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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. Precipitation is a major part of the hydrologic cycle, and is responsible for depositing most of the fresh water on the planet. Precipitation is generated in clouds, which reach a point of saturation; at this point larger and larger droplets (or pieces of ice) form, which then fall to the earth under gravity. It is possible to 'seed' clouds to induce precipitation by releasing a fine dust or appropriate chemical (commonly silver nitrate) into a cloud, encouraging droplets to form, and increasing the probability of precipitation.
Orographic precipitation
Orographic precipitation, also known as relief precipitation, is precipitation generated by a forced upward movement of air upon encountering a physiographic upland (see anabatic wind). This upwards movement cools the air, resulting in a cloud formation and rainfall. 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 formed.
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 20 to 30 inches (500 to 750 mm) per year, and interior uplands receiving over 100 inches (2.5 m) per year. Leeward coastal areas are especially dry (<20 in (500 mm) per year at Waikīkī), and the tops of moderately high uplands are especially wet (~475 in (12 m) per year at Wai'ale'ale on Kaua'i).
Convectional rainfall
Convectional rainfall occurs when the air is heated up, usually by the land below it (land tends to heat up faster than air or water bodies). As the air heats up it rises. Inevitably cooling will result, and water vapor will condense out of the air to form droplets, and eventually clouds, if there is enough vapour. This kind of precipitation is most commonly found in tropical areas.
Frontal Rainfall
Frontal rainfall occurs at both warm fronts and cold fronts. At a warm front the warm air, being lighter, rides up over the cold air. As it rises it also cools down (adiabatic process). Moisture in the air condenses to form clouds, and precipitation occurs. At a cold front the warm air is forced up by the cold and the same process occurs.
Rainfall Patterns
ER...Major elements are prevailing westerlies and ocean currents moving equatorward. At high lattitudes 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 westernlies are often laden with moisture from Gulf of Mexico and Atlantic. When polar and westernlie 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
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