Shadow patterns?

[2motheus]

Here's a question for all you experts - one whose answer I know will be "it depends," but maybe there is a general pattern.

I would like to know if the shadows from drifting clouds create a predictable cycle of thermals on the ground. Here in Michigan we have some perfect soaring days that include small puffy clouds drifting along at +/- 10 miles per hour.

My question is if lower down it's worth trying to find lift in a shadow, in the sunshine ahead of an approaching shadow, or in the sunshine just behind one?

I have some theories, but I would like to hear first from others.

Tim

[jkettu]

I believe the shadow could act as a thermal trigger. In other words, if there is a bubble of warm air that is still attached to the ground, the colder air produced by the shadow can wedge it free and there you have your thermal. Of course, when free the bubble will start to drift with prevailing wind and it will also take some time to rise to the altitude you are flying at. I would try crossing the leading edge of the shadow from sunny side into the shadow. As always with thermals, there is no guarantee that one will be found this way.

[2motheus]

jkettu, thanks for the input.

Your explanation agrees with something I have seen at least once, but this was a band of clouds. There was strong lift on the leading edge of the advancing shade line. As I recall, the lift did not require circling tightly; I assumed it extended all along the front of the clouds.

One thing I don't know is if air temperature is affected directly by sunlight passing through it, or if all temperature differentials are caused by ground-generated heat. Knowing this might help if you're high overhead and can't see altitude changes easily, but if you can tell if your plane is in sunshine or shadow.

Tim

[Andy W]

http://radiocarbonart.com/Pages/asecthermalmain.html
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