What Is a "Mach Number"?

The speed of sound depends on temperature--and therefore altitude. We use Mach numbers to describe an airplane's speed in terms of the speed of sound. An airplane moving at Mach 1 is traveling at the speed of sound. Mach 0.5 is half the speed of sound, Mach 2 is twice the speed of sound, and so on.

(Rev. 09/17/96)