Cirrostratus is a high, very thin, generally uniform
stratiform genus-type of
cloud, composed of
ice-crystals. It is difficult to detect and is capable of forming
halos when the cloud takes the form of thin
cirrostratus nebulosus. The cloud has a fibrous texture with no halos if it is thicker
cirrostratus fibratus. On the approach of a frontal system, the cirrostratus often begins as nebulosus and turns to fibratus. If the cirrostratus begins as fragmented fibratus it often means the front is weak. Cirrostratus is usually located above 5.5 km (18,000 ft). Its presence indicates a large amount of
moisture in the upper
atmosphere.