Pressure Gradient: The rate of change of pressure in an imaginary horizontal plane running between areas of high and low pressure on a weather map.
A favorable pressure gradient is required to maintain laminar flow. Laminar flow airfoils are designed to have long favorable pressure gradients. All airfoils must have adverse pressure gradients on their aft end.
As the airflow moves from the shoulder to the rear surface of the airfoil, the static-pressure gradient is unfavourable (increasing pressure with downstream distance).
Initially, each air parcel moves directly inward, in the direction of the pressure gradient, but whenever it moves it is subject to large sideways Coriolis forces, as shown in figure figure 20.2.
The large increase in drag is caused by the formation of a shock wave on the upper surface of the airfoil, which can induce flow separation and adverse pressure gradients on the aft portion of the wing.
 See also: Aircraft, Speed, Force, Wind, Wing
  
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