There are various kinds of cloud, and they were originally classified according to shape (Note 8.F). Luke Howard was the first to do this, in 1803, recognising three main groups—streaks, sheets and heaps. Streak clouds were called cirrus (which is Latin for 'hair'), sheet clouds were designated stratus ('layer') and a heap cloud is called cumulus ('pile'). Layered clouds are called stratiform, whereas billowing clouds are cumuliform.
The modern International System of cloud classification adopted by the World Meteorological Organisation (WMO) in 1956, refers first to the cloud's altitude, and then to the descriptive terminology of Howard (Table 8.2). There is a separate class for clouds of great vertical extent. All the various kinds are illustrated in Figure 8.4 and typical values of water content, thickness and equivalent water depth are summarized in Table 8.3.
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