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