Posts Tagged ‘Fluid dynamics’

Classic clouds #1 – Kelvin-Helmholtz billows

January 21, 2010

My last few posts have been about fairly meaty climate issues or on science policy.  These are important subjects but one of the things I love about atmospheric science is that there are some beautiful things in our skies.  This post is about one of my personal favourites, Kelvin-Helmholtz Billows:

These billows occur when a layer of cloud is found beneath a layer of warmer air and the two layers are flowing in different directions. When this situation occurs, the lower cloud layer is not bouyant so it can’t push up into the warm layer.  However, under the rare circumstances that the two layers are flowing just right, the interaction between the two layers will form an instability and the cloud layer will “break” into the upper layer in the wave-like pattern seen above.  These cloud formations are so rare that they rank as the highest scoring cloud in the Cloud Appreciation Society’s Cloud Collector’s Handbook.  (Confession: I have never actually seen one in the sky.)

But, I have seen one in the lab!  The amazing video below shows an experiment I did during a summer school at the University of Cambridge where the same conditions can be set up.  Enjoy.

[Thanks to Tor Smith (University of Leeds) for the footage and to the University of Cambridge Fluid Dynamics labs for the equipment.]

There are some nice observations of these cloud using the Chilbolton radar either in Chapman and Browning (1997) or on this University of Reading webpage.

UPDATE: I recently saw something that looked like KH billows in the sky where I live. They formed on an aeroplane contrail so maybe this pattern was formed more by turbulence from the plane than atmospheric shear that was already there. Either way, it made my day!

Reference:

ResearchBlogging.orgChapman, D., & Browning, K. (1997). Radar observations of wind-shear splitting within evolving atmospheric kelvin-helmholtz billows Quarterly Journal of the Royal Meteorological Society, 123 (541), 1433-1439 DOI: 10.1002/qj.49712354114