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9 scary images of shelf clouds

Shelf cloud over Wichita, Kansas

Photo: KSN-TV/National Weather Service

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Wichita, Kan.

This shelf cloud was part of a thunderstorm system that formed over western Kansas on May 6, 2008, gaining strength as it found more low-level moisture to the east, according to a National Weather Service storm summary.

The system eventually became a "bow echo," in which several storms unify into a squall line that resembles an archer's bow in overhead radar images. Shelf clouds often develop along bow echos, which sometimes generate dangerous winds like derechos and gustnados.

This NWS diagram shows how a bow echo forms: