Wind shear
Wind shear is a difference in relative speed between two adjacent air masses. An excessive wind shear produces vortices, and when the wind shear is of sufficient degree, the air will tend to move chaotically. As is explained elsewhere in this article, temperature decreases and wind velocity increase with height in the troposphere, and the reverse is true within the stratosphere. These differences cause changes in air density, and hence viscosity. The viscosity of the air thus presents both inertias and accelerations which cannot be determined in advance.