Above your head at this very moment, enormous quantities of water hang suspended as clouds. Each one is composed of tiny droplets of water, so small that at least a million are needed to form a single raindrop. However, in aggregate, every cloud is a formidable reservoir.
A small cumulus, for example - those little tufts of cotton-wool so common on a sunny day - may contain anything from 100 to 1,000 tonnes of water.
At the other end of the scale, the great towering cauliflowers which produce heavy showers and thunderstorms, the cumulonimbus clouds, are veritable mountains of water and ice; if the whole body of water contained in such a cloud could be put into a gigantic bucket and weighed, it would turn the scales at something like 100,000 tonnes.
And the amount of water in a typical mid-Atlantic depression is so vast that no meaningful figure for it can be calculated.
Clouds, however, assume a limited number of shapes no matter where they form. Indeed, this was ascertained in 1887 by the English meteorologist Ralph Abercromby, when he travelled around the world apparently with no other objective than to make sure that clouds looked the same everywhere.
The ability to distinguish between the cloud types helps meteorologists to understand the current structure of the atmosphere and the likely future progress of the weather.
The first serious attempt to classify them was by the French naturalist Jean Lamarck in 1802, which proposed five main divisions: hazy, massed, dappled, broomlike and grouped. The Latin nomenclature devised shortly afterwards by the English meteorologist Luke Howard, however, proved to be more popular and more enduring.
Howard assigned to each cloud type a Latin name according to its general appearance. Cirrus, meaning "curl", was used to describe wispy, fibrous strands; cumulus or "mass" was ascribed to flat-based "heap-clouds" extending upwards through the atmosphere; and stratus, as in "spread out", was the label attached to low, horizontal sheets.
These three primary categories, in various combinations, became the basis for the 10 main cloud families: cirrus, cirrocumulus, and cirrostratus; altocumulus, altostratus, and nimbostratus; and stratus, stratocumulus, cumulus and cumulonimbus.
The aspiring weatherperson nowadays must not only be able to easily distinguish these 10 types of cloud but must also be able to subdivide them further using words from a lengthy litany of Latin adjectives, in much the same way as botanists classify their plants.
And meteorologists further distinguish cloud-types by their height which, with an ingenuous and unexpected clarity, they describe as simply low, medium and high.