Precipitation (meteorology)

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 and hail. 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 (see also rain shadow) is precipitation generated by upward movement of air upon encountering a physiographic upland. This upwards movement cools the air, resulting in cloud formation and rainfall. In parts of the world subjected to consistent winds (for example the Tradewinds), 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 on the descending (generally warming), leeward side.