The collision coalescence process is the process of cloud droplets increasing in size by colliding and coalescing. When droplets bump into each other, it is called collision, and when they stick together, it is called coalescence.
Cloud droplets must increase in size about a million times in order to become raindrops. Raindrops are formed in warm clouds exclusively by the collision coalescence process. However, this process has little importance in clouds where temperatures are below freezing.
Droplets can only coalesce or combine if they have an opposite electrical charge; otherwise, they simply bounce off one another. As the droplets grow larger and heavier, they begin to fall through the cloud. Larger drops fall faster and collect the smaller drops as they fall, causing the droplets to grow even larger.
Clouds with temperatures below freezing use the Bergeron process to increase the size of droplets. This process was first proposed by Tor Bergeron in the early 1920s. In this process, ice crystals form high in the clouds and increase in size by attracting water vapor. As the ice crystals become large enough to fall towards the Earth's surface, they pass through the lower, warmer portion of the cloud and grow even larger.