ROGER HILL / SCIENCE PHOTO LIBRARY ROGER HILL / SCIENCE PHOTO LIBRARY
Mammatus clouds over fields. These clouds were left behind after by a supercell thunderstorm. A supercell is a severe long-lived storm within which the wind speed and direction changes with height. This produces a strong rotating updraft of warm air, known as a mesocyclone, and a separate downdraft of cold air. This storm produced hail the size of softballs. Photographed near Bismark, North Dakota, USA, on 19th July 2008.
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