Atmospheric Moisture

The atmospheric branch of the hydrologic cycle comprises moisture in three forms: (1) water vapor, (2) condensed (liquid) and sublimated (ice) water in clouds, and (3) precipitation (primarily rain and snow). Although radiation and energy budgets are the fundamental basis of physical [Page 144]geographic processes, by themselves they are insufficient to generate weather and climate: Moisture is essential. Latent heat absorbed when water evaporates at Earth's surface, and released in condensation when clouds form, links the energy and moisture budgets. Moreover, atmospheric moisture modulates the incoming solar (i.e., shortwave, SW) radiation and Earth-emitted long-wave (LW) radiation streams, significantly affecting the surface net radiation (SW + LW) and energy budget. Much like CO2 (carbon dioxide) and methane, water vapor is a greenhouse gas (GHG): More of it in the atmosphere reduces the surface diurnal temperature range (DTR) by limiting the overnight temperature drop compared with when there is little water vapor. The liquid water comprising most clouds lowers the clear-sky daytime temperature at Earth's surface and helps heat the atmosphere by absorbing SW radiation. Also, thick or multilayered clouds behave like “blackbodies” for LW radiation; they efficiently absorb and reradiate that energy back to Earth's surface, contributing to the natural greenhouse effect and, thereby, reducing the DTR compared with cloud-free skies. Clouds are integrators and tracers of atmospheric moisture and energy (Figure 1), and their large-scale patterns show associations with climatic teleconnections, particularly the El Niño Southern Oscillation (ENSO).

Most clouds exert a negative radiative forcing (i.e., the surface temperature is lower in their presence) because their reduction of the SW radiation received at Earth's surface exceeds their enhancement of the greenhouse effect. Cirrus clouds are the exception: They both transmit SW radiation and enhance the greenhouse effect, for most situations. Precipitation influences the atmospheric energy budget by cooling the air via evaporation.

Compared with Earth's surface and subsurface components of the water budget, atmospheric moisture amounts are very small (<0.05% of all freshwater), and residence times are only on the order of days; yet they are highly significant, participating directly in both weather and climate fluctuations. Considered globally, the dominant surface sources (sinks) of atmospheric moisture are the oceans, especially in the tropics and subtropics (land surfaces, especially in the subtropics and middle latitudes). Because the atmospheric storage of moisture is negligible, the water budget can be estimated as the difference between evaporation (E) and precipitation (P). Maps of the long-term, annually averaged E-P show maximum positive values (i.e., E >> P) over subtropical oceans, and maximum negative values (E << P) in the tropics and over middle-latitude oceans and adjacent land areas. Values are weakly negative over polar regions and weakly positive over subtropical land areas and the interior midlatitude deserts. The vertical exchanges of moisture (as E, P) between land surfaces and atmosphere, or “recycling,” characterize the tropics all year and middle latitudes in summer. Elsewhere and at other times (e.g., middle latitudes in winter), the quasi-horizontal movement of moisture by the winds (i.e., advection) from warmer and/or moister regions dominates.

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