Wind Shear (windshear, wind gradient) is described as difference in wind speed and direction over a short distance in the atmosphere. Wind shear is broken down into vertical and horizontal components. Horizontal wind shear is seen across weather fronts and near coastal regions. A vertical wind shear is found near the surface, or at upper levels of the atmosphere. Wind shear is a meteorological phenomenon that occurs over short distances, although it can be associated with larger scale cold fronts.
Wind shear is commonly observed near microbursts or downbursts caused by thunderstorms, or other weather events. Wind shear may also be observed around mountains, buildings, wind turbines, and even sailboats. Radiation differences that occur in clear skies can trigger wind shear as well. Aircraft that is taking off or landing can be affected by wind shear which is sometimes a cause for aircraft accidents. Varying levels of turbulence often associated with wind shear are the main culprit in many aircraft accidents.
Wind shear has a dramatic effect on sound movement through the atmosphere. This can contribute to the bending of sound waves, causing sound to travel in directions it was not intended to go. Wind shear is the common reason why we hear things from a distance that we would not normally pick up on. Strong vertical wind shear in the troposphere is capable of slowing the development of tropical cyclones, but may organize storm cells to give them longer life cycles that can produce severe weather. Thermal wind shear plays a role in horizontal temperature differences, and explains the existence of the jet stream.
There are many weather situations where wind shear is commonly observed. Weather fronts with a big difference in temperature (9 ËšF or more) across the front and movement of 30 knots or more cause significant wind shear. Frontal shear can be observed at any altitude between the surface and troposphere, and is often seen horizontally or vertically. Vertical wind shear is more common at higher altitudes. Wind shear associated with upper-level jet streams is a phenomenon known as clear-air turbulence (CAT). This is caused by horizontal and vertical wind gradients that are connected at the edge of the jet streams. The CAT is strongest on the cold air side of the jet stream, which is usually next to or just below the axis of the jet. Low-level jet streams occur overnight ahead of cold fronts. Low-level vertical wind shear occurs near the surface at the lower end of the jet. Winds that blow over mountains causes vertical wind shear on the lee side. If the flow is strong enough, it causes severe turbulence that poses a threat to ascending or descending aircraft. Nocturnal radiation inversion occurs near the surface when it is calm and clear. This can produce wind shear that does not affect the wind above the inversion layer. These nocturnal flows tend to be greatest toward sunrise. A downburst occurs when a shallow layer of rain-cooled air spreads out near the ground level from a parent storm cell. Stronger outflows result in stronger downbursts which, in turn, result in stronger vertical wind shear.
Cold fronts produce stronger horizontal wind shear than warm fronts. A shear line is produced when a front becomes stationary and a line of differing wind speeds occur in separate regions of the front, though the wind direction typically remains constant across the line. Tropical waves moving from east to west across the Atlantic and Pacific basins can trigger horizontal wind shear. Horizontal shear also occurs along land breeze and sea breeze boundaries. Offshore winds are nearly double that of the speed of onshore winds. This is due to friction between land and sea masses. Thermal wind is a meteorological term not associated with actual wind, but a difference in the geostrophic (forces caused by the Earth’s rotation and horizontal pressure) wind between two pressure levels. It is present in the atmosphere when horizontal changes in temperature occur.
Thunderstorms require wind shear to organize the storm in such a way to maintain the storm cell for a longer period of time. Wind shear occurs when the storm’s inflow separates from the rain-cooled outflow. A nocturnal low level jet-stream can increase the chances of severe weather by increasing the vertical wind shear in the troposphere. Thunderstorms with virtually no vertical wind shear in the atmosphere will quickly diminish as it sends out an outflow in all directions. This will quickly cut off the inflow of warmer, moist air and thus kills the storm.
The effects of wind shear play a significant role on sound travel. The audibility of sounds from distant sources is dependent on the amount of wind shear. Noise barriers have been implemented in areas where significant noise pollution occurs due to wind shear. Roadways and airports are common areas where noise barriers are used. The speed of sound varies with temperature. Sound speed normally decreases as it travels in altitude.
Wind engineering is a field of engineering based on the effects of wind and wind shear on the natural and manmade environment. Wind engineering draws upon meteorology, aerodynamics and a other specialist engineering disciplines. Climate models, wind tunnels, and other numerical models are used to study how wind affects objects.