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Shelf Clouds

23 May 2016, 07:01 UTC
Shelf Clouds
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The slideshow above presents 3 images of an approaching shelf cloud. A shelf cloud is a low, horizontal, rolling cloud that is “attached” to the base of a parent cloud, usually a thunderstorm. It requires strong convection (updrafts and downdrafts) and warm humid air ahead of the storm. Click on the thumbnail below for a simplified graphic of the dynamics.
Horizontal rotation can often be seen in the leading part of the shelf cloud, while the underside often appears turbulent and wind-torn. The rotation is caused by wind shear between the downdraft and updraft. Cool air from a storm cloud’s downdraft spreads out across the ground, creating a gust front. This outflow cuts under warm air being drawn into the storm’s updraft. As the lower cooler air lifts the warm humid air, its water condenses, creating a cloud which often rolls from the differential winds above and below.
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