Wind shear

Wind shear, sometimes referred to as wind gradient, is a difference in wind speed and/or direction over a relatively short distance in the atmosphere. Atmospheric wind shear is normally described as either vertical or horizontal wind shear. Vertical wind shear is a change in wind speed or direction with a change in altitude. Horizontal wind shear is a change in wind speed with a change in lateral position for a given altitude.
Wind shear is a microscale meteorological phenomenon occurring over a very small distance, but it can be associated with mesoscale or synoptic scale weather features such as squall lines and cold fronts. It is commonly observed near microbursts and downbursts caused by thunderstorms, fronts, areas of locally higher low-level winds referred to as low-level jets, near mountains, radiation inversions that occur due to clear skies and calm winds, buildings, wind turbines, and sailboats. Wind shear has significant effects on the control of an aircraft, and it has been the only or a contributing cause of many aircraft accidents.
Sound movement through the atmosphere is affected by wind shear, which can bend the wave front, causing sounds to be heard where they normally would not. Strong vertical wind shear within the troposphere also inhibits tropical cyclone development but helps to organize individual thunderstorms into longer life cycles which can then produce severe weather. The thermal wind concept explains how differences in wind speed at different heights are dependent on horizontal temperature differences and explains the existence of the jet stream.
File:Downdraft wind shear clouds illuminate at twilight over the Mojave Desert.jpg|thumb|right|Down draft winds with associated virga allow these clouds in the eastern sky at civil twilight to mimic aurora borealis in the Mojave Desert.

Definition

Wind shear refers to the variation of wind velocity over either horizontal or vertical distances. Airplane pilots generally regard significant wind shear to be a horizontal change in airspeed of for light aircraft, and near for airliners at flight altitude. Vertical speed changes greater than also qualify as significant wind shear for aircraft. Low-level wind shear can affect aircraft airspeed during takeoff and landing in disastrous ways, and airliner pilots are trained to avoid all microburst wind shear. The rationale for this additional caution includes:

microburst intensity can double in a minute or less,
the winds can shift to excessive crosswinds,
is the threshold for survivability at some stages of low-altitude operations, and
several of the historical wind shear accidents involved microbursts.

Wind shear is also a key factor in the formation of severe thunderstorms. The additional hazard of turbulence is often associated with wind shear.

Occurrence

Weather situations where shear is observed include:

Weather fronts. Significant shear is observed when the temperature difference across the front is or more, and the front moves at or faster. Because fronts are three-dimensional phenomena, frontal shear can be observed at any altitude between surface and tropopause, and can therefore be seen both horizontally and vertically. Vertical wind shear above warm fronts is more of an aviation concern than near and behind cold fronts due to their greater duration.
Upper-level jet streams. Associated with upper-level jet streams is a phenomenon known as clear air turbulence, caused by vertical and horizontal wind shear connected to the wind gradient at the edge of the jet streams. The CAT is strongest on the anticyclonic shear side of the jet, usually next to or just below the axis of the jet.
Low-level jet streams. When a nocturnal low-level jet forms overnight above Earth's surface ahead of a cold front, significant low-level vertical wind shear can develop near the lower portion of the low-level jet. This is also known as non-convective wind shear as it is not due to nearby thunderstorms.
Mountains.
Inversions. When on a clear and calm night, a radiation inversion is formed near the ground, the friction does not affect wind above the top of the inversion layer. The change in wind can be 90 degrees in direction and in speed. Even a nocturnal low-level jet can sometimes be observed. It tends to be strongest towards sunrise. Density differences cause additional problems to aviation.
Downbursts. When an outflow boundary forms due to a shallow layer of rain-cooled air spreading out near ground level from the parent thunderstorm, both speed and directional wind shear can result at the leading edge of the three-dimensional boundary. The stronger the outflow boundary is, the stronger the resultant vertical wind shear will become.
Horizontal component

Weather fronts

Weather fronts are boundaries between two masses of air of different densities, or different temperature and moisture properties, which normally are convergence zones in the wind field and are the principal cause of significant weather. Within surface weather analyses, they are depicted using various colored lines and symbols. The air masses usually differ in temperature and may also differ in humidity. Wind shear in the horizontal occurs near these boundaries. Cold fronts feature narrow bands of thunderstorms and severe weather and may be preceded by squall lines and dry lines. Cold fronts are sharper surface boundaries with more significant horizontal wind shear than warm fronts. When a front becomes stationary, it can degenerate into a line that separates regions of differing wind speed, known as a shear line, though the wind direction across the front normally remains constant. In the tropics, tropical waves move from east to west across the Atlantic and eastern Pacific basins. Directional and speed shear can occur across the axis of stronger tropical waves, as northerly winds precede the wave axis and southeast winds are seen behind the wave axis. Horizontal wind shear can also occur along the local land breeze and sea breeze boundaries.

