Although all clouds contain water, only some produce precipitation. A raindrop large enough to reach the ground without evaporating contains roughly a million times the water of a cloud droplet. Therefore, in order for precipitation to form, millions of cloud droplets must somehow join together into drops large enough to sustain themselves during their descent. There are two mechanisms to explain this phenomenon: the Bergeron process and the collision-coalescence process.
The Bergeron process is named for its discoverer, Tor Bergeron. Because water suspended in air does not freeze until it reaches nearly -40oC, much colder than it would freeze on land. This water is said to be supercooled. Airplanes collect ice when they pass through a liquid cloud made up of supercooled droplets because supercooled water freezes rapidly if agitated. Supercooled droplets will freeze quickly if they come in contact with freezing nuclei, or particles with a crystal form similar to that of ice (not unlike condensation nuclei necessary for cloud formation).
Ice crystals are solid, and therefore are held together more tightly than molecules that make up liquid water. As solid crystals form, they form freezing nuclei for more solid crystals to form. Eventually, the crystals are large enough to fall. Even a summer rain may have begun as a snowstorm in the clouds overhead.
In clouds below the freezing level, especially in the tropics, precipitation occurs as a result of a different process called the collision-coalescence process. Droplets of water larger than cloud droplets can form if giant condensation nuclei (such as sea salt) are present. As large droplets begin to form, they collide with other large droplets and eventually coalesce into a droplet large enough to fall to the ground without evaporating.