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Atmospheric Moisture

Transfer of water into and out of the Atmosphere

Evaporation

Evaporation is the phase change of a liquid to a gas. There are three very important requirements for evaporation to take place, 1) available energy, 2) available water, and 3) a vertical moisture gradient. Approximately 600 calories of heat must be added to a gram of water for it to evaporate into the air.  This energy is called "the latent heat of vaporization". Latent heat is used to break the hydrogen bonds that bind water molecules together. In doing so, the energy is "locked up" in the water molecules. The energy remains "latent" in the molecules until they combine during the condensation process to form a liquid. When this happens latent energy is released into the surrounding environment as sensible heat. Sensible heat warms the surrounding air, and thus is an important source of energy to heat the atmosphere.

Even if you have all the energy required for evaporation it will not occur unless there is water present. Desert regions are noted for their lack of precipitation. One reason is that they have little available water to evaporate to later condense and form precipitation. Eighty-eight percent of the water that is evaporated into the air comes from oceans that lie between 60^o north and south latitude. Most of the evaporation occurs in the tropical and subtropical oceans where the highest amounts of net radiation occur. 

Video: Evaporation
Courtesy NASA/Goddard Space Flight Center Scientific Visualization Studio
(Source)
A day-to-night pulsing of evaporation (light blue color) occurs over the continents
while a more constant rate of evaporation occurs over the ocean can be seen in the video above.

The third requirement for evaporation is the presence of a vertical moisture gradient. That is, there is a difference in moisture content with increasing height above the surface (Figure 7.3). All this means is that the humidity is high at the evaporating surface (liquid water), and the air above has a lower humidity. Evaporation will be the predominate phase change until the air is saturated with moisture. Though not required, wind aids in the evaporation process. Wind transfers water molecules away from the evaporating surface and hence maintains a vertical moisture gradient. 

Figure 7.3 Moisture gradient and the influence of wind

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