The USGS Water Science School
In science we learn that one question often leads to another, or several others. Before we can discuss raindrop sizes, we must understand what a raindrop is. How is a raindrop made? How big can a raindrop be?
In order to have rain you must have a cloud--a cloud is made up of water in the air (water vapor.) Along with this water are tiny particles called condensation nuclei--for instance, the little pieces of salt leftover after sea water evaporates, or a particle of dust or smoke. Condensation occurs when the water vapor wraps itself around the tiny particles. Each particle (surrounded by water) becomes a tiny droplet between 0.0001 and 0.005 centimeter in diameter. (The particles range in size, therefore, the droplets range in size.) However, these droplets are too light to fall out of the sky. How will they get big enough to fall?
Picture a huge room full of tiny droplets milling around. If one droplet bumps into another droplet, the bigger droplet will "eat" the smaller droplet. This new bigger droplet will bump into other smaller droplets and become even bigger--this is called coalescence. Soon the droplet is so heavy that the cloud (or the room) can no longer hold it up and it starts falling. As it falls it eats up even more droplets. We can call the growing droplet a raindrop as soon as it reaches the size of 0.5mm in diameter or bigger. If it gets any larger than 4 millimeters, however, it will usually split into two separate drops.
The raindrop will continue falling until it reaches the ground. As it falls, sometimes a gust of wind (updraft) will force the drop back up into the cloud where it continues eating other droplets and getting bigger. When the drops finally reach the ground, the biggest drops will be the ones that bumped into and coalesced with the most droplets. The smaller drops are the ones that didn't run into as many droplets. Raindrops are different sizes for two primary reasons.
This information is courtesy of the University of Idaho.
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Rain Rainfall accumulator activity Follow a drip through the water cycle