Raindrop

Raindrops are liquid water droplets that fall from clouds during precipitation. They form when moist air rises and cools, causing water vapor to condense into droplets. In warm clouds, droplets collide and coalesce to create larger drops; in cooler clouds, ice crystals may melt into liquid droplets as they descend.

Most raindrops range from about 0.5 to 5 millimeters in diameter. Small drops are nearly spherical, while larger ones become flattened at the bottom and take a saucer-like shape due to air resistance. As they fall, raindrops reach a terminal velocity determined by their size, typically a few meters per second for small drops and up to about 9 meters per second for large drops. Very large drops are fragile and tend to break apart before reaching the ground.

Raindrop formation and behavior are central topics in meteorology and hydrology. Rain intensity is measured in millimeters of water equivalent per hour (mm/h) with rain gauges; radar and satellites help estimate spatial distribution and rainfall rates. The descent of raindrops through air also produces optical effects such as rainbows and halos, depending on droplet size and sunlight. In some conditions, evaporation occurs before the drops reach the surface (virga).

Ecological and hydrological roles include replenishing freshwater, supporting plant and ecosystem processes, and influencing soil infiltration versus runoff. The term drizzle refers to lighter precipitation composed of finer droplets, typically smaller than about 0.5 millimeters, which produces a gentler, mist-like rainfall.