DO you remember that zany conversation, just after the "play within a play", between Polonius and Hamlet? "Do you see yonder cloud that's almost in the shape of a camel?" inquires the Prince. "By the mass, and `tis like a camel indeed," agrees Polonius. "Methinks it is like a weasel," says Hamlet, just to confuse the counsellor, and then he changes tack again: "It is backed like a whale." "Very like a whale!" responds Polonius, ever anxious to restore calm to the troubled waters of El-sinore.
Now the chances are that the pair were gazing at a cumulus cloud. Such a cloud may well appear at first glance to be fixed in shape, but it is in fact constantly changing. In a developing cumulus, the cloud bubbles upwards, as the updrafts in which it has its origins surge higher and higher; in a decaying cumulus, on the other hand, the cloud dissipates, as the vertical currents which caused it die away.
Cumulus clouds look something like cauliflowers, and are arranged randomly around the sky, usually with plenty of blue to be seen in between. Each one is an isolated tower, perhaps only a few hundred yards across, but extending vertically for many hundreds or even thousands of feet.
They form in vertical currents of air in the atmosphere, these in turn being brought about by a process known to meteorologists as convection.
Convection occurs when a volume of air near the ground has been warmed, often by contact with the surface, so that it acquires a certain buoyancy relative to the air surrounding it. As warm bubbles of air begin to drift upwards, the rising air becomes cooler with increasing height, and its temperature eventually falls to the dew-point, the temperature at which condensation takes place. The result is a cumulus cloud, in which, as the cloud develops further, the individual droplets may coalesce to form water-drops, which may in due course fall to the ground as showers.
When a cumulus cloud becomes very tall, meteorologists call it a cumulonimbus, and from such clouds come very heavy showers, hail, or even thunderstorms. A characteristic of a well-developed cumulonimbus is the spreading out of the cloud at the top into a shape which closely resembles blacksmith's anvil. The "anvil" is caused by a layer of very strong winds aloft, which shear off the top of the cloud in a forward direction. Trails of ice and snow crystals fall out of this downwind extension, evaporating as they do so, and giving the anvil a much "wispier" texture than the lower parts of the cloud that are well-defined and rounded.