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:''for the Marvel Comics character, click here''
'Wind shear', sometimes referred to as 'windshear' or wind gradient, is a difference in wind speed and/or direction over a relatively short distance in the atmosphere. Wind shear can be broken down into vertical and horizontal components, with horizontal wind shear seen across weather fronts. Wind shear itself is a microscale meteorological phenomenon, but it may be associated with mesoscale or synoptic scale weather features. It is commonly observed near microbursts and downbursts, weather fronts, low level wind maxima known as low level jets, near mountains, radiation inversions, buildings, wind turbines, and sailboats. Wind shear has a significant effect during take-off and landing of aircraft, and was a significant cause of aircraft accidents involving large loss of life within the United States.
Airplane pilots generally regard significant windshear to be a change in airspeed of 15 knots (7.7 m/s) and/or a change in azimuth of 30 degrees or greater per thousand feet (300 m) of altitude change. Wind shear can affect aircraft airspeed during take off and landing in disastrous ways. It is also a key factor in severe thunderstorms. An additional hazard is turbulence often associated with wind shear.
Sound is significantly distorted by wind shear, becoming significantly bent downward where temperature inversions exist within the atmosphere. Strong vertical wind shear within the troposphere also inhibits tropical cyclone development, but helps to organize individual thunderstorms into living long life cycles and producing severe weather. The meteorological concept of thermal wind deals with how differences in wind with height are dependent on horizontal temperature differences.
Weather situations where shear is observed include:
★ Weather fronts. Significant shear is observed, when the temperature difference across the front is 5 °C or more, and the front moves at 15 kt or faster. Because fronts are three-dimensional phenomena, frontal shear can be observed at any altitude between surface and tropopause, and therefore be seen both horizontally and vertically.
★ Low Level Jets. When a nocturnal low-level jet forms above the boundary layer 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 nonconvective wind shear.
★ Mountains. When winds blow over a mountain, vertical shear is observed on the lee side. If the flow is strong enough, turbulent eddies known as rotors associated with lee waves may form, which are dangerous to ascending and descending aircraft.[1]
★ Inversions. When on a clear and calm night, a radiation inversion is formed near the ground, the friction does not affect wind above the inversion top. Change in wind can be 90 degrees in direction and 40 kt in speed. Even a nocturnal low level jet can sometimes be observed. Density difference causes additional problems to aviation.
★ Downbursts. When an outflow boundary moves away from a thunderstorm due to a shallow layer of rain-cooled air spreading out at ground level, both speed and directional wind shear can result at the leading edge of the three dimensional boundary. The stronger the outflow boundary, the stronger the resultant vertical wind shear.
The atmospheric effect of surface friction with winds aloft force surface winds to slow and back counterclockwise near the surface of the Earth blowing inward across isobars, when compared to the winds in frictionless flow well above the Earth's surface.[2] This layer where friction slows and changes the wind is known as the planetary boundary layer, and 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 and thereby increases wind shear. These wind changes force wind shear between the boundary layer and the wind aloft, and is most emphasized at night.
In gliding, wind gradient affects the takeoff and landing phases of flight of a glider.
Wind gradient can have a noticeable effect on ground 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.[3]
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. Gliding: a Handbook on Soaring Flight, , Derek, Piggott, Knauff & Grove, 1997,
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.[4]
Soaring related to wind shear, also called dynamic soaring, is a technique used by soaring birds including albatrosses. If the wind shear is of sufficient magnitude, a bird can climb into the wind gradient, trading ground speed for height, while maintaining airspeed.[5] By then turning downwind, and diving through the wind gradient, they can also gain energy.[6]
In the United States, a string of fatal accidents near thunderstorms downed passenger airliners during final descent and initial ascent, including Eastern Air Lines Flight 66 in New York (1975), Pan Am Flight 759 in New Orleans (1982), and Delta Air Lines Flight 191 at Dallas-Fort Worth (1985). The common cause in these air disasters was low level windshear.
