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A comment on Boeing 787 Reaches 801 MPH as a Jet Stream Packs Record-Breaking Speeds. Parent comment

Re:Boeing 787 Reaches 801 MPH as a Jet Stream Packs Record-B... by cyberferret on Tuesday February 19, 2019 @04:50PM (#19203196)
Yes, for those readers who may not be familiar - the speed of sound changes with altitude. At sea level, it is around 750mph but at 40,000 feet it is around 620mph IIRC. It is affected by the density of the air (which is a combination of the air pressure and temperature) - hence why they use Mach numbers to denote airspeed above certain altitudes as that takes into consideration the air density to give you a speed relative to the speed of sound at that altitude.
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Re:Boeing 787 Reaches 801 MPH as a Jet Stream Packs Record-B... by theothermkn on Tuesday February 19, 2019 @05:15PM (#19203407)
> [The speed of sound] is affected by the density of the air (which is a combination of the air pressure and temperature)...

This is incorrect. The speed of sound in an ideal gas is a function of temperature only, and not its density. Namely, it is sqrt(gamma x R x T), where R and gamma are unchanging properties of the gas.

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Re:Boeing 787 Reaches 801 MPH as a Jet Stream Packs Record-B... by cyberferret on Wednesday February 20, 2019 @01:24AM (#19206027)
Thank you. I stand corrected.
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