World Weather Web

Clouds play a very important role in the climate system. On this page we will explain the different forms of water on Earth and in the atmosphere, what are clouds made of and why precipitation sometimes occurs. We'll also see how they form and the many different sorts of clouds.

You could never walk on a cloud as it is only water in the air: clouds form as a result of water vapour turning into droplets or ice crystals light enough to float in the air. When air containing water vapour is cooled to its saturation point, then the water vapour condenses into visible water droplets, called clouds. In other words, when the temperature is lowered, the saturation point corresponds to the moment at which condensation occurs or dew forms. In order to form clouds, water vapour also needs tiny particles in the atmosphere to condense on, called Cloud Condensation Nuclei.

High level clouds: Cirrus (Ci) Cirrostratus (Cs) Cirrocumulus (Cc)

Mid level clouds: Altostratus (As) Altocumulus (Ac)

Low level clouds: Cumulus (Cu) Stratus (St) Stratocumulus (Sc) Nimbostratus (Ns) Cumulonimbus (Cb) Mammatuscumulus (Mc)

The names of the clouds give a clue about shape of the cloud. There are three families of clouds: cirrus, cumulus, and stratus. These are the Latin names given to clouds in 1804. Cirrus means 'curl of hair', cumulus means 'heap', and stratus means 'layer'. Clouds can also be classified by their altitude, whether they are located low, middle or high in the sky. The seasons also affect clouds. Clouds are always slightly lower in the sky during winter months, and higher during the summer.

Formation of Fog and Clouds

Water vapour is an invisible gas, and a certain amount is always present in the atmosphere; there is a limit to the amount of water vapour which air can hold, and this limit is lower when the air is cold than when it is warm. Thus when air is cooled, the point may be reached when the amount of water vapour it holds is the maximum possible at the lower temperature; any further cooling will result in the condensation of water vapour into liquid water droplets. The steam from a kettle and the misting over of kitchen or bathroom windows are every day examples of condensation taking place in this way. During spring and summer, the sea surface round is usually much colder than the land, thus cooling by contact any warm moist air which passes over it. By gentle stirring in the wind, this cooling spreads upwards and, after a time, may be sufficient to cause condensation throughout a layer of air and produce sea fog.

Clouds are formed when condensation occurs in the atmosphere well above the ground, and in this case the cooling needed to produce it is brought about simply by moving the air upwards. Because pressure decreases with height, rising air must expand; just as energy is required to push the piston in a car cylinder, so too energy is needed to move away the surrounding air when a bubble of rising air expands. The only supply of energy readily available in the atmosphere is the heat of the air itself, which thus always cools when it ascends.

Cloud Types

When the atmosphere is in what is termed an unstable state in which temperature falls rapidly with height, air given an initial upward impetus will continue to move upwards spontaneously. This is a process of convection similar to that which heats water in a kettle; upcurrents, (the so-called thermals sought as lift fur gliders), in which air is cooled are balanced by downcurrents in which air is warmed. Clouds, looking at first like puffs of cotton wooI, may form where the air is ascending. If the instability extends to great heights, the clouds tower upwards and may produce showers or thunderstorms. The typical rounded top of a large Cumulus cloud is becoming fibrous and angular at the edges. A fully developed Cumulonimbus cloud shows an anvil shaped top; a thunderstorm is in progress and rain can be seen beneath the centre of the cloud.