Strong outflow from thunderstorms causes rapid changes in the three-dimensional wind velocity just above ground level. Air Force One landed five minutes before one of the strongest downbursts ever recorded in the Washington, D.C. area at Andrews Air Force Base, with President Ronald Reagan onboard.[7] Initially, this outflow causes a headwind that increases airspeed, which normally causes a pilot to reduce engine power if they are unaware of the wind shear. As the aircraft passes into the region of the downdraft, the localized headwind diminishes, reducing the aircraft's airspeed, and increasing its sink rate. Then, when the aircraft passes through the other side of the downdraft, the headwind becomes a tailwind, reducing airspeed further, leaving the aircraft in a low-power, low-speed, descent. This can lead to an accident if the aircraft is too low to effect a recovery before ground contact.[8]
As the result of the accidents in the 1970s and 1980s, in 1988 the U.S. Federal Aviation Administration mandated that all commercial aircraft have on-board windshear detection systems by 1993. Three airlines, United Airlines, Continental Airlines, and Northwest Airlines received extensions until the end of 1995 so to install predictive wind shear sensors rather than reactive systems in their aircraft. The results of these efforts was immediate. Between 1964 and 1985, wind shear directly caused or contributed to 26 major civil transport aircraft accidents in the U.S. that led to 620 deaths and 200 injuries. Of these accidents, 15 occurred during take-off, three during flight, and eight during landing. Since 1995, the number of major civil aircraft accidents caused by wind shear has dropped to approximately one every ten years due to the mandated on-board detection, as well as the addition of Doppler radar units on the ground.
The design of buildings must account for wind loads, and these are affected by wind shear. For engineering purposes, a power law wind speed profile may be defined as follows: Steel Buildings, , Stanley, Crawley, Wiley, 1993, Guidelines for Design of Low-Rise Buildings Subjected to Lateral Forces, , Ajaya, Gupta, CRC Press, 1993,
:
where:
: = speed of the wind at height
: = gradient wind at gradient height
: = exponential coefficient
In sailing, wind shear affects sailboats by presenting a different wind speed and direction at different heights along the mast. Sailmakers may introduce sail twist in the design of the sail, where the head of the sail is set at a different angle of attack from the foot of the sail in order to change the lift distribution with height. The effect of wind shear can be factored into the selection of twist in the sail design, but this can be difficult to predict since wind shear may vary widely in different weather conditions. Sailors may also adjust the trim of the sail to account for wind gradient, for example using a boom vang.[9]
Wind shear can have a pronounced effect upon sound propagation in the lower atmosphere. The audibility of sounds from distant sources, such as thunder or gunshots, is very dependent on the amount of shear. Shear can have a pronounced effect upon sound propagation in the lower atmosphere, where waves can be "bent" by refraction phenomenon. 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.[10] 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.[11]
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, creating an acoustic shadow at some distance from the source.[12] A wind gradient of 4 m/s/km can produce refraction equal to a typical temperature lapse rate of 7.5 °C/km.[13] Higher values of wind gradient will refract sound downward toward the surface in the downwind direction,[14] eliminating the acoustic shadow on the downwind side. This will increase the audibility of sounds downwind. This downwind refraction effect occurs because there is a wind gradient; the sound is not being carried along by the wind.[15]
For sound propagation, the exponential variation of wind speed with height can be defined as follows: Engineering Noise Control; Theory and Practice, , David, Bies, Spon Press, 2003,
:
:
where:
: = speed of the wind at height , and is a constant
: = exponential coefficient based on ground surface roughness, typically between 0.08 and 0.52
: = expected wind gradient at height
In the 1862 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,[16] because they could not hear the sounds of battle only six miles downwind.[17]
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. Grid Integration of Wind Energy Conversion Systems, , Siegfried, Heier, John Wiley & Sons, 2005, The wind gradient can create a large bending moment in the shaft of a two bladed turbine when the blades are vertical.[18] The reduced wind gradient over water means shorter and less expensive wind turbine towers can be used in shallow seas. Wind Turbine Operation in Electric Power Systems : Advanced Modeling, , Zbigniew, Lubosny, Springer, 2003,
For wind turbine engineering, an exponential variation in wind speed with height can be defined relative to wind measured at a reference height of 10 meters as:
:
where:
: = velocity of the wind at height,[m/s]
: = velocity of the wind at height, = 10 meters[m/s]
: = Hellman exponent
Tropical cyclones require low values of vertical wind shear so that their warm core can remain stacked above their surface circulation center, and further development as a warm-core cyclone can continue. Strongly sheared tropical cyclones tend to either level in intensity or dissipate due to the breakdown of their internal heat engine. [19]
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 of time by separating the storm's inflow from its rain-cooled outflow. Thunderstorms in an atmosphere with virtually no vertical wind shear weaken as soon as they send out an outflow boundary in all directions, which quickly cuts off its inflow of relatively warm, moist air and subsequently kills the thunderstorm.[21]
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 gradients of 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
:,
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 f is small or zero there, the equation reduces to stating that is small.
