Mach number

If an object travels through some medium, then its Mach number is the ratio of the object's speed to the speed of sound in that medium. It is a dimensionless number, typically used to describe the speed of fast aircraft. Mach 1 is equal to the speed of sound, Mach 2 is twice that speed, etc. Since the speed of sound increases as the temperature increases, the actual speed of an aircraft traveling at Mach 1 will depend on its altitude and the atmospheric conditions. High speed flight can be classified in six general categories

Subsonic            M < 1
Sonic               M = 1
Supersonic          M > 1
Transsonic     .8 < M < 1.3
Hypersonic      5 < M < 10
Hypervelocity       M > 10 and above

Airplanes (and other objects) that move at less than Mach 1 build up sound waves in front of them. Upon reaching Mach 1, they meet the sound. It (the sound) is now known as a sound barrier. Above Mach 1, the plane must penetrate the sound barrier. When it does so, it "breaks" the barrier. Upon breaking the sound barrier, a loud booming noise, known as a sonic boom is produced. The streamlined shape makes it easier for an airplane to break the sound barrier.

Aerodynamic scientists love the Mach number because that one number generates much information like the: temperature, pressure, power, density of air.

The Mach number is named after Austrian physicist and philosopher Ernst Mach.