On average, a raindrop has a diameter of approximately 0.1 to 4.9 millimetres. There are some exceptions: raindrops of 8 millimetres have been known to occur. Larger sizes of raindrop are rare because at a larger size the particles break up into smaller pieces. There are many factors that contribute to the determination of the diameter or size of a raindrop. The speed at which a raindrop particle falls downwards is proportional to its diameter. Larger particles thus fall faster than smaller ones. (If you drop a cannonball and a smaller ball they will accelerate at the same rate, but with raindrops you have to take into account wind resistance.) The maximum distance a raindrop falls before it evaporates is dependent on the process by which a liquid turns into a gas, which is also proportionate to the size of the drop. As a raindrop falls downwards it meets air resistance. Frictional drag caused by the air molecules increases proportionately to the acceleration of the raindrop. Eventually the forces of friction and gravitation balance out and the raindrop reaches its terminal velocity, which is the velocity with which its fall terminates. Smaller drops don’t reach as high a terminal velocity as larger drops. Large raindrops tend to signify a higher level of turbulence in the air and stronger upward draughts or air meeting resistance in the clouds above. Raindrops aren’t usually shaped like tears, which is often how people depict them in drawings and other images. Instead, they are oblate spheroids, which look like spheres with a dent, or saucers. A hundred years ago scientists thought the size of a raindrop was determined by forces at work within the cloud before the raindrop falls. More recently it has been shown that raindrops can form or disintegrate many times as they fall. A raindrop occurs when water vapour within a cloud coalesces around tiny particles within the same cloud. The tiny drops remain closely packed together and this means that they tend to coalesce into larger drops.