Fascinating Clouds over Chicago: Undulatus Asperatus Blanket City

Undulatus Asperatus as seen over Chicago, Illinois on July 1, 2011. Credit: Diana Kost

A dramatic display of a relatively rare cloud type blanketed the city of Chicago on Friday afternoon, drawing the attention of millions of residents.

Undulatus Asperatus as seen over Chicago, Illinois on July 1, 2011. Credit: Jough Dempsey

The clouds are frequently described as how the surface of a body of water looks from the viewpoint of a swimmer beneath the surface. These clouds have recently been deemed undulatus asperatus by meteorologists.  The Latin term translates loosely as “turbulent undulation.”

Undulatus Asperatus as seen over Chicago, Illinois on July 1, 2011. Credit: Lynn Reidl

Though the appearance of such turbulence frequently invokes concern that a severe storm may be approaching, these clouds actually appear to indicate the opposite: a decaying or otherwise dissipating thunderstorm complex. As shown on the satellite photograph below, a large thunderstorm complex had formed over southern Lake Michigan and was then dissipating and moving away from Chicago at the time these clouds were spotted.

Visible satellite image showing a decaying thunderstorm complex east of Chicago on July 1, 2011.

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