The unusual cloud formation that occurred Friday, December 10th, 2010, stirred quite a bit of discussion nationwide on social and broadcast media.
The basic formation was a rotor cloud in the lee (downwind) side of the Wet Mountains, caused by air flow over the mountains. The amazing corkscrew appearance was a manifestation of Kelvin-Helmholtz waves named after Lord Kelvin (1824-1907) and Hermann von Helmholtz (1821-1894).
The rolling eddies seen at the top of the rotor cloud were evenly spaced, which is typical of KH billows. This cloud formation is an indicator of atmospheric instability and the presence of turbulence. When two different layers of air are moving at different speeds in the atmosphere, a wave structure can form. The upper layer of air is moving at higher speeds and can form the top of the cloud layer into these wave-like, rolling structures. They can be seen if other conditions are just right. If there is a marked difference in densities of the air, such as with a temperature inversion, the mild, strong winds aloft blowing over cold air can develop the wave pattern at the top of the cold, relatively still air.
looking southwest - courtesy K. Torgerson
As the rotor cloud and KH waves continued to evolve, the waves in the cloud broke, similar to waves on an ocean beach or lakeshore.
looking southwest - courtesy K. Torgerson
Other images submitted to WFO Pueblo were from Canon City looking southeast...
courtesy Judith Sanchez
Shortly after the rotor cloud dissipated, strong westerly winds came to the ground in Pueblo County with winds gusting around 40 mph at WFO Pueblo.
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In June of 2007, the author took a photo of a wave cloud feature in the lee of Cheyenne Mountain over southwest Colorado Springs. It was around sunrise, with a cool, still air mass at and near ground level, and mild air aloft with strong westerly winds. This feature, shown below, is more subtle than the images above, but still very interesting to see.
Tom Magnuson, WCM, WFO Pueblo (updated 12/15/2010 at 2:40 p.m.)
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