 Coalescence is the process by which two or more droplets, bubbles or particles merge during contact to form a single daughter droplet, bubble or particle. It can't take place in many processes, ranging from meteorology to astrophysics. For example, it is seen in the formation of raindrops as well as planetary and star formation. In meteorology, its role is crucial in the formation of rain. As droplets are carried by the updrafts and dump drafts in a cloud, they collide and coalesce to form larger droplets. When the droplets become too large to be sustained on the air currents, they begin to fall as rain. Adding to this process, the cloud may be seated with ice from higher altitudes, either via the cloud tops reaching negative 40 degrees C negative 40 degrees F or via the cloud being seated by ice from syrups clouds. In cloud physics the main mechanism of collision is the different terminal velocity that between the droplets, the terminal velocity is a function of a droplet size. The other factors that determine the collision rate is the droplet concentration and turbulence.