During your special on the low level of Lake Michigan, you mentioned that evaporation takes a lot of water from the lake. Once in the air, where does all that moisture go? Where does it come down, and in what form?
Linda Eichhorst, DeKalb, Ill.
Water that enters the atmosphere by any means — evaporation from the oceans and from inland sources like soil, lakes and rivers, and water transpired from vegetation — eventually condenses out of the atmosphere as precipitation such as rain and snow. About 74 percent of that precipitation falls on the oceans and 26 percent on land. The 48 conterminous states of the United States add 21 inches of water to the atmosphere yearly from evaporation and transpiration, but annually receive an average of 30 inches of precipitation. The difference, seven inches, condenses from water vapor imported by wind.