MOST clouds consist of a myriad of water droplets so tiny, that as many as 200 million of them could merge together in a single teaspoon, without the risk of overflow. But not all clouds consist wholly, or even in part, of droplets of this kind. Some for example contain raindrops, each of which is equal in mass to many thousands of cloud particles. Others contain no water droplets at all, but only ice in one or more of its many forms, ranging from discrete tiny columns or needles to matted tufts an inch or more across, they are commonly called snowflakes.
Cirrus clouds the familiar "mare's tails" that in fine weather often stand out in bright contrast to the brilliant blue of an otherwise cloudless sky, nearly always consist entirely of ice particles. This is because they form six or seven miles above the ground at altitudes where the temperature of the air may be 400C or less and where water cannot survive in liquid form.
Cirrus clouds look like swirls or "hooks" of cotton wool an appearance that is reflected in their name which comes from the Latin word to "cud". They are wispy filaments of brilliant white, fibrous or hair like in appearance, and too thin to cast a distinct shadow or to blur the outline of the sun or moon.
The wispy appearance of cirrus has a number of contributory causes. In the first place, at those heights the air is so cold it cannot hold sufficient water vapour to form thicker clouds. Also, when the water vapour is transformed into particles of ice, it is distributed in the vertical over a deep layer of the atmosphere the wind at the bottom of this layer may not be blowing in the same direction as the wind at the top, a circumstance which gives the cloud its characteristic "twist" or "hook". The tufty appearance is accentuated by the oblique angle at which we are normally obliged to view the cloud.
When cirrus slowly disappears, it is often an indication of fair weather in the immediate future. But when it grows denser, it is frequently the harbinger of rain and wind, a kind of scout or avant garde leading the way for the serried ranks of lower, heavier cloud advancing in its wake. As such, it is often the first visible precursor of an invading front, a feature graphically captured by Shelley in Ode to the West Wind when he describes the sky in such a situation as bearing
Even from the dim verge
OF the horizon to the zenith's height.
The locks of the approaching storm.