Stratus (ST) is a layer cloud with large horizontal extent but little vertical development. It generally has a very low cloud base (below 1000 ft) and covers the whole sky. The typical depth is 1000 – 1500 ft. The base can be quite diffuse with veils hanging down beneath the cloud.

It is a turbulence cloud, often found in the warm sector of polar front depressions. It can also be formed when the low fog lifts.

ST consists of water droplets that are sub-zero in winter but are not very dense, so light to moderate icing can be expected. Precipitation may occur as drizzle, freezing drizzle, or snow grains.


A stratocumulus (SC) cloud is a stratiform cloud caused by turbulence. It can be found between heights 1000 ft and 6500 ft. As it is formed by turbulence, light to moderate turbulence when flying in or below the cloud might be expected. Conditions are calm above the cloud.

Like stratus, this cloud consists of water droplets, so light to moderate icing, drizzle, freezing drizzle, or snow grains can be expected. In addition, ice pellets and, from the thicker stratocumulus, intermittent rain or snow might be expected. Heavy snowfall can be experienced in winter.


This photo features heap clouds, which are clouds that generally have greater vertical than horizontal extent. They are formed convectively and the base can be found between 3000 and 7000 ft in the summer and 700 and 4000 ft in the winter. The tops can extend to 25 000 ft.

Cumulus clouds consist of water droplets, which are supercooled above the freezing level.

Precipitation can be present when the cloud has a vertical extent greater than 10 000 ft. It can take the form of rain or snow showers.

When the cloud becomes towering without being ‘iced’ (cirrus forming) at the top, it is called towering cumulus (TCU).

Strong vertical currents can be present and larger cumuliform clouds should be avoided.

Moderate to severe icing conditions can be encountered, but because the time taken to traverse the cloud is usually short, any ice build-up tends to be small.


Cumulonimbus is a towering cumulus cloud with a top that has turned into cirrus. This is called the anvil and extends in the direction of the wind. The anvil is fibrous and diffuse in appearance.

This cloud is very hazardous to aircraft. It is very dense and consists of water droplets of varying sizes, so moderate to severe icing may be expected. Moderate to severe turbulence is also likely.

CB can give precipitation in the form of rain or snow showers and hail.

Due to the severe weather conditions associated with this cloud, it is discussed in detail in a separate chapter on thunderstorms.


Altostratus is similar to nimbostratus but is less deep and less dense. This type of cloud can cover the whole or a major part of the sky and is an indication of the approach of a warm front.

Altostratus contains water droplets and ice crystals, therefore, it can cause light to moderate icing. Light to moderate turbulence can also be expected. Precipitation can take the form of continuous or intermittent rain or snow.


Cirrus is a thin wispy cloud. It is associated with the approach of a warm front. It can also indicate the line of a jet stream.

It consists of ice crystals and does not produce icing or precipitation. Likewise, there is no turbulence.


Cirrostratus is a sheet-like cloud, sometimes with a wispy veil underneath. It causes a bright ring around the sun and the moon, known as the halo phenomenon. It is associated with warm fronts.

Like cirrus, it consists of ice crystals and does not produce icing, precipitation, or turbulence.


Cirrocumulus is divided into smaller cloud elements that look like the scales of a mackerel. It is formed when there is turbulence within cirrus or cirrostratus.

Cirrocumulus consists of ice crystals and occasionally freezing water droplets. There is no icing or precipitation. There may be light turbulence.

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