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Category: Archive 2011

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1. Jr Team Selection Notice
2. Ground Inversions
3. W + O Spring meet
4. We have you covered

As I suspected -- not much knowledge among us about temperature inversions. But, the more I dig into it the more I realize that inversions occur almost every day and they are key to lift, wind velocity, direction and turbulence. The best reference I have found is the book "Air Rider's Weather" by Alan Watts, copyright 1992. It's specially written for pilots of paragliders, hang gliders, sailplanes and balloons. It can be bought used on the Internet for about $10. I got mine from Half Price Books for $4.98.   Some excerpts:

"When an overnight inversion exists, as it can right up to the middle of the morning, only turbulent eddies can mix up the deck below the inversion layer, and so the wind remains light. Yet as soon as the sun can heat the ground enough to induce strong thermals these punch through the inversion and suddenly you feel the wind increase. Very often it will shift its direction clockwise (veer) at the same time. So, the usual way the surface wind behaves on a good flying morning is to be light and fairly constant in direction up to the time when the inversion breaks, and from then on to increase. This increase will normally continue right through lunch-time into the afternoon. This is because, as the day progresses, deeper mixing hauls down faster wind from the top of the mixed layer."

"Before the layer breaks, thermal currents are inhibited and the wind above is cut off from the wind below."

"Wind statistics for the top of the Eiffel Tower prove that when the surface wind goes down, the wind at 1000' goes up and vice versa."

"As the sun sinks, the depth of the air-deck under the inversion will increase. As it does the surface wind has only its own momentum to sustain it and deprived of the help of faster wind from above, it dies."

" If inversions exist not far from the ground, as they can in the early evening when stability is setting in, the angle between the ground wind and that at 200' - 300'  may be as high as 45 degrees."

So, where's the support for the model? My take is that in the morning it is thermals from the rising sun until the temperature inversion is no more, about 10 am. In the evening, as inversions are forming, I think its not thermal activity but light laminar wind conditions that provide laminar flow over the model wings for greater lift than experienced in the turbulent air during the day.

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