Added: 02/19/2006 |
Average precipitation - feature of climate conditionsClimate is the long-term effect of the sun's radiation on the rotating earth's varied surface and atmosphere. It can be understood most easily in terms of annual or seasonal average precipitation and average temperature.
In meteorology, precipitation is any kind of water that falls from the sky as part of the weather. This includes snow, rain, sleet, freezing rain, hail, and virga. Average precipitation is a major part of the hydrologic cycle, and is responsible for depositing most of the fresh water on the planet.
Precipitation begins forming when relatively warm, moist air rises. As the air cools, water vapor begins to condense on condensation nuclei, forming clouds. After the water droplets grow large enough, two processes can occur to form precipitation.
Coalescence occurs when water droplets merge to create larger water droplets, or water droplets freeze onto an ice crystal. Because of air resistance, the water droplets in a cloud generally remain stationary. When any form of turbulence presents itself, water droplets collide, forming larger water droplets. As these larger water droplets fall, they coalesce more water droplets, and become heavy enough to overcome air resistance and fall as rain. Coalescence generally happens most often in clouds above freezing.
The Bergeron Process occurs when ice crystals acquire water molecules from nearby supercooled water droplets. As these ice crystals gain mass, they may begin to fall, acquiring more mass as coalescence occurs between the crystal and neighboring water droplets. This process is temperature dependent, as supercooled water droplets only exist in a cloud that is below freezing. In addition, because of the great temperature differential between cloud and ground level, these ice crystals may melt and become rain as they fall.
There are some types of average precipitation.
Convective precipitation occurs from convective clouds, e.g., cumulonimbus or cumulus congestus. It falls as showers, with rapidly changing intensity and at one moment only over patches, as convective clouds have limited horizontal extent. Convective precipitation is most important in the tropics. Graupel and hail always indicate convection. In midlatitudes, convective precipitation is associated with cold fronts (often behind the front) and squall lines.
Large-scale precipitation occurs as a consequences of slow (cm/s) ascent of air in synoptic systems, such as fronts, especially ahead of warm fronts.
Orographic precipitation occurs on the windward side of mountains and is caused by the rising air motion of a large-scale flow of moist air across the mountain ridge. Orographic precipitation is well known on oceanic islands, such as the Hawaiian Islands, where much of the rainfall received on an island is on the windward side, and the leeward side tends to be quite dry, almost desert-like, by comparison. This phenomenon results in substantial local gradients of average rainfall, with coastal areas receiving on the order of 500 to 750 mm per year (20 to 30 inches), and interior uplands receiving over 2.5 m per year (100 inches). Leeward coastal areas are especially dry 500 mm per year (20 inches)at Waikiki), and the tops of moderately high uplands are especially wet ~12 m per year (~475 inches) at Wai'ale'ale on Kaua'i).
Many scince centers deal with the average precipitation in extended studies of the climate variability and water resources assessment for future water projects and prepare precipitation data.
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