Saturday, October 29, 2005


I picked up a book called Meteorology for Glider Pilots. It's old - 1961 - but it looks really good. The author seems to have been a British professional meteorologist and a glider pilot. I'm about halfway through it. The first several chapters are general meteorology, fairly technical. The latter part is how it applies to soaring. Here's the one thing that has piqued my interest so far.

There's a section on katabatic and anabatic flow. Katabatic is downward flow of cool air through a valley... not terribly useful to the glider pilot but something to be aware of. (See a description of katabatic wind in on glaciers in the popular novel Deception Point by Dan Brown.) Anabatic flow is upward flow of heated air along a slope. If a whole slope of a hill or mountain gets heated, the whole lower layer of air along the slope can "slide" up the hill. Not vertically away from the slope as a classic thermal, but along the slope, toward the top. (Maybe there's no "dripping point" to cause the heated air to break loose from the slope.) I don't think I've seen a description of it in any of the other books I've read. They all talk about thermals, ridge lift (orographic), wave, and sometimes shear lines.

Last year when I flew at Tehachapi with three pilots, we flew up the mountains. At the time I was a bit confused as to the sources of lift we were using. The mountains there did not form a distinct linear ridge, as the classic diagrams of orographic lift show. There were thermals that we used, and sometimes we could see the sources of them (rocky areas), but I could not really imagine a line of thermals coming off of all points of a sloping ridge. Sometimes we were flying right up the "spine" of a sloping ridge, and not in a direction that I could say was within 30 degrees of being perpendicular to the wind direction. So now I'm thinking that maybe anabatic lift was happening on the slopes. And if both sides of a ridge got heated, and anabatic flow happened on both sides, the flows could meet at the top (spine), collide, and be forced straight up. That lift could be twice as strong as each slope's anabatic flow. And unless there was a strong wind to one side, it would not have a turbulent area in the lee of the ridge: the lift is going straight up off the ridge, not orographically being forced up and over at an angle. I think this model explains our mysterious ability to fly up the spine of a ridge all the way to the top of the mountain.

I would welcome other pilots (and meteorologists') comments and experiences!


Anonymous said...

Lift is where you find it.

Bill said...

"Understanding Flying Weather" by Derek Piggott (you'll find it on Amazon) is another good introductory reference to meterology with respect to gliding. Found it very useful when I was swotting up for my Bronze C paper.