Football can be dangerous...in more ways than expected!

A shelf cloud. Photographer: Jake Deremer.
 Source http://epod.usra.edu
on November 18, 2010.

A wonderful source of potential topics and images for this blog is Earth Science Picture of the Day at http://epod.usra.edu, which is where I found this great photo today.  This photo was taken at a football game between Kansas State University and the University of Central Florida on September 25, 2010 and, as far as I can tell, even though it looks somewhat like a watercolor painting, it has not been Photoshop'd!  Due to the danger of lightning and damaging winds, the football game was suspended for about 90 minutes.

from this WWW site 

This cloud formation is a so-called "shelf cloud", a not uncommon cloud in the midwest. Google it and you get a stunning array of images! It is a type of "arcus cloud", a low, horizontal cloud formation.  There are two types of arcus clouds, one associated with outflows of cold air from cold fronts and the other specifically associated with thunderstorms. The type of  cloud shown here is associated with thunderstorms.  The cartoon to the right shows the dynamics leading to the formation of the shelf cloud.  Cold air, descending from a thunderstorm on the left, sweeps out along the ground (blue arrows), displacing warm humid air toward the right.  The warm air rises and droplets of moisture nucleate to form the cloud.  It was named a shelf cloud because of the "uniform flatness of the cloud base region where moist air that usually originates from the lowest part of the boundary layer achieves homogeneous condensation (discussed in Fankhauser, et al., Bull. Amer. Meteoritical Soc., 64(5), 450-462, 1983)." In this photo you can see up under and behind the roll cloud. Note that the shelf cloud is "connected" to the thunderstorm cloud driving the system.

A roll cloud in Uraguay.  Photo by Daniela Mirner Eberl
taken in January 2009.

A bit of practical advice: A shelf cloud precedes a storm. It is generally associated with a squall line, and marks the place where strong winds begin.  Take note and take shelter!

The second type of arcus cloud is a "roll cloud".  These form when cold air from an advancing cold front advances under a warmer humid layer, typically over relatively long distances.  The humid air roles up around a horizontal axis.  These waves are a form of soliton, having a single peak and maintaining the shape as they advance.  Unlike shelf clouds, the roll clouds are not connected to another set of clouds. Note the clear air behind the cloud (to the left).