A sunset-lit cumulus cloud over the Guatemalan coastline

      Quite apart from their aesthetic value, human life would not exist were it not for clouds. They make all terrestrial life possible. Clouds are the freshwater delivery mechanism that transports water from the oceans to the land and makes possible the growth of plants and animals.

      A cloud is a visible concentration of minute water droplets, ice particles, or a mixture of both, floating in the atmosphere. The vast majority of them are in the lowest layer of the atmosphere, which is called the troposphere. On average, each water droplet has a diameter of about 0.01 mm, but droplets may be much smaller or larger. There are hundreds in each cubic centimetre, millions in every cubic metre. Surface tension keeps them spherical, and currents keep them airborne. Because they are so small, the droplets can remain in liquid form at temperatures as low as -30°C. When still liquid at sub-zero temperatures, they are described as supercooled. At higher levels in the troposphere, clouds are made of ice crystals, which come in many shapes and sizes.

      An English amateur meteorologist and artist called Luke Howard named the clouds in the early nineteenth century. He proposed three principle categories—cumulus, stratus, and cirrus—and their modifiers. There have been a few additions, but his names pretty much stuck. When trying to identify a cloud, the key pointers to look for are whether it is heaped (a type of cumulus), flat (a type of stratus), or wispy (a type of cirrus); how low its base is; and whether rain or snow is falling from it. Clouds are named from a combination of their appearance, altitude, and ability to produce precipitation. The names are derived from five Latin words: Alto – rain; Cirrus – curled; Cumulus – heaped; Nimbus – rain; Stratus – layered. Additionally, lenticularis is Latin for lens-shaped; mammatus means breast-shaped; and pyro means fire.

      How clouds form

       A “tablecloth” of orographic cloud sits on Cape Town’s Table Mountain

      A parcel of air is a little like a sponge: it can hold water vapour. Warm air can hold more than cool air, but whatever its temperature it can only accommodate so much before it becomes saturated. Then, tiny water droplets condense around even tinier particles of dust or salt and form cloud. One of two things can cause the air to become saturated: either the amount of water vapour in the air parcel increases, or its temperature falls. For example, if air moves over the ocean or a lake, it picks up evaporating water vapour as it goes. Warm air will pick up more than cold air. If this air then rises, for example if it reaches land and passes over a mountain, it will cool. If the cooling continues, the air eventually reaches the point (its dew point) where its water vapour condenses—and cloud develops.

      Three primary types of uplift cause cloud to form. Orographic uplift happens when wind pushes air over hills and mountains. Frontal uplift occurs when a wedge of air undercuts another, pushing it higher. Convectional uplift takes place when the ground is heated and causes the air in contact with it to become warmer and rise vertically. Of course, none of these scenarios will produce cloud if the air is bone dry. However, if it is moist, one form of cloud or another will form sooner or later.

      The type of cloud that develops depends on whether orographic, frontal, or convective processes are dominant; how much water vapour the air contains; how quickly the air temperature decreases with altitude; and the wind speed.

      CUMULUS FAMILY

       Characteristics

       Cumulus Abbreviation: Cu

       Appearance: Heaped

       Cloud-base: From 2,000 ft (600 m)

       Cloud-top: Up to 70,000 ft (21,000 m) in the tropics

       Cause: Warming of land and convection

       Weather: Wide range, from fair to stormy, and from dry to torrential rain

      Cumulus clouds are probably the most familiar. The Latin root of the word cumulus, meaning ‘heap’, is a good descriptor of the appearance of these clouds, but they are very variable. They range from the benign, fluffy “cotton wool” clouds—fair-weather cumulus, or cumulus humilis—of a fine summer’s day to towering, menacing cumulonimbus clouds, which bring hailstones, thunder, and lightning, and sometimes even spawn tornadoes.

      Whatever their scale, all cumulus clouds develop as a result of convection, warm air rising. As it rises, it cools and any water vapour it contains condenses to produce cloud. Depending on local atmospheric conditions, convection may be very limited, producing small clouds, which may later evaporate again. If the warming process continues, bigger clouds will form. In extreme circumstances these may tower to the top of the troposphere, where they flatten out in gleaming white anvils. Such clouds produce storms.

      Cumulus and cumulonimbus have relatively low cloud bases, but the suffix “cumulus” is also added to mid- and high-altitude clouds, altocumulus and cirrocumulus respectively.

      1 Fair-weather cumulus

       Characteristics

      Cumulus humilis Abbreviation: Cu hum

      Appearance: Shallow and flattish

      Cloud-base: 1,000–5,000 ft (300–1,500 m)

      Cause: Warming of land and convection

      Composition: Water droplets, sometimes supercooled

      Weather: Fine

      These “cotton-wool” clouds of fair days may be widely spread out or almost touching each other. Often occurring with only light breezes, they have flattish bases, all at the same height. They look mid-grey from beneath although their tops can be dazzlingly white if there are no higher clouds to obscure the Sun.

      As with other cumulus clouds, they form where rising air currents caused by a warming land cause water vapour in moist air to condense as tiny water droplets. Fair-weather cumulus does not contain ice crystals. These clouds often form just inland of coastlines, where summer sea breezes carry moist air over warm land. The bottom of the clouds shows the condensation level in the atmosphere. Although fair-weather cumulus clouds never produce rain or snow, they can be the precursors of thicker cloud and rain.

      The rising currents within these clouds are generally fairly light. If the air is stable, they remain shallow, sometimes pancake-flat, and may evaporate within a few minutes. However, fair-weather cumulus can also indicate unstable conditions, in which case they are likely to grow bigger and taller—and could develop later into rain-bearing forms of cumulus.

      2 Medium cumulus

       Characteristics

      Cumulus mediocris Abbreviation: Cu med

      Appearance: Gently heaped

      Cloud-base: 1,000–5,000

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