The first step in raindrop formation is condensation of water vapour onto small particles such as dust, soot, or sea salt. These particles are called cloud condensation nuclei, or cloud seeds, and are required to form the tiny water droplets that are called cloud droplets.
Did you know? Raindrops aren’t actually shaped like teardrops. The teardrop shape is likely used because it suggests that drops are falling.
Cloud droplets have a diameter of only 0.01 to 0.02 millimetres and are so named because clouds are made up of billions of these tiny water droplets. Most clouds do not produce rainfall because air resistance keeps the tiny cloud droplets from falling. However, when turbulence causes cloud droplets collide, a process called “coalescence” occurs. Coalescence is when small water droplets fuse into larger droplets. When these larger droplets become heavy enough to overcome air resistance, they fall through the cloud, continuing to coalesce, and eventually fall to the ground as rain.
Did you know? "Drizzle" is defined as raindrops with diameters smaller than 0.5 millimetres that fall close together from thin stratus clouds.
Raindrops range in size from 0.5 millimetres to about five millimetres, with the shape of the raindrop corresponding to its size. Droplets smaller than two millimetres are spherical, droplets larger than two millimetres flatten out at the bottom like a hamburger bun, and even larger droplets indent in the bottom to form a parachute or jellyfish-like shape. As droplets larger than five millimetres fall, they will become deformed by air resistance and fragment into smaller droplets.
Did you know? A five millimetre raindrop falls as fast as nine meters per second. That equals 32 kilometres per hour!
United States Geological Survey: Raindrops