Originally Posted by
rickair7777
Gradual, and it becomes meaningless when the atmosphere becomes so thin as to have negligible affect on vehicle performance.
On re-entry, the shuttle started measuring mach at M 25. Nope, didn't forget the decimal.
So does this not happen until reaching the Karman line (300K I think)? Or is it where the practical end is, perhaps at around 100K?
I'd love to know what IAS M25 is during reentry!