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Modified 9-Jul-14
Created 9-Jul-14
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A classic view of the waves of fog, cresting over Windy Hill just after the sun set on us. The sun still lights the fog. PLUS an encounter wih a coyote in the early morning -- pretty much of a yawn, but interesting to me and the dogs. A buck in velvet. Lifting morning fog.
Here is an explanation of the fog waves. They are a Kelvin-Helmholtz instability!
"Thanks for the stunning photos, Dan.
"Danna: I’m not a weather expert, but, perhaps, more relevantly, my academic training is in instabilities in fluid mechanics. This is a beautiful example of a Kelvin-Helmholtz instability. The conditions required are two layers of fluid, one more dense than the other with the more dense one on the bottom (stable configuration), and the two layers moving at different speeds. As the fluid on top moves across the one on the bottom, some of the lower fluid is entrained and lifted up just a bit. Then, the upper fluid has to move a little faster over that bump which makes the pressure lower in that spot (like an airplane wing). That encourages the bump to grow further upwards in a positive feedback loop. Once the peaks start to get pushed over by the moving upper fluid, they move downward due to gravity (because that fluid was heavier) making the waves. The peaks form at regular intervals, because once one nascent peak forms, the pressure is slightly higher in the neighboring regions and that is enough to prevent any other peaks from forming nearby.
"The phenomenon actually occurs frequently in the atmosphere but is rarely observed because clouds (or fog) have to be in the right place to make it visible and the waves are short-lived. I’ve seen the Kelvin-Helmholtz instability in clouds a few times in my life – alas, I never got a photo.
"If you search on “Kelvin-Helmholtz” in Google images, you’ll see other good pictures of these waves in clouds/fog. I was particularly entranced with the giant ones in Alabama in 2011 – taller than a multi-story building!
"This is my favorite movie of the laboratory version of the phenomenon. Just amazing.
<https://www.youtube.com/watch?v=UbAfvcaYr00>
- Anne Kopf-Sill, PV Forum, 7/2014
The Kelvin-Helmholtz instability is also responsible for ocean waves.