How do clouds become heavy enough to rain?

Clouds are made of tiny water droplets or ice crystals, but most of those particles are far too small to fall as rain. In warm clouds, droplets collide and merge, a process called collision and coalescence, until they become large enough to drop out of the cloud. In colder clouds, ice crystals can grow at the expense of liquid droplets in the Bergeron process, which was described in the 1930s by Tor Bergeron. Once precipitation falls into air that is warm enough, snow can melt into rain before reaching the ground.