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 (or in an ocean with horizontal gradients of
density), 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 (and high) temperature in the same manner as the
geostrophic wind flows around areas of
low (and
high)
pressure. The
thermal wind equation is :f \mathbf{v}_T = \mathbf{k} \times \nabla \left( \boldsymbol{\varphi}_1 - \boldsymbol{\varphi}_0 \right) \,, 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.|235x235px 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 (lines of equal pressure) 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.
Parachuting Wind shear or wind gradients are a threat to parachutists, particularly to
BASE jumping and
wingsuit flying. Skydivers have been pushed off of their course by sudden shifts in wind direction and speed, and have collided with bridges, cliffsides, trees, other skydivers, the ground, and other obstacles. Skydivers routinely make adjustments to the position of their open canopies to compensate for changes in direction while making landings to prevent accidents such as canopy collisions and canopy inversion.
Soaring Soaring related to wind shear, also called
dynamic soaring, is a technique used by
soaring birds like
albatrosses, who can maintain flight without wing flapping. If the wind shear is of sufficient magnitude, a bird can climb into the wind gradient, trading ground speed for height, while maintaining airspeed. By then turning downwind, and diving through the wind gradient, they can also gain energy. It has also been used by
glider pilots on rare occasions. Wind shear can also produce
wave. This occurs when an
atmospheric inversion separates two layers with a marked difference in wind direction. If the wind encounters distortions in the inversion layer caused by
thermals coming up from below, it will produce significant shear waves that can be used for soaring.
Impact on passenger aircraft tail section after a microburst slammed the aircraft into the ground. Another aircraft can be seen flying in the background past the crash scene. Windshear can be extremely dangerous for aircraft, especially during takeoff and landing. Sudden changes in wind velocity can cause rapid decreases in
airspeed, leading to the aircraft being unable to maintain altitude. Windshear has been responsible for several deadly accidents, including
Eastern Air Lines Flight 66,
Pan Am Flight 759,
Delta Air Lines Flight 191, and
USAir Flight 1016. Windshear can be detected using
Doppler radar. Airports can be fitted with
low-level windshear alert systems or
Terminal Doppler Weather Radar, and aircraft can be fitted with
airborne wind shear detection and alert systems. Following the 1985 crash of Delta Air Lines Flight 191, in 1988 the U.S.
Federal Aviation Administration mandated that all commercial aircraft have airborne wind shear detection and alert systems by 1993. The installation of high-resolution Terminal Doppler Weather Radar stations at many U.S. airports that are commonly affected by windshear has further aided the ability of pilots and ground controllers to avoid wind shear conditions.
Sound propagation Wind shear can have a pronounced effect upon sound propagation in the lower atmosphere, where waves can be "bent" by
refraction phenomenon. The audibility of sounds from distant sources, such as
thunder or
gunshots, is very dependent on the amount of shear. The result of these differing sound levels is key in
noise pollution considerations, for example from
roadway noise and
aircraft noise, and must be considered in the design of
noise barriers. This phenomenon was first applied to the field of
noise pollution study in the 1960s, contributing to the design of urban highways as well as
noise barriers. plot of wind vectors at various heights in the troposphere. Meteorologists can use this plot to evaluate vertical wind shear in weather forecasting. (Source:
NOAA) The
speed of sound varies with temperature. Since temperature and sound velocity normally decrease with increasing altitude, sound is
refracted upward, away from listeners on the ground, producing an
acoustic shadow at some distance from the source. In 1862, during the
American Civil War Battle of Iuka, an acoustic shadow, believed to have been enhanced by a northeast wind, kept two divisions of Union soldiers out of the battle, because they could not hear the sounds of battle only six miles downwind.
Effects on architecture Wind engineering is a field of
engineering devoted to the analysis of
wind effects on the natural and
built environment. It includes strong winds which may cause discomfort as well as extreme winds such as
tornadoes,
hurricanes, and storms which may cause widespread destruction. Wind engineering draws upon
meteorology,
aerodynamics, and several specialist engineering disciplines. The tools used include climate models, atmospheric boundary layer wind tunnels, and numerical models. It involves, among other topics, how wind impacting buildings must be accounted for in engineering.
Wind turbines are affected by wind shear. Vertical wind-speed profiles result in different wind speeds at the blades nearest to the ground level compared to those at the top of blade travel, and this, in turn, affects the turbine operation. This low-level wind shear can cause a large bending moment in the shaft of a two-bladed turbine when the blades are vertical. The reduced wind shear over water means shorter and less expensive wind turbine towers can be used in shallow seas. ==See also==