Wind shear is the difference in speed or direction between two layers of air in the atmosphere. Wind shear may occur in either a vertical or horizontal orientation. An example of the former situation is the case in which one layer of air in the atmosphere is traveling from the west at a speed of 31 mph (50 kph) while a second layer above it is traveling in the same direction at a speed of 6.2 mph (10 kph). The friction that occurs at the boundary of these two air currents is a manifestation of wind shear.
An example of horizontal wind shear occurs in the jet stream where one section of air moves more rapidly than other sections on either side of it. In this case, the wind shear line lies at the same altitude as various currents
in the jet stream, but at different horizontal distances from the jet stream's center.
Wind shear is a crucial factor in the development of other atmospheric phenomena. For example, as the difference between adjacent wind currents increases, the wind shear also increases. At some point, the boundary between currents may break apart and for eddies that can develop into clear air turbulence or, in more drastic circumstances, tornadoes and other violent storms.
Wind shear has been implicated in a number of disasters resulting in property damage and/or loss of human life. The phenomenon is known as a microburst, a strong localization down draft (down burst) which, when it when reaches the ground, continues as an expanding outflow. For example, it is associated with the movement of two streams of air at high rates of speed in opposite directions. An airplane that attempts to fly through a microburst passes through the wind shear at the boundary of these two air streams. The plane feels, in rapid succession, an additional lift from headwinds and then a sudden loss of lift from tailwinds. In such a case, a pilot may not be able to maintain control of the aircraft in time to prevent a crash.