The Original uk.sci.weather FAQ

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

Use these navigation bars to move around the FAQ:

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)
none
Section 2A:( Basic information and definitions. )
(added) 2A.27: How do I convert millibars to inches etc?
Section 2B:( Background and special topics. )
none
Section 3: Questions relating to sources of information.
none
Section 4: Pointers to other sources of information.
none
Section 5A: Some books that are worth reading.
none
Section 5B: Some magazines/periodicals etc.
none
Section 6A: Recording/reporting a weather event
none
Section 6B: A weather event ...what to look out for
none
Section 6C: Some thoughts on 'non-standard' instrument siting
none
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
Glossary:
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:

Index:
1. General introduction.

1.1 About this FAQ
1.2 About uk.sci.weather
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.

4.1 ... SCI.GEO.METEOROLOGY NEWSGROUP FAQs
4.2 ... TORRO
4.3 ... ROGER BRUGGE'S WEATHER LINKS SITE
4.4 ... ROYAL METEOROLOGICAL SOCIETY
4.5 ... UNIV. OF EAST ANGLIA - SCHOOL OF ENVIRONMENTAL SCIENCES
4.6 ... BBC WEATHER CENTRE
4.7 ... UK WEATHER INFORMATION SITE
4.8 ... STRATUSNET
4.9 ... WESTWIND WEATHER SERVICES
4.10 ... WILL HAND'S WEB SITE
4.11 ... IRISH METEOROLOGICAL SOCIETY
4.12 ... PITSFORD HALL WEATHER STATION
4.13 ... PRODATA ASSOCIATES/AUTOMATIC WEATHER STATIONS
4.14 ...WEATHER INFORMATION PAGES
4.15 ... FROSTED EARTH WEATHER SITE
4.16 ... NORTH-EAST MEDIA OF ATMOSPHERIC SCIENCE (NEMAS)

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:

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: http://www.booty.demon.co.uk/metindex.htm
and via the UK Weather Information Site at: http://www.weather.org.uk/resource/ukswxfaq.htm
There is also now a simple 'frames' version of this FAQ, with easier access to other meteorology related FAQ's held at: http://www.booty.demon.co.uk/metinfo/uswfaqfr.htm
Queries, comments, suggested corrections etc., relating to the content, to the author:
Martin Rowley: martinr@booty.demon.co.uk

and notification of problems with the structure of the web page, html errors etc., to :
webmaster@booty.demon.co.uk

1.2 About uk.sci.weather: 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 uk.sci.weather 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:

SECTION 2A: Basic information and definitions.

2A.1
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.

2A.2
Q. Where are the principal jetstreams in the atmosphere, and what are their characteristics?
A.
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.)

2A.3
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:http://www.booty.demon.co.uk/metinfo/uppair.htm)

2A.4
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).

2A.5
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: http://www.booty.demon.co.uk/tthkfaq.htm

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

2A.6
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).

2A.7
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:

For full details, see: http://www.booty.demon.co.uk/metinfo/isa.htm This site also has the definitions of QFE, QNH, QFF and QNE.

2A.8
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.

2A.9
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)

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:

2A.10
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.

2A.11
Q. There are other 'trails' visible from aircraft - what are they?
A.
>> 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.

2A.12
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.

2A.13
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.

2A.14
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.

2A.15
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.

2A.16
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).

2A.17
Q. What's the difference between Humidity and Relative Humidity?
A.
(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.

2A.18
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).

2A.19
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: http://www.booty.demon.co.uk/metinfo/uppair.htm

Use these navigation bars to move around the FAQ:

2A.20
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.

2A.21
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.

2A.22
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:... http://www.booty.demon.co.uk/tthkxtrm.htm

2A.23
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.

2A.24
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

2A.25
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.

2A.26
Q. So, what are the factors that can modify the temperature structure in the lowest few hundred metres in marginal snow situations?
A.
(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.

**2A.27
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. )

Use these navigation bars to move around the FAQ:

SECTION 2B: Background and special topics.

2B.1
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: http://www.aoml.noaa.gov/hrd/tcfaq/tcfaqHED.html.

2B.2
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:

- GALE WARNINGS IN FORCE
- GENERAL SITUATION
- AREA FORECASTS: WIND DIRECTION/SPEED:WEATHER:VISIBILITY:(SHIP ICING IF APPROPRIATE)
- COASTAL WEATHER REPORTS AROUND BRITISH/IRISH COASTS (*)

(*) 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)

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

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:

For more details, visit the BBC Weather Centre site at: http://www.bbc.co.uk/weather/

and click here for the LATEST FORECAST.

2B.3
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: http://www.met-office.gov.uk/sec1/sec1pg7.html

2B.4
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).

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.

2B.6
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".

2B.7
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.
http://www.cru.uea.ac.uk/~mikeh/datasets/uk/
(*) Professor Lamb died on Friday, 27th June, 1997

2B.8
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:http://itu.rdg.ac.uk/rms/rms.html
and Roger Brugge's site: http://www.met.reading.ac.uk/~brugge/

2B.9
Q. Would anyone be interested in my weather observations?
A. Indeed YES! For a start, the uk.sci.weather 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: http://www.torro.org.uk/ 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: http://www.met.rdg.ac.uk/~brugge/col.html for more details.

also, a UK Weather Diary can be viewed and added to, which can be viewed at:
http://www.met.rdg.ac.uk/~brugge/diary.html
[ 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:

2B.10
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: http://www.cru.uea.ac.uk/tiempo/floor2/data/nao.htm

2B.11
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:
http://www.cru.uea.ac.uk/~mikeh/datasets/uk/

2B.12
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 ...at least for the present, and certainly seems to be the one favoured by the majority of subscribers to uk.sci.weather. 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]
http://www.wmo.ch/

[TORRO statement re:El Nino and Severe Weather]
http://www.torro.org.uk/

[El Nino theme page sponsored by NOAA/TOGA-TAO]
http://www.pmel.noaa.gov/toga-tao/el-nino/home.html

... 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 uk.sci.weather asked for
... to go directly to this see
http://www.booty.demon.co.uk/metinfo/ENSO_ref.htm

2B.13
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:..

http://www.torro.org.uk/sfinfo.htm

(@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.)

2B.14
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 greatly...in 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: http://www.weather.org.uk/ ), 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.

2B.15
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.)

2B.16
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.).

2B.17
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.

2B.18
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.

2B.19
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-movi