**M = V/a**

**M** is the local Mach number,

**V** is the local flow velocity with respect to the boundaries (either internal, such as an object immersed in the flow, or external, like a channel),

**a** is the speed of sound in the medium, which in air varies with the square root of the thermodynamic temperature.

**Mach number** (**M** or **Ma**) is a dimensionless quantity in fluid dynamics representing the ratio of flow velocity past a boundary to the local speed of sound – WIKIPEDIA