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