mach (number)

[Etymology: E. Mach; Austria 1838-1916] rheology. Symbol Ma. Relating to momentum transport, the dimensionless ratio of the speed of the object to the speed of sound, in the same situation and conditions.International Standards Association
[ ISO 31-12:1992 Quantities and Units: Characteristic Numbers]
[Mills I., Cvitas T., Homan K., Kuchitsu K. Quantities, Units and Symbols in Physical Chemistry, 2nd edn (Oxford: Blackwell, 1993)] Values less than 1 are termed ‘subsonic’ and those greater than 1 ‘supersonic’ or, if significantly higher, ‘hypersonic’.

Since the speed of sound varies considerably with both pressure and volumic mass of a medium, translation between Mach number and more familiar expressions of speed is local in context. In the atmosphere the speed of sound, and hence the meaning of ‘Mach 1’, varies from about 340 m·s^-1 (1 115 ft s^-1, 760 m.p.h.) in air at 20°C near sea level to about 290 m·s^-1 (950 ft s^-1, 650 m.p.h.) at an altitude of 11 000 m (35 000 ft).