a record at the point you have installed the anemometer head, and as such does provide interest. You will find though that the poorer the exposure, the greater the variability in wind direction.

And finally ... it is pertinent to note that there are occasions when limitations of exposure are a positive advantage. For example, a useful field study for students is to set up a series of temperature recording devices within a stand of trees, both horizontally and vertically. Readings from such an array would obviously be used for the study of the heat/humidity budget of the wood and any need to 'standardise' as above is not a factor, apart from ensuring of course that the thermometers or other devices are correctly calibrated and 'zero-referenced' for the range of values required. And there are specialised applications where the sensor site must depart from the WMO standards: for example, temperature and wind sensors set adjacent to a major motorway route are there precisely to monitor the disturbed airflow and heat characteristics consequent upon heavy traffic flow passing a couple of metres away. For these specialised applications, advice should be sought from the manufacturers of the equipment and other relevant authorities.

Use these navigation bars to move around the FAQ:

0. Index 1. Intro. 2A. Q/A's[The basics] 2B. Q/A's[Background] 3. Topics 4. Sites 5A. Books 5B. Mags. 6. Obs. 7. Suppliers 8. Glossary

SECTION 7: Some suppliers of meteorological equipment and associated services.

[ An entry in this section does not imply approval or endorsement of the product, service etc. Entries are alphabetical, by name of supplier. If you are a supplier of goods/services in support of weather observing (not weather forecasting, data analysis etc.), and you wish to be included, let me know, with the details as under. "n/a" means either 'not available' or 'not applicable' as appropriate.]

This section is sub-divided into:
7A: General suppliers of equipment & services.
7B: Supplier of software and associated support service.

**7A: General suppliers of equipment & services.
1. Name of Supplier: Campbell Scientific Ltd.
2. Address (for correspondence) of Supplier: Campbell Park, 80, Hathern Road, Shepshed. Leics. LE12 9RP (UK)
3. Location of showroom etc. (if available) and opening times:n/a
4. Telephone number and fax (if available): Tel: 01509 601141 (intl: +44 1509 601141) Fax: 01509 601091 (intl: +44 1509 601091)
5. e:mail address and/or web site url (if available):
6. Equipment or services supplied: The supply of professional automatic weather stations, including configurations approved by The Met.Office. The systems use high-quality components and so are not the cheapest. However, they are accurate, rugged and reliable and can be configured according to requirements.
7. Is there a free leaflet/brochure available: YES
1. Name of Supplier: Casella Limited.
2. Address (for correspondence) of Supplier: Regent House, Wolseley Road, Kempston, Bedford. MK42 7JY (UK)
3. Location of showroom etc. (if available) and opening times:n/a
4. Telephone number and fax (if available): Tel: 01234 841441 (intl: +44 1234 841441) Fax: 01234 841490 (intl: +44 1234 841490)
5. e:mail address and/or web site url (if available):
6. Equipment or services supplied: An extensive range of Classical Meteorological Instrumentation for monitoring temperature, sunshine, airflow, humidity, pressure and rainfall. Along side the classical range, Automatic Weather Stations are available. These resilient units use new technologies with easy to use Windows(TM) software and a wide range of sensors to measure meteorological variables.
7. Is there a free leaflet/brochure available: YES
1. Name of Supplier: Diplex Ltd.
2. Address (for correspondence) of Supplier: P.O. Box 172, Watford, Hertfordshire. WD1 1BX (UK)
3. Location of showroom etc. (if available) and opening times: (Watford: trade only, by prior appointment)
4. Telephone number and fax (if available): Tel: 01923 231784 (intl: +44 1923 231784) Fax: 01923 243791 (intl: +44 1923 243791) Fax: 01923 236300 (intl: +44 1923 236300) Fax: 0181 950 8503(intl: +44 181 950 8503)
5. e:mail address and/or web site url (if available): (not yet available - to be added later)
6. Equipment or services supplied: Diplex have many years experience in the supply of an extensive range of quality equipment at affordable prices to both the amateur and professional/educational community. Conventional thermometers, hygrometers, barometers and rain-gauges are of course available, but rugged, easy-to-use distant-reading electronic units to monitor temperature, relative humidity and rainfall are also offered. Of particular interest is the inexpensive instrument screen for self-assembly and a nice range of recording instruments in attractive mountings, including barographs and thermographs.
7. Is there a free leaflet/brochure available?: A list of nearly 40 items of literature is provided free of charge, from which your specific needs can then be identified. A small charge (to cover postage etc.) is made where several brochures are subsequently requested.
1. Name of Supplier: ICS Electronics Ltd.
2. Address (for correspondence) of Supplier: Unit V, Rudford Industrial Estate, Ford, Arundel, West Sussex BN18 0BD (UK)
3. Location of showroom etc. (if available) and opening times: As above by appointment, 9-12.30 13.30-17.30 Monday- Friday
4. Telephone number and fax (if available): Tel: 01903 731101 (intl: +44 1903 731101) Fax: 01903 731105 (intl: +44 1903 731105)
5. e:mail address and/ or web site url (if available):
6. Equipment or services supplied: The Davis range of Weather stations from ICS Electronics offer high precision Weather Monitoring solutions for Home and Industry users. Sensor options include: Anemometer, pressure, rainfall, temperature and humidity. Industrial systems also offer UV, Solar radiation and Leaf wetness. A comprehensive Windows (TM) computer data logging interface and a wide range of installation options are available for all models.
7. Is there a free leaflet/brochure available: YES
1. Name of Supplier: Prodata Associates Ltd
2. Address (for correspondence) of Supplier: 14B Egremont Street, Ely, Cambs CB6 1AE (UK)
3. Location of showroom etc. (if available):n/a
4. Telephone number and fax (if available): Tel/Fax: 01353 664175 (intl: +44 1353 664175)
5. e:mail address and/or web site url (if available):
6. Equipment or services supplied: (1) The Davis range of weather instrumentation; (2) Software and expertise for linking weather stations to personal computers.
7. Is a free leaflet/brochure available?: YES (for the Davis range of weather stations.)
1. Name of Supplier: Sales and Service Company
2. Address (for correspondence) of Supplier: 59A, Station Road, Chingford. London. E4 7BJ (UK)
3. Location of showroom etc. (if available):n/a
4. Telephone number and fax (if available): Tel: 0181 505 3280 (intl: +44 181 505 3280) Fax: 0181 559 0425 (intl: +44 181 559 0425)
5. e:mail address and/or web site url (if available):
6. Equipment or services supplied: Service and supply of spares, recording charts, pens, inks etc., for all types and makes of autographic instruments. Suppliers of weather instrumentation, including: barometers, rain gauges, frost predictors, thermometers, hygrometers, hand anemometers etc.
7. Is a free leaflet/brochure available: YES

**7B: Supplier of software and associated support service.
Name of Supplier: Richard H. Brockmeier
e:mail address and/or web site url:
Services supplied: Shareware for recording weather observations (Weather Log).
Name of Supplier: University of Liverpool
e:mail address and/or web site url (if available):
Services supplied: The page contains a variety of FREE software utilities which can be downloaded on-line for meteorological and remote sensing purposes. It is an official University of Liverpool web page.
Name of Supplier: Colin Tandy
e:mail address and/or web site url (if available):
Services supplied: A range of Windows9*(TM) software to plot weather maps, tephigrams and extract climate data from the GTS data available on internet using professional standards. Designed for both the amateur and professional meteorologist and private aviators and glider pilots.

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Freeze date: 09 JAN 2000