The thin, layered cirrostratus cloud is composed of ice crystals and forms a veil that covers all or part of the sky.
Height of base: 20,000 - 40,000 ft
Latin: cirrus - lock or tuft of hair; stratus - flattened or spread out
What are cirrostratus clouds?
Cirrostratus are transparent high clouds, which cover large areas of the sky. They sometimes produce white or coloured rings, spots or arcs of light around the Sun or Moon, that are known as halo phenomena. Sometimes they are so thin that the halo is the only indication that a cirrostratus cloud is in the sky.
Cirrostratus clouds can span thousands of miles, can be smooth or fibrous and are often fringed with cirrus clouds. Shadows will normally still be cast by the Sun when shining through cirrostratus clouds, which can help distinguish them from similar altostratus clouds.
How do cirrostratus clouds form?
As a result of slowly rising air, cirrostratus cloud can form. Usually generated at the forefront of frontal weather systems, the movements of cirrostratus can be used to predict what the weather will do in the next 24 hours.
Cirrostratus 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 cirrostratus clouds?
Though cirrostratus itself does not produce precipitation, it can indicate whether or not precipitation is likely. If cirrostratus nebulosus exists in the sky it is likely that an incoming warm front will bring persistent rain within a day. If cirrostratus fibratus is spotted, stratus may proceed it, bringing only light drizzle.
How do we categorise cirrostratus clouds?
There are two main 'species' of cirrostratus:
- Cirrostratus fibratus - similar to cirrus, but with more consistency. Wispy strands all tightly knitted together, making for more of an animal fur look
- Cirrostratus nebulosus - a uniform veil-like layer covering the sky. Sometimes almost unnoticeable, cirrostratus nebulosus is featureless, composed entirely of ice