On March 17, 2004, Russell from New York contacted me about calculating the miles-per-hour equivalent of Mach 0.91 which was the maximum velocity of the original Cessna Citation X aircraft. Mach numbers, which indicate velocity relative to the speed of sound, vary greatly with temperature (and therefore with altitude) but only minimally with humidity. The following links may be useful:

- Sound
of Speed in Air — the formula
v sound in air = 331.4 + 0.6T
_{c}m/s (where T_{c}is the temperature in degrees Celsius and m/s is velocity in meters per second) provides a good approximation. - Speed of Sound at nasa.gov — includes a Java applet for calculating the speed of sound at various altitudes.
- Speed of Sound, Mach Number, and the Sound Barrier at Aerospaceweb.org
- Mach vs. Altitude Tables at Aerospaceweb.org
- Measuring the Speed of Sound — Relative humidity does have an effect, but it is minimal.