How do raindrops form?
Rain occurs when particles grow within clouds, fall, and reach the ground as liquid. Cloud droplets are much smaller than raindrops. Water in a cloud does not by itself guarantee precipitation or its arrival at the surface.
Collision and ice-crystal processes
In warm clouds, droplets can grow by colliding and merging. In colder clouds, ice crystals and supercooled droplets interact; falling snow may melt into rain in warmer air below. Drops can merge or break apart during descent, while air currents keep particles aloft. Cloud temperature alone therefore cannot determine rainfall amount.
Shape, evaporation, and measurement
Small raindrops are nearly spherical; larger ones flatten because of air resistance rather than taking a pointed teardrop shape. Dry air below a cloud can evaporate rain before it reaches the ground, producing virga. One millimeter of rainfall equals one liter per square meter. Short-term intensity and daily accumulation describe different aspects of rainfall and risk.