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 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 when a warm front meets a cold front. The warm air, being lighter, will be forced over the cold air. As it rises it also cools down. Moisture in the air condenses to form clouds, and precipitation occurs.

• See also: umbrella, monsoon