Rain begins when droplets inside a cloud grow heavy enough to fall downward.

The longer version

Inside a cloud, countless droplets are far too small to fall, so they stay aloft on gentle rising air. For rain to start, droplets must merge or grow around ice crystals until they reach a size at which gravity overcomes the updraught. Collisions between droplets of different sizes speed this up considerably.

Once a droplet is large enough it leaves the cloud, and it may partly evaporate on the way down. Warmer clouds tend to rely on colliding droplets, while colder ones depend on ice crystals that grow and then melt as they descend through warmer layers.

Where this entry sits

Clouds, wind, rain, and storms shape daily life in ways that most people barely notice. This collection explains snow, lightning, rainbows, the turning seasons, temperature, the instruments forecasters rely on, and scraps of old weather lore. The full list sits on the Weather and Climate page, where every entry is listed in order.