Rain seems like such a simple phenomenon. The sky grows dark, perhaps a peal of thunder is heard, and then the rain falls. We are so used to the idea that we rarely stop to wonder how it happens. The reality is far more complicated than it seems.
Rain comes from evaporated water that collects in clouds. At high altitudes, the air is cooler, and permits the condensation of water vapor around small particles floating in the atmosphere. These basic drops of water are very small, less than 0.1 mm in diameter, more usually around 0.02 mm, and so light that they never fall to the ground. At that size, their falling speed is less than the velocity of rising air, so they are kept aloft, and a large collection of these cloud droplets makes for a visible cloud. If for some reason the fall speed of a cloud droplet should exceed the speed of rising air, the falling droplets generally evaporate long before they reach the ground. For this reason, cumulus clouds are often flat on their lower surfaces: the heavier droplets near the bottom fall, evaporate, and rise back to the top as water vapor, condensing around new particles.
In order for rain to fall to the ground, condensed water needs to grow into larger drops. Technically, a raindrop must be at least 0.5 mm in diameter, and on average raindrops tend more towards 2 mm in diameter. Less than 0.5 mm is drizzle. Raindrops begin as spheres, but as they fall, air friction causes the bottom to flatten out a bit. Larger raindrops are more strongly distorted in this process, and sometimes break apart into smaller droplets. The small size and light weight of raindrops mean that air resistance is capable of exerting an appreciable effect on their fall speeds; larger, heavier raindrops fall faster than smaller ones. The largest raindrops, around 5 mm in diameter, fall as fast as 10 meters per second, while the smallest ones (around 0.5 mm) fall at 2 meters per second. Drizzle falls slower still, from 2 meters per second down to half a meter per second.
Rain can form in two ways: through the melting of ice or snow, or through the process of collision and coalescence.
In temperate zones, rain often forms as ice or snow, melting when it hits the warmer air below the cloud. In this case, the water droplets were floating about in a supercooled state, remaining liquid until they contacted a small particle, such as a particle of dust. The water then freezes, and this tiny crystal of ice or snow expands as other droplets touch it and freeze. When it is large enough, it falls, but soon melts in the warmer air below. In particular, the rain that comes in thunderstorms formed in this fashion.
Collision and coalescence is a rather more complicated process. Air temperatures are warmer in this process, so condensation is a slower phenomenon than is the formation of ice crystals in the earlier example. Scientific observers have noted, however, that rainfall develops more quickly than it should if each raindrop had to mature to its eventual size through condensation alone. As droplets form that are large enough to fall despite the rising air beneath them, they will collide with slightly smaller droplets passing beneath them. These droplets can only be slightly smaller; if they are too much smaller, the pocket of displaced air just beneath the falling drop will push the smaller neighbor aside. As the falling drop absorbs smaller droplets on its path, it grows, sometimes leading to the cascade effect. When the drops get very large, around 5 mm in diameter, the flattening effect caused by air resistance can actually tear the drop apart. The smaller fragments then resume the process of collision and coalescence as they pass through the cloud on their way to the ground below.
If there are enough of them, we have rain.