III.

Evaporation

Evaporation is the process by which liquid water changes to water vapor and enters the atmosphere as a gas. Evaporation of ice is called sublimation. Evaporation from the leaf pores, or stomata, of plants is called transpiration. Every day about 1,200 cu km (about 290 cu mi) of water evaporates from the ocean, land, plants, and ice caps, while an equal amount of precipitation falls back on the earth. If evaporation did not replenish the water lost by precipitation, the atmosphere would dry out in ten days.

The evaporation rate increases with temperature, sunlight intensity, wind speed, plant cover, and ground moisture, and it decreases as the humidity of the air increases. The evaporation rate on the earth varies from almost zero on the polar ice caps to as much as 4 m (as much as 13 ft) per year over the Gulf Stream. The average is about 1 m (about 3.3 ft) per year. At this rate, evaporation would lower sea level about 1 m per year if the water were not replenished by precipitation and runoff.