HAVE you ever wondered why rain should fall from certain clouds, but not from others?
The answer lies in the size of the drops of water that make up the cloud. If they are so small and light that the air around them offers great resistance to their movement, they "float" in the atmosphere rather than drifting down to earth. But if they are big, they fall as rain.
Cloud particles are very small, indeed. A typical "droplet" is only one fiftieth of a millimetre in diameter, but there are something like a billion of them in a cubic metre of the average cloud. A small cumulus, for example - those little tufts of cotton wool so common on a sunny day - may hold anything from 100 to 1,000 tonnes of water.
Each cloud droplet, looked at individually, does in fact fall through the air, but its rate of descent is almost imperceptible, and negligible compared to the other motions in the atmosphere - the swirls and eddies of the moving body of air.
Sometimes, however, when sufficient moisture is available, clouds manage to produce bigger and heavier drops which enjoy no such immunity from the law of gravity. This often comes about as very small droplets collide with each other, and merge together to form larger ones.
As each drop gets bigger it drifts downwards, and since those of different sizes fall at different speeds, the tendency for collision increases, resulting in larger and larger drops. If they become sufficiently big, they eventually fall to earth as rain or drizzle.
Drizzle drops are typically half a millimetre in diameter, and make it to the ground if the updrafts are gentle, the cloud base low, and the relative humidity high in the air below the cloud.
They soon evaporate, however, if the air below the cloud is dry. Sometimes raindrops can even be seen to evaporate as they fall earthwards, appearing in the sky as streamers hanging obliquely from a cloud - phenomena known to meteorologists as virga.
A typical raindrop, on the other hand, is about two millimetres in diameter. They rarely exceed five millimetres because large drops become deformed as they fall through the air, and ultimately disintegrate into a number of smaller ones - which puts an upper limit on drop size.
Moreover the "tear shaped" drop so beloved of cartoonists does not exist in nature: very small drizzle drops are spherical, while the larger raindrops take on a shape rather like a plum pudding, flattened at the bottom because the air pressure is greater there than on the sides.