__The Speed of Sound__

The speed of sound - referred to as "mach 1.0" is used as a reference for how fast a speedy airplane is moving. But it's a bit of a slippery metric, in terms of the aircraft’s movement through the air (true airspeed).

Oddly, the speed of sound in our atmosphere is solely a function of the temperature – and not anything else.

It is calculated very simply:

mach 1.0 (in knots) = 39 x square_root(temperature in Kelvin)

which is ridiculously simple. Kelvin is just Celcius with a bias of +273.

Examples:

+15C: 39 x sqrt(15+273) = 662 KTAS or 761 mph

+40C: 39 x sqrt(40+273) = 690 KTAS or 793 mph

0C: 39 x sqrt(273) = 644 KTAS or 741 mph

-20C: 39 x sqrt(-20 + 273) = 620 KTAS or 713 mph

-40C: 39 x sqrt(-40 + 273) = 595 KTAS or 685 mph

-60C: 39 x sqrt(-60 + 273) = 569 KTAS or 654 mph

Mach 1.0 can vary by 150 mph depending upon whether it is hot or cold out!

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Dec 2014