Shelf Cloud
slopes downward and away from the rain area

This is the first of four photographs of an approaching thunderstorm to help visualize the difference between gust front outflow "push" and wall cloud inflow "pull." To the distant west-southwest, note the suspicious cloud lowering at the south flank of an isolated severe thunderstorm. Is it a wall cloud or a portion of a shelf cloud?

Photograph by: Doswell

A subtle, but important clue is that the lowering slopes downward away from the rain area, rather than into the rain. This is the slope that a shelf cloud usually takes. As cold air is "pushed" out of the precipitation area by the downdraft, warm air slides up and over the gust front forming the concave-shaped shelf cloud.

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Credits and Acknowledgments for WW2010.
Department of Atmospheric Sciences (DAS) at
the University of Illinois at Urbana-Champaign.