Cirrus (Ci) - the name derives from the Latin cirrus = curl of hair, tuft or wisp. Cirrus cloud is a member of the ten fundamental cloud types (or cloud genera) and are wispy white high-altitude cloud formations occurring between about 5 to 13km (16,600 to 40,000ft). In fact, they are the highest of the main cloud genera, popularly known as 'mares' tails they may even form in the upper troposphere.