Clouds indicate how the day's weather will unfold

摘要

From time immemorial clouds have fascinated mankind. But only in the last 200 years have clouds been studied closely, categorised and named. As aids to short-term weather prediction, cloud types and movements can be very valuable, even in this day ofinstant satellite pictures and radar images. Coupled with intelligent barometer watching and armed with a little local knowledge, there is no reason why anyone needs to plead ignorance when it comes to the weather. Basically, all clouds form due to evaporation from water surfaces and transpiration from vegetation. As this moist air rises it cools and expands, reaching condensation level or dew point. When the water vapour begins to condense, very tiny droplets appear in the air, thus producing the white and grey colours of a cloud. All clouds are white, but some seem dark and even black due to their thickness and their shading from the sun by higher level clouds.