Water in the Atmosphere

While the amount of water in the atmosphere of the Earth is small compared to the oceans, it is nevertheless a very large quantity at any given time and has a profound effect on the climate of the Earth. This view of the water in the atmosphere is drawn from Franks, who spent much of his adult life studying water.

Studies of the average amount of water in the atmosphere leads to figures like 60,000 cubic kilometers. If all of that suddenly came down as rainfall, it would cover the Earth's surface to a depth of about half a meter. In a year's time the total rainfall on the Earth is about 2.25 million cubic kilometers, or about 37 times the water content of the atmosphere at one time. This tells us that the vast amount of water that evaporates from the oceans and the lesser amount from land areas has an average residence time in the atmosphere of about ten days.

Earth's Water Resources (km3) Rivers 1200 Topsoil 25,000 Annual runoff 34,000 Lakes 100,000 Glaciers 200,000 Fresh groundwater to 1km depth 4,000,000 Artic ice cap 3,000,000 Antarctic ice cap 30,000,000 Annual rainfall 2,000,000 Moisture in atmosphere 60,000 Photosynthesis (annual) 100 Oceans 1,300,000,000 From Franks, Table 1.1 Compare with Graedel & Cruzen

About 70% of normal rainfall re-evaporates back into the atmosphere, while the remaining 30% is involved in runoff and penetration into the surface and follows a more indirect pathway before being available for re-evaporation into the atmosphere. Because of the large heat of vaporization of water, this evaporation process has a major cooling effect upon the Earth's surface.

The water vapor in the atmosphere plays a major protective role in blocking harmful high-energy radiation from the Sun and elsewhere. A narrow window of transmission of electromagnetic radiation allows the Sun's peak of radiation to pass, but the absorption increases by nine orders of magnitude just above that window. This offers protection against harmful UV radiation.