Rainfall

Rainfall is liquid precipitation in the form of raindrops that fall from clouds when atmospheric moisture cools and condenses, and the air becomes saturated. It is a central component of Earth's hydrological cycle, supporting freshwater supplies, ecosystems, and weather patterns.

Rain forms when moist air rises, expands and cools, and water vapor condenses into droplets. When these droplets grow too large to be supported by the air, they fall as precipitation. Rainfall is measured with rain gauges that collect droplets and yield totals in millimeters. Reports can cover hourly, daily, or event-based amounts. Rainfall intensity is often given in millimeters per hour.

Rainfall types include convective rainfall, driven by intense surface heating and leading to thunderstorms; orographic rainfall, produced as air is lifted over mountains; and frontal rainfall, where contrasting air masses lift warm, moist air along a front. Other forms include drizzle, light showers, and heavy downpours.

Spatial and temporal rainfall patterns vary with latitude, elevation, and season. Tropical regions typically receive high and often daily rainfall; mid-latitudes show seasonal rainfall; deserts are very dry. Monsoon systems and tropical cyclones can generate large totals in short periods. Climate change is expected to alter rainfall distribution and increase the frequency of extreme precipitation in some regions.

Rainfall supports agriculture, replenishes rivers and aquifers, and maintains ecosystems. It can cause hazards such as floods and erosion when precipitation is intense or prolonged. Data on rainfall are collected from ground gauges, radar, and satellite measurements and are used to create rainfall maps and inform water management, agriculture, and weather forecasting. Isohyets are lines of equal rainfall used in regional climatology.