Friday, July 23, 2010
Strange Clouds Explained!
Since last night I have gotten nearly a dozen emails from people in Seattle who reported seeing "weird", "strange", and even "scary" clouds. Above and below are nice examples of the view that many Seattle residents had Thursday evening in the hours around sunset.
Some thought the clouds looked like question marks, but in my mind they resemble breaking waves on a beach, and as we will see they have a lot in common with the seashore phenomenon.
I can tell you exactly what you are seeing--Kelvin Helmholtz instability clouds.
Now that will impress your friends!
These clouds form when there is a large change in wind with height...we call this wind shear in the business. When the wind shear builds up sufficiently an instability forms in which wavelike disturbances are created that can amplify like a breaking wave on the beach. And like the breaking wave there is often a lot of turbulent motion.
Yesterday this was a large change in wind direction centered around 1300 meters (roughly 4300 feet). At low levels there was southerly flow and above northerly flow.
Want to see it? Here is the output from vertical wind profiler at Sand Point in Seattle. Height in meters is on the y axis and time on the x axis. The symbols are the typical meteorological wind barbs. The time is in Greenwich Mean Time, so look at around 23/03 GMT...that is around 8 PM. You see the big shift in wind direction between 1000 and 1500 meters?
Another way to see the big change of wind direction with height is from the time lapse video taken from my department yesterday. You can find it
Wait until near sunset and you can see low level clouds going one direction and higher level clouds in another...lots of shear! And if you look carefully in the lower left you will see the Kelvin Helmholtz clouds form!
Or perhaps you would like to view a computer simulation of the phenomenon:
As noted earlier, these clouds are often associated with substantial turbulence--if you fly into one you can really have a rough ride. I remember a few years ago I saw such clouds ahead of me on a flight. I tightened my seat belt and told the person next to me that they she should be prepared for turbulence. She laughed. Well, people often laugh at weathermen. We are used to it. Then we hit it. Drinks went flying. The seat belt sign went on. She wasn't laughing anymore!