JIM REED PHOTOGRAPHY / SCIENCE PHOTO LIBRARY JIM REED PHOTOGRAPHY / SCIENCE PHOTO LIBRARY
Mammatus clouds. These pouch-shaped clouds usually form on the underside of large cumulonimbus thunderclouds. A well-developed mammatus formation may be a sign that a tornado is developing. The mammatus forms on the underside of the anvil of a thunderstorm, so can be quite distant from the centre of the storm. Photographed in New Mexico, USA.
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