cloud

Standard types of cloud. The height and nature of a cloud can be deduced from its name. Cirrus clouds are at high levels and have a wispy appearance. Stratus clouds form at low level and are layered. Middle-level clouds have names beginning with ‘alto’. Cumulus clouds, ball or cottonwool clouds, occur over a range of height.

This cumulus cloud has the classic domed stack shape that appears to rise threateningly high and suggests the possibility of a rain shower. But it would have to be rather broader and to rise perhaps three times higher still for it to become a cumulonimbus cloud, and for rain to become likely.

Altocumulus clouds resemble the tufted cirrocumulus clouds of the classic ‘mackerel sky’, but they are rather larger. They will not necessarily appear larger to an observer on the ground, because they are at a greater altitude (the prefix alto- is Latin, meaning high). The mackerel pattern may also seem more regularly aligned.

A bank of cumulus cloud, technically termed ‘cumulus congestus’. This phenomenon is particularly prevalent over a coastline with hills or mountains immediately behind, and with a strong breeze normally blowing off the sea. Such banks are also found elsewhere as the first sign of a change in atmospheric pressure or the approach of a weather system.

Cumulus clouds, gathering together, do not necessarily threaten rain. They are named ‘cumulus’ because they gather together, or accumulate. Such clouds contain a high proportion of water vapour at around the altitude of condensation. The altitude of condensation depends on temperature and humidity; atmospheric pressure also influences temperature.

High-altitude clouds. Clouds are formed when moisture in the air condenses, and are composed of millions of water droplets suspended in air. Different types of cloud are found at high and low altitudes, and they are also affected by upper air movements, such as thermal winds.

Thirlmere in the Lake District, northwestern England. In the sky overhead are clouds at high altitude, formed from water vapour condensed into millions of tiny water droplets suspended in air.

Water vapour condensed into minute water particles that float in masses in the atmosphere. Clouds, like fogs or mists, that occur at lower levels, are formed by the cooling of air containing water vapour, which generally condenses around tiny dust particles.

Clouds are classified according to the height at which they occur, and by their shape. Cirrus and cirrostratus clouds occur at around 10 km/33,000 ft. The former, sometimes called mares'-tails, consist of minute specks of ice and appear as feathery white wisps, while cirrostratus clouds stretch across the sky as a thin white sheet. Three types of cloud are found at 3–7 km/10,000–23,000 ft: cirrocumulus, altocumulus, and altostratus. Cirrocumulus clouds occur in small or large rounded tufts, sometimes arranged in the pattern called mackerel sky. Altocumulus clouds are similar, but larger, white clouds, also arranged in lines. Altostratus clouds are like heavy cirrostratus clouds and may stretch across the sky as a grey sheet. Stratocumulus clouds are generally lower, occurring at 2–6 km/6,500–20,000 ft. They are dull grey clouds that give rise to a leaden sky that may not yield rain. Two types of clouds, cumulus and cumulonimbus, are placed in a special category because they are produced by daily ascending air currents, which take moisture into the cooler regions of the atmosphere. Cumulus clouds have a flat base generally at 1.4 km/4,500 ft where condensation begins, while the upper part is dome-shaped and extends to about 1.8 km/6,000 ft. Cumulonimbus clouds have their base at much the same level, but extend much higher, often up to over 6 km/20,000 ft. Short heavy showers and sometimes thunder may accompany them. Stratus clouds, occurring below 1–2.5 km/3,000–8,000 ft, have the appearance of sheets parallel to the horizon and are like high-level fogs.

In addition to their essential role in the water cycle, clouds are important in the regulation of radiation in the Earth's atmosphere. They reflect short-wave radiation from the Sun, and absorb and re-emit long-wave radiation from the Earth's surface.