Any form of water that falls from the atmosphere to the Earth's surface, including rain, snow, sleet, and hail. It is the primary input to the hydrologic cycle.
Precipitation encompasses all forms of water, whether liquid or solid, that fall from the atmosphere and reach the Earth's surface, including rain, snow, sleet, hail, drizzle, and freezing rain. It is the primary input to the terrestrial hydrologic cycle and the ultimate source of all freshwater resources. Precipitation forms when water vapor in the atmosphere condenses around nuclei particles to form cloud droplets, which then grow through collision-coalescence or ice crystal processes until they are heavy enough to fall. Global precipitation averages approximately 1,000 mm per year over land, but varies enormously from less than 25 mm/year in extreme deserts to over 11,000 mm/year in the wettest locations. Precipitation is measured using rain gauges (point measurements) and estimated over larger areas using weather radar, satellite sensors, and reanalysis products. Accurate precipitation data are essential for water balance calculations, flood forecasting, drought monitoring, agricultural planning, and climate change assessment. The spatial and temporal variability of precipitation is a primary driver of hydrological variability, making its characterization one of the most important challenges in hydrology.