Very often, as you sit inside an airliner after the reassuring news that all the doors are armed and cross-checked, the captain may tell you precisely the kind of weather awaiting you at your destination.
He might say, for example: "It is a bright and sunny day in Buenos Aires, with a light north-easterly breeze and a temperature of 25 degrees." But have you ever wondered how he knows?
Has he a friend or two in all exotic places who obligingly telephones the news? Or does he merely chance his arm and trust in the apparent dependability of the weather everywhere but here.
The answer lies in the Metar, which stands for Meteorological Aerodrome Report, and which gives in coded form a summary of the weather experienced at a particular airport in the very recent past.
A typical one might read as follows:
METAR EGLL 1100Z 21015G27KT 3000 RA SCT006 OVC025 12/11 Q1014.
The message is instantly recognisable for what it is - and, more importantly, for what it means - to anyone involved in aviation. Roughly translated, it reads: "At London's Heathrow Airport at 11 o'clock GMT there was a fresh, south-westerly wind, gusting occasionally to 27 knots; the visibility was three kilometres in rain; there were scraps of cloud 600 ft above the ground, and the sky was completely overcast at a height of 2500 ft; the temperature was 12C; the dew point - a measure of the humidity - was 11 degrees, and the correct pressure altimeter setting for an aircraft landing at Heathrow is 1014 hectopascals."
Such codes are used worldwide for the exchange of reports on existing weather conditions, and forecasts of weather at thousands of airports around the world. At every major airport routine reports on the current weather are prepared every 30 minutes, on the hour and half hour, and zapped immediately to join the vast river of similar reports which flows incessantly through the aviation telecommunications channels.
These reports on current weather are supplemented every three hours or so by forecasts for every airport for several hours ahead, each prepared in a similar code form by forecasters working on site or in the vicinity.
Together these two messages, which give all the necessary information about the present and future weather at the destination, form a vital part of the documentation provided for any pilot about to undertake a flight.
The codes have the advantage of being easily understandable with a little practice, no matter what the nationality or mother tongue of the recipient. They are also both comprehensive and concise and can be easily handled by computer.