★ Air safety
★ Convergence zone
★ Hodograph
★ Low level windshear alert system
★ Microburst
★ Refraction
★ Tropical cyclone
★ Thunderstorm
★ Turbulence
1. National Center for Atmospheric Research. T-REX: Catching the Sierra’s waves and rotors Retrieved on 2006-10-21.
2. Glossary of Meteorology. E. Retrieved on 2007-06-03.
3. Glider Flying Handbook, , , , U.S. Federal Aviation Administration, 2003, FAA-8083-13_GFH
4. Glider Basics from First Flight to Solo, , Thomas, Knauff, Thomas Knauff, 1984,
5. Principles of Animal Locomotion, , R., Alexander, Princeton University Press, 2002,
6. Bird Migration, , Thomas, Alerstam, Cambridge University Press, 1990,
7. National Weather Service Forecast Office, Riverton, Wyoming. http://www.crh.noaa.gov/riw/severe/microburst.php Downburst.] Retrieved on 2006-10-22.
8. NASA Langley Air Force Base. Making the Skies Safer From Windshear. Retrieved on 2006-10-22.
9. The Symmetry of Sailing, , Ross, Garrett, Sheridan House, 1996,
10.
11. C.Michael Hogan, '' Analysis of highway noise'', Journal of Water, Air, & Soil Pollution, Volume 2, Number 3, Biomedical and Life Sciences and Earth and Environmental Science Issue, Pages 387-392, September, 1973, Springer Verlag, Netherlands ISSN 0049-6979
12. The Master Handbook of Acoustics, , F., Everest, McGraw-Hill, 2001,
13. Lightning, , Martin, Uman, Dover Publications, 1984,
14. Handbook of Atmospheric Electrodynamics, , Hans, Volland, CRC Press, 1995,
15. Noise Pollution and Control Strategy, , S., Singal, Alpha Science International, Ltd, 2005,
16. Grant as Military Commander, , Sir, Cornwall, Barnes & Noble Inc, 1996,
17. The Darkest Days of the War: the Battles of Iuka and Corinth, , Peter, Cozzens, The University of North Carolina Press, 2006,
18. Large Wind Turbines, , Robert, Harrison, John Wiley & Sons, 2001,
19. University of Illinois. Hurricanes. Retrieved 2006-10-21.
20. Fundamentals of Weather and Climate, , J., Mcilveen, Chapman & Hall, 1992,
21. University of Illinois. Vertical Wind Shear Retrieved on 2006-10-21.
★ National Science Digital Library - Wind shear
Cirrus uncinus ice crystal plumes showing high level wind shear, with changes in wind speed and direction.
'Wind shear', sometimes referred to as 'windshear' or wind gradient, is a difference in wind speed and/or direction over a relatively short distance in the atmosphere. Wind shear can be broken down into vertical and horizontal components, with horizontal wind shear seen across weather fronts. Wind shear itself is a microscale meteorological phenomenon, but it may be associated with mesoscale or synoptic scale weather features. It is commonly observed near microbursts and downbursts, weather fronts, low level wind maxima known as low level jets, near mountains, radiation inversions, buildings, wind turbines, and sailboats. Wind shear has a significant effect during take-off and landing of aircraft, and was a significant cause of aircraft accidents involving large loss of life within the United States.
