Describe how the process of collision and coalescence produces precipitation. Warm clouds form above the freezing point of water. Precipitation is formed by collisions. Larger droplets fall faster than smaller droplets due to a greater terminal velocity. It is the larger droplet falling through the smaller ones that grows by coalescing.
Coalescence is enhanced when colliding drops have opposite electric charge.
Size is also enhanced by the time a falling drop spends in a cloud. Thick clouds with strong vertical updrafts form the largest drops. A warm towering cumulus congestus because the strong updrafts increase the time spent in colliding and coalescing. Clouds have to be present. Heavy showers have big drops and need more time in a cloud.