this post was submitted on 21 Dec 2023
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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)?

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[–] [email protected] 44 points 11 months ago

Yes, because its calculation relies on the local speed of sound, which is varies based on several factors, including altitude.

https://en.wikipedia.org/wiki/Mach_number

[–] [email protected] 22 points 11 months ago (1 children)

Yes a fair amount. At ground is about 760mph at 40000 feet about 659.

Temp and humidity factor some as well.

[–] [email protected] 6 points 11 months ago (2 children)

Interesting. So orbital Mach is like what?

[–] [email protected] 7 points 11 months ago* (last edited 11 months ago)

Technically it would reach an altitude where it would become 'null' due to dividing by zero. You would eventually hit the vacuum of space where there is no speed of sound and any speed is faster than it.

[–] [email protected] 5 points 11 months ago* (last edited 11 months ago) (1 children)
[–] [email protected] 1 points 11 months ago (1 children)

So what's Mach in outer space

[–] [email protected] 2 points 11 months ago

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

[–] [email protected] 22 points 11 months ago* (last edited 11 months ago)

Mach number depends on the (local) speed of sound, which depends on the temperature of the air - therefore on the altitude.

[–] [email protected] 13 points 11 months ago

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.)

[–] [email protected] 9 points 11 months ago

@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?