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.
See also umbrella.