Not Surfable: Kelvin-Helmholtz Waves Manifest In Alabaman Sky

Seen here is an image taken yesterday by a hillbilly Alabama resident of a fairly intimidating meteorological phenomenon. The waves you see are referred to as Kelvin-Helmholtz waves, and are seen in various fluid situations. Basically they occur when velocity shear is created between two separate fluids. Shear can be described as putting a force at the top of an object to one direction while also applying a force at the bottom of the object in the other direction. Usually the object would spin, but if it's immobile this is a shear force that could tear the object. A common example would be wind blowing against the flow of a stream. Right at the surface these Kelvin-Helmholtz waves would form within the water and the air. You generally can't see them because the two fluids are clear or homogenous in color. What we have here is a layer of fog that had settled in during the morning and a cold front that came in high over a ridge. The cold air was low pressure and pulled the fog up but it was also moving quickly to pull the streams of fog with it. Cool huh? This sort of stuff always makes me think of what ancient peoples would have thought when they saw this stuff in the distance. "We have angered the fog god...quick sacrifice a goat!"

Image: (Kelso, Lim & Perry: JFM, Vol. 306)

Sources...
Giant Tsunami-Shape Clouds Roll Across Alabama Sky [LiveScience]
Jet In A Cross-Flow [eFluids Image Gallery]