Rain

Rain is liquid precipitation consisting of water droplets that form in clouds when atmospheric water vapor condenses and coalesces. As these droplets grow, they become too heavy to be suspended by updrafts and fall to the Earth's surface, delivering fresh water to ecosystems and landscapes.

Raindrop formation occurs primarily through two processes. In warm clouds, collision-coalescence causes droplets to merge into larger drops. In cold clouds, the Bergeron process converts supercooled water droplets and ice crystals; ice crystals grow at the expense of surrounding droplets and melt into raindrops near the surface.

Rainfall intensity is commonly categorized as light, moderate, or heavy. Light rain ranges roughly from 0.5 to 2.5 millimeters per hour, moderate from about 2.5 to 7.6 mm/h, and heavy above 7.6 mm/h. Drizzle consists of finer droplets smaller than 0.5 mm. Rain is measured by rain gauges and reported as daily totals or hourly rates in millimeters or inches.

Global rainfall distribution varies with latitude, season, and topography. The tropics receive abundant and frequent rain, while subtropical regions are drier. Mountainous areas and rain shadows produce localized patterns. Rain sustains freshwater supplies and agriculture but can also trigger floods, landslides, and erosion. It is a central component of the hydrological cycle, connecting atmospheric water, rivers, soils, and living organisms.

Human societies study and forecast rain to manage water resources and mitigate hazards. Historical records of rainfall inform climate research and drought monitoring, while rainwater collection remains a practice in various cultures.