All high clouds are a type of cirrus, a common cloud that can be seen at any time of the year.
Height of base: 20,000 - 40,000 ft
Shape: layered, tufty or patchy
Latin: cirrus - lock or tuft of hair
What are cirrus clouds?
Cirrus clouds are short, detached, hair-like clouds found at high altitudes. These delicate clouds are wispy, with a silky sheen, or look like tufts of hair. In the daytime, they are whiter than any other cloud in the sky. While the Sun is setting or rising, they may take on the colours of the sunset.
How do cirrus clouds form?
Cirrus clouds form from the ascent of dry air, making the small quantity of water vapour in the air undergo deposition into ice (to change from a gas directly into a solid). Cirrus is made up completely of ice crystals, which provides their white colour and form in a wide range of shapes and sizes.
Cirrus clouds can also form through contrails, the vapour trails left by planes as they fly through a dry upper troposphere. These streaks can spread out and become cirrus, cirrostratus and cirrocumulus.
What weather is associated with cirrus clouds?
They often form in advance of a warm front, where the air masses meet at high levels, indicating a change in the weather is on the way.
Technically these clouds produce precipitation but it never reaches the ground. Instead, it re-evaporates, creating virga clouds.
How do we categorise cirrus clouds?
Cirrus clouds have five defined 'species';
- Cirrus fibratus - Thin and fibrous, cirrus fibratus are often aligned with the high altitude wind direction, making for white parallel stripes which streak across the sky. These are the most common type of cirrus cloud
- Cirrus uncinus - With its trademark hook shape, cirrus uncinus is famous for looking like a horse's tail. These wispy streaks of cirrus cannot be seen without a characteristic 'flick' at the end of its tail
- Cirrus spissatus - These clouds sit right at the top of the troposphere. A thick, dense cirrus layer that dominates much of the sky above, often formed by passing warm fronts or the remnants of a cumulonimbus incus
- Cirrus floccus - Ragged cirrus patches which are much larger than cirrocumulus floccus. These have a more cotton wool-like appearance than the rest of the cirrus family
- Cirrus castellanus - More vertically developed than cirrus floccus, cirrus castellanus have turret-like tops and are taller than they are wide.