How do clouds like this form (see Photo, above)?
This cloud form is known as mamma (the Latin word for 鈥渂reast鈥� or 鈥渦dder鈥�) and is technically described as a 鈥渟upplementary cloud form鈥�. It is created when downdraughts bring cold air from higher levels, causing the air to reach its dew point and condense into cloud droplets. Compensating warm air rises between the individual pouches of falling air.
Mamma can form beneath various cloud types, including cirrus, cirrocumulus, altocumulus, altostratus and stratocumulus, where they often appear irregular in shape. However, beneath the overhanging 鈥渁nvils鈥� of cumulonimbus, where heat has been lost to the atmosphere from the top of the anvil, they are often sharply defined pouches, as shown here. Mamma sometimes take the form of long contorted tubes that resemble the intertwined trunks of elephants.
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Storm Dunlop, Visiting research fellow, Sussex University, Brighton, UK
The pendulous features at the base of the cloud appear to be mamma (also known as mammatus or mammatocumulus), and are probably on the base of a cumulonimbus or storm cloud. Mamma occur when the upper parts of the cloud radiate heat into the atmosphere, cool and sink. If the sinking air is relatively warm and humid, the water vapour it contains will condense into cloud droplets as it mixes with colder, drier air beneath the cloud.
The process is an upside-down version of the way cumulus clouds form 鈥� the air associated with the latter warms at ground level and rises, its water vapour condensing to form clouds. Mamma air in the troposphere cools and sinks to form the clouds. Mamma attached to a cumulonimbus are associated with severe weather conditions, and aviators are strongly advised to avoid them.
鈥淢amma are associated with severe weather, and aviators are strongly advised to avoid them鈥�
A good summary of this phenomenon can be found in Gavin Pretor-Pinney鈥檚 The Cloudspotter鈥檚 Guide (Sceptre, 2006).
Ed Hutchinson, Cambridge, UK