Explanation:

The collision-coalescence model applies to warm clouds that form in the tropics. Warm clouds are those that form at altitudes where the air temperature is above freezing. For precipitation to form under this model, there needs to be a variety of different size condensation nuclei. Large condensation nuclei will create large water droplets while smaller condensation nuclei create small ones. In order for the droplets to make their way to the surface they have to be heavy enough to overcome the resistance imposed by upwardly rising air that is fueling the development of the cloud. The smaller, lighter droplets are easily suspended in the updrafts of air, while the larger heavy collector droplets fall and collide with the smaller ones. Upon collision, the droplets coalesce into a bigger droplet. As the droplet falls, resistance by the air flattens the droplet to the point where it becomes unstable and breaks apart. With enough collisions, the droplet achieves a size sufficient to fall all the way to the surface.