Does Mach speed change depending on the altitude?
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)?
Yes, because its calculation relies on the local speed of sound, which is varies based on several factors, including altitude.
Yes a fair amount. At ground is about 760mph at 40000 feet about 659.
Temp and humidity factor some as well.
Mach number depends on the (local) speed of sound, which depends on the temperature of the air - therefore on the altitude.
In essence? Yes.
Technically, it’s other factors- temperature, pressure, humidity, and whatever else it’s actually composed of. But all of those are affected by altitude.
Mostly? It’s temperature and composition that affect it (and if air was an ideal gas, pressure would have no effect. Open air is not an ideal gas, however.)
@gedaliyah yes. Mach number is defined in terms of the local speed of sound, and that varies depending on pressure and temperature. I don’t know exactly how much, but several percent at least?