Wind shear

Wind shear, also called wind shear or wind gradient, is a difference in wind speed and / or direction over a relatively short distance in the atmosphere. Wind shear can be divided into vertical (wind change over a vertical distance), horizontal (wind change over a horizontal distance) or both.

Wind shear may be associated with unstable lines and cold fronts. They are also observed near micro-explosions, mountains, buildings and wind turbines. Wind shear has a significant effect during takeoff and landing due to its effects on aircraft control, and has been a contributor to many accidents.

Pilots are trained to avoid any kind of wind shear, as several dangerous situations can occur: in a micro-explosion, the wind intensity can double in a minute or less. Winds can change and cross quickly, causing a lot of turbulence and uncontrolled airplanes.

An even bigger problem for pilots is clear sky turbulence, a turbulent wind movement caused by shear, but with no visual cues like clouds. The atmospheric region most susceptible to this type of turbulence is the high troposphere, at altitudes between 7 and 12 km. At lower altitudes, it may also occur near mountain ranges.

It is usually impossible to detect with the naked eye and very difficult to detect with conventional radar this turbulence. As a result, it is difficult for aircraft pilots to detect and avoid them.

Although altitudes near the tropopause (12km) are generally cloudless, thin Cirrus clouds can form where there are sudden changes in wind speed, for example. Cirrus lines perpendicular to the wind flow indicate possible clear sky turbulence, especially if their ends are scattered.

Wind shear can be a threat to skydivers, who can be pushed off course by sudden changes in wind direction and speed, and can collide with bridges, buildings, trees, other skydivers and the ground.

The most common weather situations where wind shear is observed include:

Fronts: Significant shear is observed when the temperature difference across the front is 5 ° C or more, and when the front moves at 55 km / h or faster. Vertical wind shear above hot fronts is more of concern for aviation than near and behind cold fronts because of its longer duration.

Mountains: When winds blow over a mountain, a vertical shear is observed. If the flow is strong enough, turbulent swirls (known as "rotors") associated with lenticular clouds can form, and they are very dangerous for aircraft to climb and descend.

Thermal inversions: When on a clear, calm night, an inversion is formed near the ground, friction does not affect the wind above the top of the inversion layer, causing a difference in wind direction below the layer from the layer above.

Micro Explosions: When a micro explosion occurs, winds near the surface change direction, pointing outward from the center of the downstream. The stronger the wind, the stronger the vertical shear of the resulting wind.

Wind shear also influences tropical cyclones. Tropical cyclones are basically "thermal motors" powered by the temperature difference between the ocean surface and the upper atmosphere. Tropical cyclone development requires low vertical wind shear so that its hot core can remain above its surface circulation center, thus promoting strengthening. Vertical wind shear "rips" the heat engine "machine" causing it to break.

Vertical wind shear is a very important factor in determining thunderstorms and their severity. Vertical shear will increase or decrease the forces of air currents, which will favor or not the occurrence of thunderstorms. Thunderstorms that occur in light vertical wind shear usually appear "flat" vertically, scattered on both sides.

These storms last for a short time because the downward airflow cuts the upward airflow from below. Cumulonimbus occurring in strong vertical wind shear are longer lasting and stronger because the two air currents (upward and downward) will not be exactly above each other, because at the top of the cloud the wind direction or speed will be different from the wind direction. or wind speed at the base of the cloud.

Severe thunderstorms, which can generate tornadoes and hail, require wind shear to organize the storm in such a way as to maintain the thunderstorm over a long period of time. Thunderstorms in an atmosphere with virtually no vertical wind shear weaken as the strongest rain stream descends, which then quickly cuts off the relatively hot air inlet and "kills" the storm.

Wind shear can also create waves. This occurs when an atmospheric inversion separates two layers with a very strong difference in wind direction. If wind encounters distortions in the inversion layer caused by bottom-up thermal currents, it will create significant shear waves, also known as Kelvin-Helmholtz undulations.