When the atmosphere is stable (i.e. temperature not decreasing rapidly or even increasing with height), air will only rise upwards when forced to do so. The clouds which result in these circumstances form in layers or large patches rather than indiscrete puffs with blue sky in between. A simple example of this process is when stable air is forced upwards to rise over a hill, forming what is described as orographic cloud. If the surface air is very moist, Stratus cloud may envelop the hill top but equally, the hill induced upward motion may cause a patch or band of cloud to form well above the hill. In both cases however, it is characteristic of orographic cloud that it will appear stationary despite a wind blowing through it; this is because cloud is constantly formed in the region of ascent upwind of and over the hill, and is dispersed by warming downwind where the air is descending. Another, very common type of layer cloud arises from the turbulence generated in the lower part of the atmosphere by the friction between the air and the ground. In this zone (which may be several thousand feet thick), large eddies form exactly as they do, visibly, in water flowing alongside the bank of any stream. In the atmosphere the eddies are vertical rather than horizontal and involve a lot of upward movement of air in which cloud can form in layers, perhaps covering a large area. Convection, brographic and turbulence clouds can all be distinguished on the excellent satellite cloud photographs now regularly presented on television. These pictures also not infrequently show broad bands of thick cloud which can be many hundreds of miles lang. The association already mentioned between depressions and rising air, together with the fact that rising air is a pre-requisite for cloud formation, suggest a connection between cloud and low pressure systems; but although the satellite pictures and the accompanying weather charts often show thick cloud bands passing through or into depression centres, it is clear that the connection does not depend on the pressure value alone. To understand the formation of these important clouds it is necessary to consider the origin and characteristics of the air involved.

Clouds with Vertical Development

Fair weather cumulus have the appearance of floating cotton and have a lifetime of 5-40 minutes. Known for their flat bases and distinct outlines, fair weather cumulus exhibit only slight vertical growth, with the cloud tops designating the limit of the rising air. Given suitable conditions, however, harmless fair weather cumulus can later develop into towering cumulonimbus clouds associated with powerful thunderstorms.

Puerto Rico, 19/06/2004

Cirrus (Ci), high level

The most common form of high-level clouds are thin and often wispy cirrus clouds. Typically found at heights greater than 20,000 feet (6,000 meters), cirrus clouds are composed of ice crystals that originate from the freezing of super cooled water droplets. Cirrus generally occurs in fair weather and point in the direction of air movement at their elevation.

La Graciosa, Lanzarote Playa Amadores, Gran Canaria

Cirrostratus (Cs), high level

Sheet-like and nearly transparent. Cirrostratus are sheet-like, high-level clouds composed of ice crystals. Though cirrostratus can cover the entire sky and be up to several thousand feet thick, they are relatively transparent, as the sun or the moon can easily be seen through them. These high-level clouds typically form when a broad layer of air is lifted by large-scale convergence. Sometimes the only indication of their presence is given by an observed halo around the sun or moon. Halos result from the refraction of light by the cloud's ice crystals. Cirrostratus clouds, however, tend to thicken as a warm front approaches, signifying an increased production of ice crystals. As a result, the halo gradually disappears and the sun (or moon) becomes less visible.

Cirrocumulus (Cc), high level

A patch or layer of cloud consisting of tiny individual cloudlets at high-level is called cirrocumulus. The cloudlets may make a regular dappled or rippled pattern. Sometimes they look like the scales on a fish - a "mackerel" sky that may mean that unsettled weather is on its way. Like all high-level clouds, cirrocumulus is made of ice crystals. It forms when cirrus or cirrostratus is warmed gently from below. This causes air to rise and sink inside the cloud. Some of the ice crystals change into water vapour, and gaps appear. It can be difficult to tell cirrocumulus from altocumulus. Cirrocumulus has no shading (which altocumulus usually has), and because it is so much higher, the cloudlets of cirrocumulus seems much smaller than those of altocumulus.

Puerto Rico, Gran Canaria 25/4/2004 Playa Amadores, Gran Canaria

Altostratus (As), mid level

Altostratus clouds are named because they are middle height clouds ("alto") and are spread out, thin, and layered ("stratus"). Sunlight is often visible through these clouds as in this picture. Altostratus clouds form when a front of warm, moist air meets a body of cold, dry air. If these clouds thicken, then precipitation is likely.

Puerto Rico, Gran Canaria 7/11/2004

Altocumulus (Ac), mid level

Altocumulus may appear as parallel bands or rounded masses. Typically a portion of an altocumulus cloud is shaded, a characteristic which makes them distinguishable from the high-level cirrocumulus. Altocumulus clouds usually form by convection in an unstable layer aloft, which may result from the gradual lifting of air in advance of a cold front. The presence of altocumulus clouds on a warm and humid summer morning is commonly followed by thunderstorms later in the day.

