SOME meteorologists, as a rule, are shy and diffident, but sometimes use long, pretentious and exotic words to describe the different types of cloud. They are not intending to impress;the objective is to be specific, so the cloud described can be visualised by any meteorologist, anywhere in the world.
When the weather is calm and warm, and the sky relatively clear, surface heating by sunshine produces ascending columns of air. If the atmosphere is somewhat humid, a cloud forms near the top of each of these invisible pillars - a bulbous rounded cloud with a flat base, resembling a clump of cotton wool or giant cauliflower. This is a cumulus cloud. When it becomes very tall, towering perhaps 10 or 15 thousand feet into the sky, meteorologists call it a cumulonimbus, from which come heavy showers, or even thunderstorms.
The very highest clouds, at 20,000 feet or more above the ground, are cirrus clouds. They are thin wispy streaks of brilliant white, fibrous or hair-like in appearance, and composed entirely of ice crystals which give them a silky sheen. These feathery filaments are called "mares' tails", the proverbial precursors of a spell of frontal rain.
Cirrostratus cloud is similar in texture to cirrus, but forms a continuous sheet, a thin, diaphanous curtain drawn across the sky, often hiding a warm sun attempting to shine faintly through it. Cirrocumulus, another close relation, is rather more patchy in appearance than the other two, and comprises closely spaced "pebbly" elements, resembling cobblestones or fish scales.
Altocumulus and stratocumulus look alike, and are distinguished mainly by their height. The former is typically 7,000 or 8,000 feet above ground level, while stratocumulus is much lower - approximately half that. Both are common on a dry cloudy day, where there is no particular threat of rain - but not much sun apparent either.
Altostratus, however, brings a definite threat of rain. It is normally associated with a front, and appears at first as a uniform layer of grey, relatively thin cloud, but thickens rapidly as rain approaches. And the very lowest cloud, stratus, is the cloud you see clinging on to the mountaintops in soft drizzly weather, or scurrying across the sky in ragged patches on a wet and windy day; it may be anywhere from 2000 feet down to ground level - in which latter case, of course, we call it fog.