17 December 2011

Magic Clouds In The Magic City!

Posted by Dan Satterfield


These are Kelvin-Helmholtz waves over Birmingham, Al. on Friday. Image ctsy. James Spann's Alabama Weather Blog.

Birmingham, Alabama is known as the magic city, and they had a magical cloud display yesterday to go along with the name! I have mentioned Kelvin-Helmholtz waves before on this blog, but there was an AMAZING display of them yesterday in Birmingham. I’m talking a jaw dropping display.

These clouds look like sculpted waves in the ocean and are caused by strong wind shear (especially a change in velocity) over a thin layer in the atmosphere. In this case you had light winds near the surface, but the winds were much stronger a few thousand feet up. If there are clouds present they will flow with the shape of the air and you can see the instability that develops. Wikipedia has a decent article on Kelvin-Helmholtz waves as well. The dynamics of these waves can get rather complex, and they are named after the two scientists who first described them in a mathematical sense.

When one layer moves past another in the atmosphere the velocity shear can cause this type of instability. The YouTube video below shows how you can create the effect in a lab. Sci-Quest Science museum (and many others I’m sure) in Huntsville, AL. has an exhibit that makes great Helmholtz waves, and I always play with it when there!

James Spann the meteorologist for the ABC affiliate in Birmingham has more amazing pics of the clouds on his Alabama weather blog as well. The Birmingham NWS office sends up two RAOB (weather balloon) soundings a day from the Alabaster area (just south of the city), and the data from 6am CST yesterday called up the data from the RAOB balloon launch at 6am yesterday tells the story (see below):


RAOB sounding from BHM at 12Z 16 Dec. 2011 (6am CST). Notice, the atmosphere is saturated in the low levels and there is a noticeable increase in wind from the surface to just a few thousand feet.

Was Van Gogh inspired to paint starry night from a display of Kelvin-Helmholtz clouds??

The winds were around 10 knots at the surface but just a few hundred feet higher the RAOB balloon measures a 25 knot wind. A cool front had passed a few hours before and the skies were beginning to clear behind it, but the atmosphere was still saturated from the surface to 700 mb (about 3km in height).  It’s amazing what the wind looks like when the clouds are present to show us the air flow!

These type of clouds are seen rather more frequently in the San Francisco area, and many art experts have long suspected that the impressionist masterpiece “Starry Night” was at least partially inspired by a display like this witnessed by Vincent Van Gogh.