When liquid water droplets in a cloud grow large enough to fall, the result is rain. Droplets typically need to reach about 0.5 millimeters or more in diameter to overcome air resistance and reach the ground without evaporating.

Rain forms through collision and coalescence in warm clouds or through the melting of ice particles that fall into warmer air. Intensity ranges from light to extreme. Prolonged heavy rain can produce flooding; brief intense rain can cause flash floods in vulnerable areas.

Rain is the primary source of freshwater for most terrestrial ecosystems and human societies. Its timing, amount, and intensity affect agriculture, water supply, and natural hazards. Measuring and forecasting rain remain central tasks of meteorology and hydrology.

The sound and smell of rain have cultural significance in nearly every region of the world.