Precipitation (meteorology)
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Image:Regnbyge.jpg In meteorology, precipitation is any form of water that falls from the sky as part of the weather to the ground. This includes snow, rain, sleet, freezing rain, hail, and virga. Precipitation is a major part of the hydrologic cycle, and is responsible for depositing most of the fresh water on the planet.
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How Precipitation Forms
Condensation
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
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.
Bergeron Process
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.
Types of Precipitation
Convective precipitation
Image:Konvektionsregen.jpg 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
Large-scale precipitation occurs as a consequence of slow (cm/s) ascent of air in synoptic systems, such as fronts, especially ahead of warm fronts.
Orographic precipitation
Image:Steigungsregen.jpg 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, resulting in adiabatic cooling and condensation. In parts of the world subjected to relatively consistent winds (for example the trade winds), a wetter climate prevails on the windward side of a mountain than on the leeward (downwind) side as moisture is removed by orographic precipitation, leaving drier air (see katabatic wind) on the descending (generally warming), leeward side where a rain shadow is observed. Similarly, the interior of larger mountain zones is often quite dry, such as the Great Basin in North America.
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).
Rainfall Patterns in the United States
Western
Major elements are prevailing westerlies and ocean currents moving equatorward. At high latitudes the current is warmer than land, westerlies pick up moisture and cool when moving over land. When the land is warmer than the ocean clouds don't drop precipitation, but pick up additional moisture; it rains in the mountains. When the land is cooler than the ocean, then westerlies cool as they move inland and rain occurs in the lowlands.
Eastern and Central
Polar air masses (above 50° latitudes) are distinct from lower latitude air masses. The westerlies are warm air masses that move poleward from 30°N. In Eastern US the westerlies are often laden with moisture from Gulf of Mexico and Atlantic. When polar and westerly air masses meet, precipitation occurs.
Inland Continental Areas
Areas not proximate to large bodies of water, warm faster than surrounding areas. Hot air masses rise from the center of the continent forming a low pressure area. This low pressure draws water laden clouds from the coasts. This area is heated & rises, this causes cooling adiabatically and precipitation. Resulting in summer rain, and less winter precipitation.
See also
External links
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