Weather satellites playa critically important role in today's high-tech weather forecasting by providing a reliable, routine set of 'images' of clouds and moisture, round the clock. Not only do the high-quality real-time pictures aid forecasters, but they are invaluable in supporting research into the nature of potentially damaging weather systems. I n addition, many other 'fields' are derived from their images, including the concentration and distribution of important trace gases such as ozone.