One of the first things that people think of when you begin to look at the weather are clouds. Clouds come in a variety of shapes, colours and patterns and a good understanding of cloud behaviour can help you predicate the weather and other related phenomena.

Almost everyone has seen the brilliant white blossoms of summer and felt the result of those glowering grey clouds in winter. The form and textures of all the different types of cloud seem endless.

Scientifically they are affected by height, density, illumination, humidity and so on. However this section not only aims to help you learn about clouds, how they are formed and why, but also to give a aesthetic appreciation of the many types.

I am sure I am not alone in recalling looking up at the clouds in the sky and forming them into images and pictures in my mind. Perhaps we should retain that sense of wonder and interest as we plod through these sections.

There are in fact only two basic forms of cloud - layered and convective, and only two basic constituents - water droplets and ice crystals.

Layered Clouds are much larger horizontally than vertically. They usually have a more or less smooth base and top and can extend for hundreds of kilometres, especially where associated with weather fronts. They can develop when the atmosphere is stable.

Convective Clouds are generally discrete and cellular. They are usually deeper than they are across. The base is quite uniform except sometimes when the cloud is dissolving, but the tops vary considerably with turrets of cloud projecting above the main mass. They form only when the atmosphere is unstable.

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Meteorologists use a classification that is similar to the Latin names employed for plants and animals. Clouds are arranged in genera (the ten main types described first), species and varieties.

Genus Type of cloud
species Cloud shape and structure
variety Arrangement of elements and transparency

Cloud Forms - The three basic cloud forms are

cu cumulus heap clouds

st stratus layer clouds

ci cirrus hair-like or feathery clouds

These are also the names of three specific types. There was once a fourth form, 'nimbus', meaning 'rain-bearing'. The name still remains in some types (such as nimbostratus) although other clouds also produce rain or snow.

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Stratus - This is a low grey, uniform featureless cloud, usually with a base below 500 m always formed entirely of water droplets. It often has a ragged base and top and is identical to fog, which is stratus at ground level. It is sometimes shallow and tenuous. Generally there are no optical phenomena associated with it.

Stratus forms either by the slow uplift of moist air or when a gentle wind carries nearly saturated air across a cold land or sea surface. There is not much precipitation from stratus, because it is a shallow cloud, but it may produce slight to moderate drizzle.

Stratocumulus - Perhaps the most common cloud, it frequently forms in the lowest few kilometres when wind-driven turbulence lifts air above its condensation level. It is a low, grey or whitish sheet of cloud with a definite structure. These clumps, broader pancakes or rolls show dark shading and are outlined by thinner and paler regions of cloud or by blue sky.

It is formed completely of water droplets, and although not itself associated with rain, may markedly increase rain falling through it from higher cloud. Generally there are no optical phenomena associated with it.

Cumulus - Cumulus is easy to recognise, they are the fluffy clouds that float across the sky on a fine day and are often known as fair- weather clouds. They have rounded tops and flat, darker bases. They are formed when the unstable layer is shallow with plenty of blue sky between.

Cumulonimbus - This is the largest and most energetic of the cumulus family. They are the deepest and most vigorous convective clouds and produce not only showers but thunderstorms, hail, squally winds and occasionally tornadoes. They usually form by heating from below, but the most intense cumulonimbus are helped by cold winds at high levels.

Cumulonimbus are avoided where possible by aircraft because strong up and down currents within and near them create severe turbulence, while their high water content can rapidly produce thick layers of ice on cold airframes. Sometimes has rainbows in rain beneath cloud.

Altostratus - This is a dull, medium level, white or blueish grey cloud. It forms by slow ascent of air over a wide area, especially ahead of a warm front or occlusion where it is often a precursor of rain.

Although composed mostly of water droplets, thick altostratus can produce light rain, though most of this evaporates before reaching the ground. It may have optical phenomena such as coronae or iridescence when thin.

Nimbostratus - This is a dark, grey, heavy sheet of cloud from which rain or snow is falling. It appears black from below but as the rain or snow becomes heavier the base may become indistinguishable with just ragged scuds of low fractostratus to be seen. The rain or other precipitation usually lasts for a long time. At a warm front it may continue for several hours, unlike the relatively short, intense rain from showers.

Altocumulus - Altocumulus is a medium level cloud which occurs as an individual rounded masses with clear sky between them. It forms in rolls or patches with or without gaps between, by turbulent mixing often in moist layers remaining from dispersed fronts.

Altocumulus cloud may contain either water droplets, which are usually supercooled, or ice crystals, or both. They may therefore exhibit a range of optical phenomena, depending on which form of water is predominant (e.g. Iridescence, corona, mock suns, sun pillars)

Cirrus - This is high cloud composed entirely of ice crystals and takes many forms. It is non-uniform and often thin and wispy, sometimes with thicker bright sheaves, and all shapes between. It forms by ascent in the upper troposphere. Occasionally it is manufactured when condensation trails from high flying aircraft seed already moist or very cold air.

It has no precipitation which reaches the ground by it actually consists of falling ice crystals. Optical phenomena such as mock suns and circumzenithal arcs may be seen.

Cirrostratus - This is a sheet of high ice-cyrstal cloud, sometimes so thin that it goes completely unnoticed because it has little effect on sunlight. It is produced by the slow ascent of air and condensation or sublimation high in the troposphere, usually well ahead of weather fronts. It appears in meteorological folklore because it is often an early indication of rain.

Cirrocumulus - This attractive ice cloud is the high-level equivalent of stratocumulus or altocumulus, and much less common than either. It is formed of cells aligned in streets rather like ripples in the sand on the beach. Cirrocumulus forms by wave motion or turbulence through a moist layer in the high atmosphere.

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Just as cloud species describe shape and form varieties define cloud transparency or the arrangement of the individual elements in the specific type. Please note that most of the descriptions are self explanatory so only those more obscure ones are detailed on the table below.

Other Varieties include: Billows, Cloud Streets

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The 14 cloud species are used to describe cloud shape and structure.

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Clouds are also classified by the height of their bases, with broad categories of high, medium and low. The names reflect the fact that specific types occur at particular levels. The range of heights and the maximum altitudes of medium and high level clouds are greater towards the equator than near the poles. Similarly, cloud heights tend to be lower in winter than in summer.

Cloud heights are difficult to estimate without a great deal of experience or equipment.

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Certain forms of cloud are not true types but always occur in association with one or more of the ten main types. These three accessory clouds are

There are also a number of other features which can describe the appearance of certain clouds. These are

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Condensation trails, or contrails are a familiar sight. They are lines of cloud that have formed from the water vapour emitted by aircraft engines. Initially the exhaust is very hot and the water vapour is invisible, so there is a clear gap behind the engines. Farther away mixing cools the exhaust sufficiently for condensation to occur.

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