Clouds are continually changing and appear in an infinite variety of forms. This is a guide to the 10 main named groups of clouds.
Clouds are continually changing and appear in an infinite variety of forms. The classification of clouds is based on a book written by Luke Howard, a London pharmacist and amateur meteorologist, in 1803. His book, The Modifications of Clouds, named the various cloud structures he had studied. The terms he used were readily accepted by the meteorological community and are still used across the world today.
The World Meteorological Organization (WMO) has extended Luke Howard's classifications to make 10 main groups of clouds, called genera. These are divided into three levels - cloud low (CL), cloud medium (CM) and cloud high (CH) - according to the part of the atmosphere in which they are usually found.
|Cloud level (ft)||Cloud type|
High clouds (CH)
Base usually 20,000 ft or above, over British Isles
Medium clouds (CM)
Base usually between 6,500 and 20,000 ft over British Isles.
Low clouds (CL)
Base usually below 6,500 ft over British Isles.
The many possibly variations in the shape of clouds and differences in their internal structure have led to the subdivision of most of the cloud genera into species. For a more detailed guide on cloud spotting including species and cloud codes, view our Cloud types for observers (PDF, 4 MB) guide.
Stratus/strato = flat/layered and smooth
Cumulus/cumulo = heaped up/puffy, like cauliflower
Cirrus/cirro = High up/wispy
Alto = Medium level
Nimbus/Nimbo = Rain-bearing cloud
Last updated: 3 October 2013