The supersonic passage of an airplane causes shock waves where the air is abruptly pushed aside to allow the passage of the airplane.  The pressure and temperature of air passing through a shock wave is abruptly increased and the air velocity is decreased.  Where the air has room to expand, there are expansion waves.  The characteristics of expansion waves are opposite to shock waves.  The pressure and temperature of air passing through an expansion wave is abruptly decreased and the air velocity is increased.  The diagram of a double-wedge airfoil (Fig. 1) portrays a supersonic wave pattern at an angle of attack.  The solid lines are shock waves and the dashed lines are expansion waves.  The pressure on top of the airfoil, behind the leading edge expansion wave is decreased.  On the bottom, behind the leading edge shock wave, the air undergoes high compression, which increases the pressure.  The pressure along the top and bottom of the airfoil is plotted in Fig. 2.

There are many shock waves from a supersonic airplane, but they usually interact and coalesce into two main shocks – one from the front and the other from the rear of the airplane.  They gradually diverge due to a slight difference in propagation speed and usually an observer hears a double sonic boom.