Cloud If the air becomes more unstable, the cloud tends to grow vertically into the species "mediocris", then "congestus", the tallest cumulus species which is the same type that the International Civil Aviation Organization refers to as 'towering cumulus'. With highly unstable atmospheric conditions, large cumulus may continue to grow into cumulonimbus "calvus" (essentially a very tall congestus cloud that produces thunder), then ultimately into the species "capillatus" when supercooled water droplets at the top of the cloud turn into ice crystals giving it a cirriform appearance. All cloud varieties fall into one of two main groups. One group identifies the
Cirrostratus cloud Cirrostratus cloud Cirrostratus is a high-level, 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