The ratio of the speed of a body or flow of a fluid to the local speed of sound in the fluid.
If the Mach number is less than 1, the flow is called subsonic and local disturbances can propagate ahead of the flow. If the Mach number is greater than 1, the flow is called supersonic and disturbances cannot propagate ahead of the flow with the result that shock waves form.
In the atmosphere, the speed of sound varies with temperature and atmospheric pressure, hence, so does mach number.
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