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
Wall cloud the wall cloud, as it is also an inflow feature. Most movement is horizontal, but some rising motion is often apparent as well. Some wall clouds also have a band of cloud fragments encircling the top of the wall cloud where it meets the ambient cloud base; this feature is a collar cloud. Another accessory cloud is the flumen, commonly known as the beaver's tail. It is formed by the warm, humid inflow of a strong thunderstorm, and is often mistaken for tornadoes. Although the presence of a flumen is associated with tornado risk, the flumen, similar to scud clouds,