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?

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. Within 20 minutes, the storm continued to approach. The ragged shelf structures has the same tilt, and although it is a bit easier to identify, there is still some question as to its nature.

Another important clue is to discern whether or not the cloud element in question remains in one spot relative to the precipitation area, or moves away from the precipitation. When it moves away, as this cloud has, it signifies "push" and shelf cloud-producing outflow. Observe carefully, for there are signatures of strong outflow winds (a steep-sloped rain foot and a small gustnado). The storm was producing 70 MPH winds at this time.

introduction

Terms for using data resources. CD-ROM available. Credits and Acknowledgments for WW2010. Department of Atmospheric Sciences (DAS) at the University of Illinois at Urbana-Champaign.

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