|A shelf cloud. Photographer: Jake Deremer.|
on November 18, 2010.
|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.
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).