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An F/A-18 Hornet creating a vapor cone at transonic speed just before reaching the speed of sound

A mach number is a number that describes speed. It is a measure of the speed of an object compared to the speed of sound. Mach number is given as a percentage compared to the local speed of sound. For example, half the speed of sound = Mach 0.5.

\displaystyle{ M=v/a }

Where is the speed of the object, and a is the speed of sound.

\displaystyle{ \mathrm{M} = \frac{u}{c}, }