Airplane pilots generally regard significant windshear to be a change in airspeed of 15 knots (7.7 m/s) and/or a change in azimuth of 30 degrees or greater per thousand feet (300 m) of altitude change. Wind shear can affect aircraft airspeed during take off and landing in disastrous ways. It is also a key factor in severe thunderstorms. An additional hazard is turbulence often associated with wind shear.
Sound is significantly distorted by wind shear, becoming significantly bent downward where temperature inversions exist within the atmosphere. Strong vertical wind shear within the troposphere also inhibits tropical cyclone development, but helps to organize individual thunderstorms into living long life cycles and producing severe weather. The meteorological concept of thermal wind deals with how differences in wind with height are dependent on horizontal temperature differences.
Where and when it is strongly observed
Microburst schematic from NASA. Note the downward motion of the air until it hits ground level, then spreads outward in all directions. The wind regime in a microburst is completely opposite to a tornado.
Weather situations where shear is observed include:
★ Weather fronts. Significant shear is observed, when the temperature difference across the front is 5 °C or more, and the front moves at 15 kt or faster. Because fronts are three-dimensional phenomena, frontal shear can be observed at any altitude between surface and tropopause, and therefore be seen both horizontally and vertically.
★ Low Level Jets. When a nocturnal low-level jet forms above the boundary layer 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 nonconvective wind shear.
★ Mountains. When winds blow over a mountain, vertical shear is observed on the lee side. If the flow is strong enough, turbulent eddies known as rotors associated with lee waves may form, which are dangerous to ascending and descending aircraft.[1]
★ Inversions. When on a clear and calm night, a radiation inversion is formed near the ground, the friction does not affect wind above the inversion top. Change in wind can be 90 degrees in direction and 40 kt in speed. Even a nocturnal low level jet can sometimes be observed. Density difference causes additional problems to aviation.
★ Downbursts. When an outflow boundary moves away from a thunderstorm due to a shallow layer of rain-cooled air spreading out at ground level, both speed and directional wind shear can result at the leading edge of the three dimensional boundary. The stronger the outflow boundary, the stronger the resultant vertical wind shear.
Planetary boundary layer
The atmospheric effect of surface friction with winds aloft force surface winds to slow and back counterclockwise near the surface of the Earth blowing inward across isobars, when compared to the winds in frictionless flow well above the Earth's surface.[2] This layer where friction slows and changes the wind is known as the planetary boundary layer, and 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 and thereby increases wind shear. These wind changes force wind shear between the boundary layer and the wind aloft, and is most emphasized at night.
Effects on flight
Gliding
Glider ground launch due to wind shear.
In gliding, wind gradient affects the takeoff and landing phases of flight of a glider.
Wind gradient can have a noticeable effect on ground 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.[3]
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. Gliding: a Handbook on Soaring Flight, , Derek, Piggott, Knauff & Grove, 1997,
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.[4]
Soaring
The albatross is an expert in dynamic soaring using wind shear.
Soaring related to wind shear, also called dynamic soaring, is a technique used by soaring birds including albatrosses. If the wind shear is of sufficient magnitude, a bird can climb into the wind gradient, trading ground speed for height, while maintaining airspeed.[5] By then turning downwind, and diving through the wind gradient, they can also gain energy.[6]
Impact on passenger aircraft
In the United States, a string of fatal accidents near thunderstorms downed passenger airliners during final descent and initial ascent, including Eastern Air Lines Flight 66 in New York (1975), Pan Am Flight 759 in New Orleans (1982), and Delta Air Lines Flight 191 at Dallas-Fort Worth (1985). The common cause in these air disasters was low level windshear.
Strong outflow from thunderstorms causes rapid changes in the three-dimensional wind velocity just above ground level. Air Force One landed five minutes before one of the strongest downbursts ever recorded in the Washington, D.C. area at Andrews Air Force Base, with President Ronald Reagan onboard.[7] Initially, this outflow causes a headwind that increases airspeed, which normally causes a pilot to reduce engine power if they are unaware of the wind shear. As the aircraft passes into the region of the downdraft, the localized headwind diminishes, reducing the aircraft's airspeed, and increasing its sink rate. Then, when the aircraft passes through the other side of the downdraft, the headwind becomes a tailwind, reducing airspeed further, leaving the aircraft in a low-power, low-speed, descent. This can lead to an accident if the aircraft is too low to effect a recovery before ground contact.[8]
Effect of wind shear on aircraft trajectory. Note how merely correcting for the initial gust front can have dire consequences.
