Since the speed of sound depends on the air, which changes at different altitudes, are “Mach” speeds dependent on the altitude? Or is it basically just like 100 °C is the same even at altitudes where water boils at a lower temperature (in that it is a fixed speed based on the speed of sound at sea level)?
That’s where speed is relative. But if you had to use a Mach number, it’s be at sea level or standard temp/pressure