Wind shear

ind shear is a difference in wind speed or direction between two points in the atmosphere. Depending whether the two points are at different altitudes or at geographically different locations, shear can be either vertical or horizontal.

Wind shear can affect aircraft airspeed during take off and landing in disastrous ways. An explanation in regard to vertical wind shear can be seen here: (wind gradient). It is also a key factor governing the severity of thunderstorms. An additional hazard is turbulence often associated with wind shear. Wind shear also generally inhibits tropical cyclone development.

Weather situations when shear is observed are:

Weather fronts. Significant shear is observed, when the temperature difference across the front is 5 °C or more, and the front moves at 15 kt or faster. Remember that fronts are three-dimensional creatures; thus frontal shear can be observed at any altitude between surface and tropopause.
Obstacles to flow. When wind above a mountain blows from the direction of the mountain, vertical shear is observed on the lee side.
Inversions. When on a clear and calm night, a radiation inversion is formed near the ground, the friction does not affect wind above the inversion top. Change in wind can be 90 degrees in direction and 40 kt in speed. Even a nocturnal low level jet can sometimes be observed. Density difference causes additional problems to aviation.
Downbursts.