Page:Handbook of Meteorology.djvu/72



Water vapor and floating dust are components of the air which vary from day to day and even from hour to hour. All the waters of the land are derived from the water vapor of the air; and this in turn is brought from the oceans. Inasmuch as life in its various forms depends on the process whereby ocean waters are taken into the air and are dropped upon the land as rain or as snow, the study of the water vapor content of the air is of vital importance to humanity. Before the waters of the sea can be poured over the land, several distinct processes take place: Evaporation, diffusion, condensation, and precipitation.

Evaporation.—It is assumed that the molecules of a volume of water are in constant motion among themselves. Some of the molecules at the surface are in such rapid motion that they bombard themselves into the air, thereby becoming a part of it. This loss to the water goes on at all ordinary temperatures and even at very low temperatures. At 212° F (100° C) the pressure, or tension of the vapor is as great as that of the air, and the water is said to boil.

In meteorology, evaporation is a term applied practically to the net loss of water, or other liquid exposed to the air. Free water surfaces, soil and vegetation have each their problems; meteorology is concerned chiefly with evaporation from a free surface of water. Diffusion of the water vapor derived from the ocean, and from bodies of fresh water, is so universal that in no part of the earth is the air free from water vapor.

Various conditions affect evaporation. Under ordinary conditions of light winds and moderately dry air the rate of evaporation is proportional to the surface. It is also directly