Cirrostratus Clouds: High altitude sheet-like clouds composed of ice crystals. These thin clouds often cover the entire sky. Found in an altitude range from 5.000 – 18.000 m. It indicates a considerable amount of moisture and warm air that is advancing or has moved in at higher levels colder than freezing. Due to their high altitude, cirrostratus clouds are made up of ice crystals. It may be pointers of impending drizzle or snow storms because they often signify a warm front coming in.
They typically happen at an altitude of 5 kilometers (16,000 ft) to 12 kilometers (39,000 ft). Like lower altitude cumuliform and stratocumuli form clouds, cirrocumulus signifies convection. They are thin, sheet-like high clouds that often cover the whole sky. They are so thin that the sun and moon can be seen through them. Cirrostratus clouds generally come 12-24 hours before a rain or snow storm.
Cirrostratus clouds sometimes indication they come within reach of a humid front if they form after cirrus and extend from one region across the sky, and thus may be signs that precipitation might follow in the next 12 to 24 hours or as soon as 6–8 hours if the front is fast moving. It can length thousands of miles, and might be smooth or fibrous and is often fringed with cirrus clouds. Shadows will usually still be cast by the sun when shining through cirrostratus clouds, which can help distinguish them from similar altostratus clouds.