Sources of Atmospheric Moisture

Moisture as vapor acts the same as any other gas. It mixes with other gases in the air, and yet maintains its own identity and characteristics. It is the raw material in condensation. It stores immense quantities of energy gained in evaporation; this energy is later released in condensation. Much of the energy for thunderstorms, tornadoes, hurricanes, and other strong winds comes from the heat released when water vapor condenses. The availability of water vapor for precipitation largely determines the ability of a region to grow vegetation, which later becomes the fuel for wildland fires.

Moisture in the atmosphere is continually changing its physical state--condensing into liquid, freezing into ice, melting into liquid water, evaporating into gaseous water vapor, and condensing back to liquid. These changes are all related to temperature, the gage of molecular activity in any substance. At about -460° F (absolute zero) the molecules of all substances are motionless. As the temperature rises, they move around at increasing speeds. Water molecules move slowly at subfreezing temperatures, more rapidly at melting temperature, and still more rapidly through the boiling stage. However, at any given temperature, individual molecules, whether solid, liquid, or gas, do not have the same speeds or direction of travel. Collisions that change their speeds and directions occur continuously.

Water vapor in the air comes almost entirely from three sources: Evaporation from any moist surface or body of water, evaporation from soil, and transpiration from plants. Some water vapor results from combustion. Because the oceans cover more than three-fourths of the earths surface, they are the most important moisture source, but land sources can also be important locally.

Plants have large surfaces for transpiration; occasionally they have as much as 40 square yards for each square yard of ground area. Transpiration from an area of dense vegetation can contribute up to eight times as much moisture to the atmosphere as can an equal area of bare ground. The amount of moisture transpired depends greatly on the growth activity. This growth activity, in turn, usually varies with the season and with the ground water supply. In areas of deficient rainfall and sparse vegetation, such as many areas in the arid West, both transpiration and evaporation may be almost negligible toward the end of the dry season. This may also be common at timberline and at latitudes in the Far North.

In evaporation from water bodies, soil, and dead plant material, the rate at which moisture is given up to the air varies with the difference between the vapor pressure at the evaporating surface and the atmospheric vapor pressure. Evaporation will continue as long as the vapor pressure at the evaporating surface is greater than the atmospheric vapor pressure. The rate of evaporation increases with increases in the pressure difference. The vapor pressure at the evaporating surface varies with the temperature of that surface. Therefore, evaporation from the surfaces of warm water bodies, warm soil, and dead plant material will be greater than from cold surfaces, assuming that the atmospheric vapor pressure is the same.

Transpiration from living plants does not vary as evaporation from dead plant material. Living plants will usually transpire at their highest rates during warm weather, but an internal regulating process tends to limit the water-loss rate on excessively hot and dry days to the plants particular current needs.

In still air during evaporation, water vapor concentrates near the evaporating surface. If this concentration approaches saturation, further evaporation will virtually halt, even though the surrounding air is relatively dry. Wind encourages evaporation by blowing away these stagnated layers and replacing them with drier air. After a surface has dried to the point where free water is no longer exposed to the air, the effect of wind on evaporation decreases. In fact, for surfaces like comparatively dry soil or wood, wind may actually help reverse the process by cooling the surfaces and thus lowering the vapor pressure of moisture which these surfaces contain.