Originally Posted by Mad (Flt) Scientist
Ultimately, the speed of sound in air depends on the air temperature only.
Therefore, you can have the speed of sound be different for the same density and pressure, as long as the temperature is different. Density and pressure determine flight level and TAS-to-IAS, so you'd be at the same FL, the same IAS, the same TAS, but a different Mach number.
In a single volume of gas, of course, I can't change JUST T and not p and V. But we're talking about a case of moving from one section of the atmosphere to another, so the gas law relationship doesn't have to hold.
Er, why does the gas law not hold in atmosphere?