Pronunciation: (trop'u-sfēr", trō'pu-), [key]
the lowest layer of the atmosphere, 6 mi. (10 km) high in some areas and as much as 12 mi. (20 km) high in others, within which there is a steady drop in temperature with increasing altitude and within which nearly all cloud formations occur and weather conditions manifest themselves.
Random House Unabridged Dictionary, Copyright © 1997, by Random House, Inc., on Infoplease.