Sometimes you may see during the nightly weather forecast on television a display with the caption "Meteosat visible". Even if you recall that Meteosat is a weather satellite, you may wonder what the message means; should you rush outside, perhaps, to catch a fleeting glimpse of the spacecraft as it makes what must surely be a very rare appearance, skimming like a falling star across the evening sky?
But your dash would be in vain. Meteosat visible just tells you the kind of satellite picture shown on the screen. It means the image has been taken by an instrument sensitive to the "visible light" portion of the electromagnetic spectrum, and shows more or less what an ordinary black-and-white camera would see from space.
"Visible" pictures like these show the positions of the fronts and pressure systems very clearly, but they have their drawbacks. For one thing, there is no sunlight at night, and there are parts of the world where there is hardly any daylight at certain times of year. If the forecaster were entirely dependent on ordinary photographs, there would be times when satellites would be of little help.
But an alternative to Meteosat visible is - not Meteosat invisible - but Meteosat infra-red. The infrared images are the product of sensors sensitive to heat; the pattern which emerges reflects variations in temperature over the field of view, and although it bears a strong resemblance to the visible picture, it has a number of important advantages.
In the first place, temperature - unlike light - does not switch off at night, so infra-red pictures are available throughout the 24 hours. Moreover, since the pattern is based on temperature, and since temperature as a rule decreases with altitude, the brightness of the image of a particular cloud is a measure of its height.
The forecaster, therefore, can use the infra-red image to distinguish the very high rain-bearing clouds from the harmless, thin layers which do no more than keep the sun or frost at bay. The current Meteosat records images in three channels - visible, infra-red, and the so-called "water vapour" channel which provides information about the distribution of rain and moisture in the field of view. Meteosat Second Generation, the first of which is to be launched next year, will record images in 12 channels - one visible, nine in the infra-red, and two others. By comparing images taken in these different channels, meteorologists will be able to build up precise three-dimensional pictures of the complex structure of the atmosphere.