MUCH of the cloud we see in the sky is layered cloud. As the name implies, it covers the sky in a layer, rather like a curtain drawn horizontally across the heavens overhead. It occurs when air ascends gently over a relatively large area most notably, perhaps, in the vicinity of a weather front - instead of rising in convective lumps or currents, as happens in the formation of the isolated, towering cumulus clouds.
The very highest clouds, 20,000 ft or higher, are called cirrus the familiar "mare's tails" that in fine weather often stand out in bright contrast to the brilliant blue of an otherwise cloudless sky. They are thin wispy filaments of brilliant white, fibrous or hair-like in appearance, and often with a "silky" sheen. Cirrus looks like swirls or "hooks" of cotton wool, and is too thin to blur the outline of the sun or moon. It nearly always consists entirely of ice particles, because at the great heights at which it forms the temperature may be -40C or even less.
Cirrostratus cloud is similar in texture, but forms a continuous sheet; it appears like a thin gauze curtain drawn across the sky, with the sun shining faintly through it. And cirrocumulus is another close relation - rather more patchy in appearance than the other two, and frequently arranged in more or less regular bands across the sky.
Altocumulus and stratocumulus look alike, and are distinguished mainly by their height. The former is typically seven or eight thousand feet above ground level, while stratocumulus is generally three or four thousand feet up. Both are a whitish grey in colour, with light and dark shadows giving the appearance of "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 to be seen either.
Altostratus, however, brings a very definite threat of rain. At first it appears as a uniform layer of grey, relatively thin cloud, which may allow the outline of the sun to appear as if seen through a sheet of ground glass, but it thickens as the rain approaches, in due course completely obscuring the sun in a dark, smooth, threatening blanket.
The lowest cloud is stratus, often seen clinging to the mountain-tops in mild, soft drizzly weather, or scurrying low across the sky in ragged patches on a wet and windy day. The base of stratus cloud may be anywhere from zero to 2000 ft; indeed fog at ground level is merely stratus cloud which has formed at or near the surface of the earth.