Puerto Rico, Gran Canaria 19/06/2004 Puerto Rico, Gran Canaria 09/06/2004

Cumulus (Cu), low level

Cumulus clouds are puffy, low altitude clouds. These clouds often resemble cotton balls. Cumulus clouds can form a variety of ways. Either a front of cool, moist air meeting a body of warm, dry air or a high of cool dry air can cause their formation. Cumulus clouds can cause rain, although rain does not always follow cumulus clouds. Fair weather cumulus are fuelled by buoyant bubbles of air, or thermals, that rise upward from the earth's surface. As they rise, the water vapour within cools and condenses forming cloud droplets. Young fair weather cumulus have sharply defined edges and bases while the edges of older clouds appear more ragged, an artefact of cloud erosion. Evaporation along the cloud edges cools the surrounding air, making it heavier and producing sinking motion (or subsidence) outside the cloud.

Las Palmas, Gran Canaria 23/11/2003 Puerto Rico, 28/05/2004 Playa Amadores, Gran Canaria 28/03/2004

Stratus (St), low level

Stratus means "spread out" in Latin, and Stratus clouds are a blanket of grey clouds. These clouds are located at a low altitude of less than 5,000 feet. Stratus clouds often form when a front of warm, moist air meets a body of cold, dry air. Rain and drizzle can also be produced by Stratus clouds.

Puerto Rico, Gran Canaria 19/06/2004 Puerto Rico, Gran Canaria 09/06/2004

Stratocumulus (Sc), low level

Stratocumulus clouds generally appear as a low, lumpy layer of clouds that is sometimes accompanied by weak intensity precipitation. Stratocumulus vary in colour from dark grey to light grey and may appear as rounded masses, rolls, etc., with breaks of clear sky in between.

Puerto Rico, Gran Canaria 28/02/2005 Puerto Rico, Gran Canaria 20/04/2004

Nimbostratus (Ns), low level

Nimbostratus are dark, low-level clouds accompanied by light to moderately falling precipitation. Low clouds are primarily composed of water droplets since their bases generally lie below 6,500 feet (2,000 meters). However, when temperatures are cold enough, these clouds may also contain ice particles and snow. The word Nimbostratus means "rainstorm" and "spread out" in Latin. Nimbostratus clouds colour the sky a solid grey. These clouds develop when a front of warm, moist air meets a body of cold, dry air.

Puerto Rico, Gran Canaria 27/03/2005 Arguineguin, Gran Canaria 20/02/2003 Puerto Rico, Gran Canaria 2004

Cumulonimbus (Cb), low level

Cumulonimbus clouds are much larger and more vertically developed than fair weather cumulus. They can exist as individual towers or form a line of towers called a squall line. Fuelled by vigorous convective updrafts (sometimes in excess 50 knots), the tops of cumulonimbus clouds can easily reach 39,000 feet (12,000 meters) or higher. Lower levels of cumulonimbus clouds consist mostly of water droplets while at higher elevations, where temperatures are well below 0 degrees Celsius, ice crystals dominate. Under favourable atmospheric conditions, harmless fair weather cumulus clouds can quickly develop into large cumulonimbus clouds associated with powerful thunderstorms known as super cells. These clouds form when a front of cool air meets a body of hot, moist air.

The Netherlands, 19/11/2005 Puerto Rico, Gran Canaria 27/03/2004

Mammatuscumulus (Mc), low level

Mammatuscumulus clouds are strange-looking pouches that hang down from a thunderstorm anvil. Though most often grey, they can appear in a startling array of eerie colours, such as ghostly green or orange. Their pendant appearance suggests a funnel, but they have absolutely nothing to do with tornado formation -- except that, a robust storm with mammatus to its fore may well have a tornado lurking to its rear, somewhere. . . Mammatus clouds form in sinking air. (Most clouds form in rising air.) Although mammatus most frequently form on the underside of a cumulonimbus, they can develop underneath cirrocumulus, altostratus, altocumulus, and stratocumulus. For a mammatus to form, the sinking air must be cooler than the air around it and have high liquid water or ice content. They derive their name from their appearance. The bag like sacs that hang beneath the cloud resemble cow's udders...