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.
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.
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 and incoming warm front will being persistent rain within a day, though if cirrostratus fibratus is spotted, stratus may proceed it, bringing only light drizzle.
Cirrostratus can span thousands of miles, and may 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.
There are two main 'species' of cirrostratus, those are:
info from met office