In an average year, some 60 volcanoes disgorge their millions of tonnes of ash and lava into the atmosphere. The lava and the rest of the heavy material fall to Earth and transform the surrounding landscape in a very obvious way. More subtly, the lighter particles and gases projected high into the stratosphere may form a veil around the world to affect the global climate for a year or two.
However, in between these two extremes, great clouds of dust and ash are expelled to heights that correspond to the cruising altitudes of jet aircraft. Such debris, if caught up in jet engines, may cause multiple engine failures with the possibility of a catastrophic accident. Nothing much can be done to remove this hazard from the air, but it would be an obvious advantage if pilots had sufficient information to avoid it.
Eruptions in remote areas may go unreported for several days, and in addition, the debris is difficult to detect with on-board radar. Satellite observations, however, can be used to spot it. But the difficulty is that it is often hard to distinguish between volcanic ash clouds on satellite images and "normal" clouds composed of water droplets or ice particles. Viewed from above in a conventional photographic image, they all appear in whites of only slightly differing shades.
Multi-channel satellites, however, make the task much easier. Images taken in the "visible light" section of the spectrum record more or less what might be seen by an ordinary black-and-white camera; clouds are a brilliant white, because they reflect back towards the satellite a large proportion of the sunlight which hits them, and volcanic ash looks similar for the same reason.
But satellites also record images in various parts of the infra-red portion of the spectrum; in these cases the sensor reacts to temperature rather than to visible light, with cold objects appearing very white, and those at higher temperatures black.
The new Meteosat Second Generation series of satellites, for example - the first of which is due to be launched in 2002 - will record images in 12 channels, nine of these in the infra-red portion. Volcanic ash clouds scatter infra-red solar radiation in a quite different way to water drops or ice particles, and in addition, their radiation absorption and emission characteristics vary significantly for different wavelengths of incident infra-red radiation. Therefore, by comparing images simultaneously obtained in infra-red channels, differences in brightness make it possible to distinguish volcanic ash clouds easily from ordinary clouds which may look similar in photographs.