Q:

What are types of rainfall?

A:

Moisture precipitates out of the atmosphere primarily as three types of rain: convective, stratiform and orographic. In each case, warm, moist air is lifted up and suspended until it reaches saturation and then discharges its water content as rain.

Convective rain is the simplest form, and it occurs mainly in tropical regions. In a convection system, water evaporates off of ocean and lake surfaces, rises through the air by simple convection and falls when the air is saturated. Convective fronts are usually localized and limited in their horizontal extent.

Stratiform rain results when a moist warm front encounters cooler, drier air. The cool air is denser than the warm front, so it subducts under the warm air and drives it to a higher altitude. Then, if conditions are right for precipitation, the warm mass discharges.

Orographic rain is driven by a process very similar to stratiform rain, but with the rain-laden clouds rising against mountains rather than sliding over cool air. A typical orographic rain begins over the ocean as a convection cell, moves toward land and is suddenly lifted high by a coastal range of mountains. Orographic systems usually drop their moisture soon after making landfall and rarely have the energy to penetrate deep inland. This deprives inland areas of rain and creates rain shadow deserts such as the Atacama.


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