The very highest clouds, at 20,000 feet or higher, are called Cirrus clouds. They are thin wispy streaks of brilliant white, fibrous or hair-like in appearance, and composed entirely of ice crystals which gives them something of a silky sheen.
These benign feathery filaments, usually to be seen against a clear blue sky, are often swept by strong winds aloft into the hook-like shapes that are often 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. It appears like a thin, diaphanous curtain drawn across the sky, often hiding a wan sun attempting to shine faintly through it.
Cirrocumulus, on the other hand, another close relation, is rather more patchy in appearance than the other two, and comprises closely-spaced "pebbly" elements, resembling cobble-stones or fish-scales.
Because of the latter appearance it is often referred to as a "mackerel" sky - another traditional sign that rain is on the way.
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 height. Both are a whitish-grey, with "rolls" or undulations, often arranged in long, straight horizontal columns. 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 very definite threat of rain. It is normally associated with a front or rain-belt, and appears at first as a uniform layer of grey, relatively thin cloud, but thickens rapidly as the rain approaches, ultimately to obscure the sun completely in a dark, smooth, threatening blanket.
The very lowest type of cloud, called Stratus, is the variety you see clinging to the mountain-tops in soft drizzly weather, or scurrying across the sky in ragged patches on a wet and windy day.
Its base may be anywhere from 2,000 feet to ground level - in which case, of course, we call it fog.
Finally there are Cumulus clouds, and their big brother, the Cumulonimbus. Rather than being arranged in layers, these clouds extend upwards like towers in the sky over an area some hundreds of yards across.
They have that rounded "cauliflower" appearance so beloved of landscape artists, and are a familiar sight in bright, showery conditions. The smaller ones are white and fluffy; the larger cumulonimbus clouds are dark and threatening, and stretch upwards into the sky for tens of thousands of feet.