Well, this is an eyeopener. I always figured it was the altitude related density. And here it's the temp. Learn something new every day......

Speaking of U-2's. One of my favourite story lines about these remarkable planes is the condition that determines the plane's max altitude.

One of the books I read described how the plane could climb until the rising stall speed related to the altitude density boxed the plane's speed up against the Mach 1 barrier. They would climb until the stall to supersonic range was somewhere around 10 knots or so and at that point the auto pilot's most crititical function was to maintain the speed. In a turn, which at that height were all of a HUGE radius, any signs of buffeting could be either the faster outside wing going supersonic or the slower inner wing reaching the stall speed. The reaction was to very slightly dive the plane as it was better to go farther into the supersonic region than risk a tip stall on the inside wing. If the buffet kept up or worsened then the plane was gently coaxed to nose up to a slightly slower speed to reduce the outer tip below the critical Mach number. Making the wrong move and stalling the inner panel could quickly result in a spin that could only be recovered when the plane had reached some decent air many thousands of feet lower. I seem to recall that such a spin would flame out the engine at the same time. All in all not a nice situation to be in.

Heady stuff indeed to us ground bound folks.