This excellent question came from a second grader while learning about condensation in The Weather Program taught by Catch the Science Bug. He wanted to know if more and more water vapor molecules stuck to the water drop as it fell and if that is why the amount of rain falling can change from a drizzle to rain to a down pour.
Yes! After checking with the National Weather Service, water vapor can attach to raindrops as they fall and increase the size of the falling drop, thus increasing the amount of rain falling. The United States Geological Survey also informed us that raindrops can bump into each other and become one drop as they are falling as well. This process is called coalescence. Once a drop reaches 4 millimeters in size, it usually splits into two drops.
Additionally when a different cloud moves overhead, the amount of rain and the intensity of rain may also change. So this is another reason why, we can change from a drizzle to rain. Rain is usually from a cloud higher in our atmosphere and drizzle from a lower cloud.
Raindrops form when water vapor molecules bump into a dust particle in a cloud and condense around it becoming a droplet (smaller than a drop). Condense means that the water vapor (water in gas form) changes to water in liquid form around the dust particle. Wow- a good thing about dust! The size of the dust particle will affect the size of the droplet. A bigger particle of dusts results in a bigger droplet. The droplets must coalesce (bump and become one) with other droplets to become heavy enough to fall from the cloud.