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Named from the Latin word for “pouchy”, Mammatus clouds are often confused as being harbingers of tornadoes.
These are high-level clouds which appear on the underside of thunderstorm anvils and are above 20,000 feet, so they’re not going to produce a twister. Tornadoes generally form from a very low cloud known as a wall cloud.
Pilots avoid Mammatus because they do indicate severe turbulence within the top of the storm. This is one of the only known clouds that develop from sinking air. The prevailing theory is that pockets of intensely cold air descend from the anvil, and as they make contact with the moist environment around the storm they produce condensation.
Since the air sinks in an irregular pattern, the familiar tendrils or “udders” of the cloud take shape and it looks like the underside of a huge cow in the sky!
While Mammatus may at time be associated with tornado-producing storms, they are a mighty poor substitute for Doppler radar and storm spotters. Many storms which produce these clouds never spin up a tornado.