E. Linacre and B. Geerts
Windshear is a measure of the variation of wind speed & direction with height, or horizontal distance. Mathematically, it is the difference between two wind vectors. Meteorologists usually refer to variations of horizontal wind with height, whereas pilots understand windshear as a variation of wind from one place to the next, as in a microburst.
The large-scale windshear can be largely explained by thermal wind balance (Note 12.F). Wind shear is important in the development of thunderstorms. In a more sheared environment, thunderstorms tend to be more organized and severe. A detailed explanation of this can be found in a series of learning modules on deep convection for forecasters, by COMET.
Wind shear due to different wind speeds at various levels is important in the growth of a tornado. The shear causes a horizontal roll of the lowest 2 km of the troposphere. Then the updraught within a storm tilts the roll, so that there is a component of rotation about a vertical axis, forming a ‘meso-cyclone’ of 5 - 10 km diameter, at an elevation of 5 - 6 km. Vertical stretching due to inflow within the mesocyclone increases the speed of rotation, though still not to tornado speed.
A ‘rear-flank downdraught’ is created by evaporative cooling of precipitation on the edge of the meso-cyclone. This downdraught leads to a ‘gust front’, which in turn causes horizontal wind shear. The convectively-generated wind shear may in turn be tilted and stretched, and eventually produce a tornado.