Paul Hastings, age 11, of Fort Huachuca, Arizona, for his question:

Why does the warm desert get so cold at night?

As a rule, the beauteous desert is crowned by a clear blue sky. Few rain clouds hang around the region and the clean bright air tends to be drier than normal. All this fair weather enhances the daytime scene. However, it also creates extreme temperature changes between day and night. Clouds and misty moisture absorb the sun's rays as they pierce down through the atmosphere. A curtain of clouds may reflect back 75 per cent of the sunlight before it reaches the ground. This factor also works in the opposite direction. At night, the misty moisture prevents the earth's store of heat from escaping aloft.

During the desert days, about 75 per cent of the sun's radiation comes through the clear sunny air and strikes the surface. The ground and the air above it get very hot. After sunset, no more solar heat is added. The ground begins to radiate its store back into the atmosphere. There are no clouds, no misty moisture to prevent its escape    and quite quickly the daytime temperature may drop 60 degrees or more.