It is essential for pilots of aircraft to be provided with accurate and up-to-date information on the weather at their destination airfield. In the majority of cases details of conditions are, on request, passed directly to aircraft by ATC personnel. Information is also available through the FIS (Flight Information Service) for any airfield. For the main UK and some European airports weather information is broadcast from Heathrow airport in London. The service is known as ‘Volmet’, derived from the French word ‘Vol’, meaning weather.

Before 1981, weather broadcast from major world airports were read over the air by local persons. Because of local accents there were sometimes difficulties in clearly understanding the transmissions. Marconi Space and Defence Systems developed Automatic Volmet to overcome this problem, and since May 1981 it has been operational from Heathrow. The Marketing Manager of Marconi, Mr John West, has recorded all the standard weather report phrases, words and figures which are digitised and stored in a computer memory.

The weather reports received at Heathrow are automatically processed and converted into a code which then selects the appropriate phrases from the memory ready for transmission. Although it is a human voice which is heard, the whole process is fully automatic and operates on a continuous basis, the RAF also use this system. Volmet meteorological messages are known as METAR - routine met aerodrome reports. Other types of messages are used for different purposes.

Each broadcast consists of the following details in respect of each airport:

a) Station name; g) Cloud details;

b) Time of observation; h) Temperature;

c) Surface wind details; i) Dew point;

d) Horizontal visibility; j) QNH;

e) Runway visual range; k) Trend.

f) Weather details;

a) The name by which the airport is commonly known.

b) The time of the observation in UTC.

c) The direction of the surface wind and speed, e.g. zero seven zero degrees one five knots.

d) Below 5,000 m the visibility is expressed in metres (e. g. ‘Four zero zero metres’). Above 5,000 m it is given in kilometres.

e) Runway visual range is given in metres (e. g. ‘five zero zero metres’). Where more than one runway is in use, separate RVR readings may be given.

f) Where appropriate, a description of certain weather conditions is given in plain language, e. g. rain showers, freezing rain, etc.

g) Where applicable, the extent to which the sky is obscured by cloud is estimated. If the cloud cover is between one eighth and half the word ‘scattered’ is used to indicate height of the base of the cloud layer in hundreds of feet above aerodrome level. Where the cloud cover is between five eighths and seven eighths the word ‘broken’ is used. Where the entire sky is covered by cloud the word ‘overcast’ is used.

h) Temperature is given in degrees Celsius.

i) Dew point is given in degrees Celsius.

j) QNH is given in millibars.

k) The trend of the weather conditions may be added if a change is expected soon. If no change is expected, an abbreviation of the words ‘no significant change’ is given, pronounced ‘no-sig’.

Where significant changes are expected the TREND of the forecast will be indicated by the words ‘Becoming’ or ‘Tempo’: ‘Becoming’ is used to indicate that a change is expected to take place at either a regular or irregular rate; ‘Tempo’ is used to indicate a period of temporary fluctuations to the forecast conditions expected to the less than one hour.

For example ‘BECOMING from 11.00, 25° 35 knots, maximum 50 knots, TEMPORARILY from 06.30 until 08.30, 3000 metres. Moderate rain showers’.

In conditions where visibility is 10 km or more. the lowest cloud is at a minimum of 5,000 ft, there is no precipitation, thunderstorm, shallow fog or low drifting snow, then the relevant parts of the Volmet transmission will be replaced by the expression ‘Cavok’ derived from ‘Cloud and Visibility OK’ and pronounced ‘KAV-O-KAY’.

A typical broadcast, when conditions are good, is as follows:

‘This is London Volmet South, this is London Volmet South London Heathrow at one four five zero. One zero zero degrees nine knots. Cavok. Temperature nine, dewpoint four. QNH one zero two five. Nosig.’

A more complicated transmission, in less favourable conditions, might be:

‘Paris Charles de Gaulle at zero seven three zero. Zero eight zero, zero one knots, zero zero five zero metres. Runway visual range zero seven five metres. Fog. Sky obscured. Temperature three, dewpoint three. QNH one zero two three. Fog dispersal operations are in progress.’

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