Atmospheric humidity is constantly changing as water vapor enters and leaves the atmosphere at the Earth's surface. These water vapor fluxes are
coupled with energy exchanges that account for significant energy transfers between the Earth's surface and the atmosphere (see Fig. 2.3). The distribution of atmospheric moisture is complex, but one evident control is that moisture condenses out of saturated air (Allen and Ingram, 2002). Consequently, spatial variations in atmospheric moisture provide important insights into the atmospheric branch of the hydrologic cycle. The amount of water vapor in the atmosphere is crucial for quantifying the atmospheric water balance and for understanding the quantity of water vapor present to support the precipitation formation process. In addition, the latent heat content of atmospheric water vapor can have a profound effect on the efficiency with which heat is transported horizontally by the atmosphere (Pierrehumbert, 2002). The spatial distribution of atmospheric water vapor is a composite of surface moisture sources, atmospheric moisture sinks, and available energy to drive phase changes of water at the surface.
Methods for expressing atmospheric humidity introduced in Section 3.6 include specific humidity, which is the ratio of the mass of water vapor to the mass of humid air. Specific humidity is particularly useful for expressing the global distribution of atmospheric water vapor because its value is not influenced as the atmosphere expands or contracts in response to pressure changes. Also, specific humidity is not temperature dependent, which makes it a good indicator for comparing atmospheric water vapor at locations with different air temperatures. Specific humidity (q) is represented by
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