|Quoting Pilotpip (Reply 1):|
As air goes over the top of a wing, it accelerates. If you're going .80 mach, it's possible that some of that air will be going faster than mach 1.
Two technical terms must be understood :
freestream mach number : mach number of the undistrubed airflow that lies far ahead of the body that is moving through the air.
local mach number : mach number of the disturbed airflow that lies close to the body that is moving through the air.
The Mach number displayed on the flight display in the cockpit or in a sales brochure is the freestream mach number. It is the mach number of the undisturbed
airflow far ahead of the entire aircraft.
Logically, the freestream mach number of the entire aircraft is the free stream mach number of a particular segment of the aircraft, like say, the wing. As Pilotpip explained, due to conservation of mass, over parts of the upper surface of the wing, the air is moving faster than the "surrounding" freestream air. i.e. the local mach number is greater than the freestream mach number.
This is why even though the pilot reads that the airplane is flying at Mach 0.8, certain parts of the wing will most definately be travelling at speeds greater than Mach 0.8 (not necessarily supersonic).
For a simple to understand yet techinally correct explanation of why this is so, please read the following article written by my favourite professor.