Near coastlines

The magnitude of winds offshore is nearly double the wind speed observed onshore. This is attributed to the differences in friction between landmasses and offshore waters. Sometimes, there are even directional differences, particularly if local sea breezes change the wind on shore during daylight hours.

Vertical component

Thermal wind

Thermal wind is a meteorological term not referring to an actual wind, but a difference in the geostrophic wind between two pressure levels and, with ; in essence, wind shear. It is only present in an atmosphere with horizontal changes in temperature, i.e., baroclinicity. In a barotropic atmosphere, where temperature is uniform, the geostrophic wind is independent of height. The name stems from the fact that this wind flows around areas of low temperature in the same manner as the geostrophic wind flows around areas of low pressure.
The thermal wind equation is
where the are geopotential height fields with, is the Coriolis parameter, and is the upward-pointing unit vector in the vertical direction. The thermal wind equation does not determine the wind in the tropics. Since is small or zero, such as near the equator, the equation reduces to stating that is small.
This equation basically describes the existence of the jet stream, a westerly current of air with maximum wind speeds close to the tropopause which is the result of the temperature contrast between equator and pole.

Effects on tropical cyclones

s are, in essence, heat engines that are fueled by the temperature gradient between the warm tropical ocean surface and the colder upper atmosphere. Tropical cyclone development requires relatively low values of vertical wind shear so that their warm core can remain above their surface circulation center, thereby promoting intensification. Strongly sheared tropical cyclones weaken as the upper circulation is blown away from the low-level center.

Effects on thunderstorms and severe weather

Severe thunderstorms, which can spawn tornadoes and hailstorms, require wind shear to organize the storm in such a way as to maintain the thunderstorm for a longer period. This occurs as the storm's inflow becomes separated from its rain-cooled outflow. An increasing nocturnal, or overnight, low-level jet can increase the severe weather potential by increasing the vertical wind shear through the troposphere. Thunderstorms in an atmosphere with virtually no vertical wind shear weaken as soon as they send out an outflow boundary in all directions, which then quickly cuts off its inflow of relatively warm, moist air and causes the thunderstorm to dissipate.

Planetary boundary layer

The atmospheric effect of surface friction with winds aloft forces surface winds to slow and back counterclockwise near the surface of Earth blowing inward across isobars when compared to the winds in frictionless flow well above Earth's surface. This layer where friction slows and changes the wind is known as the planetary boundary layer, sometimes the Ekman layer, and it is thickest during the day and thinnest at night. Daytime heating thickens the boundary layer as winds at the surface become increasingly mixed with winds aloft due to insolation, or solar heating. Radiative cooling overnight further enhances wind decoupling between the winds at the surface and the winds above the boundary layer by calming the surface wind which increases wind shear. These wind changes force wind shear between the boundary layer and the wind aloft and are most emphasized at night.

Effects on flight

Gliding

In gliding, wind gradients just above the surface affect the takeoff and landing phases of the flight of a glider.
Wind gradient can have a noticeable effect on ground launches, also known as winch launches or wire launches. If the wind gradient is significant or sudden, or both, and the pilot maintains the same pitch attitude, the indicated airspeed will increase, possibly exceeding the maximum ground launch tow speed. The pilot must adjust the airspeed to deal with the effect of the gradient.
When landing, wind shear is also a hazard, particularly when the winds are strong. As the glider descends through the wind gradient on final approach to landing, airspeed decreases while sink rate increases, and there is insufficient time to accelerate prior to ground contact. The pilot must anticipate the wind gradient and use a higher approach speed to compensate for it.
Wind shear is also a hazard for aircraft making steep turns near the ground. It is a particular problem for gliders which have a relatively long wingspan, which exposes them to a greater wind speed difference for a given bank angle. The different airspeed experienced by each wing tip can result in an aerodynamic stall on one wing, causing a loss of control accident.