Precipitation refers to the deposition of atmospheric moisture from the atmosphere to the surface of ecosystems. Precipitation can occur as liquid rain, solid snow, or hail. In addition, vapor-phase water in the atmosphere can directly condense or freeze onto surfaces as dew or frost. Precipitation is a central element of Earth's hydrologic (or water) budget. Overall, the amount of water evaporated into the atmosphere from all oceanic and terrestrial surfaces is balanced by the amount returned to those surfaces by precipitation.

On the global scale, most evaporation to the atmosphere, about 86% of the total, occurs from the surface of the oceans. Much of it, about 77% of the total, precipitates back to the marine environment. The difference is transported by moving air-masses over the continents, resulting in a net import of evaporated water from Earth's oceans to the surfaces of the continental land-masses. The annual volume of precipitation can...