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
Cumulus humilis cloud Cumulus humilis cloud Cumulus humilis are cumuliform clouds with little vertical extent, common in the summer, that are often referred to as "fair weather cumulus". If they develop into cumulus mediocris or cumulus congestus, thunderstorms could form later in the day. They generally form at lower altitudes (500–3000 m (1,500–10,000 ft)), but in hot countries or over mountainous terrain these clouds can occur at an altitude of up to . They show no significant vertical development, indicating that the temperature in the atmosphere above them either drops off very slowly or not at all with altitude; that is, the adiabatic