As the result of the accidents in the 1970s and 1980s, in 1988 the U.S. Federal Aviation Administration mandated that all commercial aircraft have on-board windshear detection systems by 1993. Three airlines, United Airlines, Continental Airlines, and Northwest Airlines received extensions until the end of 1995 so to install predictive wind shear sensors rather than reactive systems in their aircraft. The results of these efforts was immediate. Between 1964 and 1985, wind shear directly caused or contributed to 26 major civil transport aircraft accidents in the U.S. that led to 620 deaths and 200 injuries. Of these accidents, 15 occurred during take-off, three during flight, and eight during landing. Since 1995, the number of major civil aircraft accidents caused by wind shear has dropped to approximately one every ten years due to the mandated on-board detection, as well as the addition of Doppler radar units on the ground.
Architecture
The design of buildings must account for wind loads, and these are affected by wind shear. For engineering purposes, a power law wind speed profile may be defined as follows: Steel Buildings, , Stanley, Crawley, Wiley, 1993, Guidelines for Design of Low-Rise Buildings Subjected to Lateral Forces, , Ajaya, Gupta, CRC Press, 1993,
:
where:
: = speed of the wind at height
: = gradient wind at gradient height
: = exponential coefficient
Sailing
In sailing, wind shear affects sailboats by presenting a different wind speed and direction at different heights along the mast. Sailmakers may introduce sail twist in the design of the sail, where the head of the sail is set at a different angle of attack from the foot of the sail in order to change the lift distribution with height. The effect of wind shear can be factored into the selection of twist in the sail design, but this can be difficult to predict since wind shear may vary widely in different weather conditions. Sailors may also adjust the trim of the sail to account for wind gradient, for example using a boom vang.[9]
Sound propagation
Wind shear can have a pronounced effect upon sound propagation in the lower atmosphere. The audibility of sounds from distant sources, such as thunder or gunshots, is very dependent on the amount of shear. Shear can have a pronounced effect upon sound propagation in the lower atmosphere, where waves can be "bent" by refraction phenomenon. 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.[10] 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.[11]
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, creating an acoustic shadow at some distance from the source.[12] A wind gradient of 4 m/s/km can produce refraction equal to a typical temperature lapse rate of 7.5 °C/km.[13] Higher values of wind gradient will refract sound downward toward the surface in the downwind direction,[14] eliminating the acoustic shadow on the downwind side. This will increase the audibility of sounds downwind. This downwind refraction effect occurs because there is a wind gradient; the sound is not being carried along by the wind.[15]
For sound propagation, the exponential variation of wind speed with height can be defined as follows: Engineering Noise Control; Theory and Practice, , David, Bies, Spon Press, 2003,
:
:
where:
: = speed of the wind at height , and is a constant
: = exponential coefficient based on ground surface roughness, typically between 0.08 and 0.52
: = expected wind gradient at height
In the 1862 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,[16] because they could not hear the sounds of battle only six miles downwind.[17]
Wind turbines
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. Grid Integration of Wind Energy Conversion Systems, , Siegfried, Heier, John Wiley & Sons, 2005, The wind gradient can create a large bending moment in the shaft of a two bladed turbine when the blades are vertical.[18] The reduced wind gradient over water means shorter and less expensive wind turbine towers can be used in shallow seas. Wind Turbine Operation in Electric Power Systems : Advanced Modeling, , Zbigniew, Lubosny, Springer, 2003,
For wind turbine engineering, an exponential variation in wind speed with height can be defined relative to wind measured at a reference height of 10 meters as:
:
where:
: = velocity of the wind at height,
: = velocity of the wind at height, = 10 meters
: = Hellman exponent
Effects on tropical cyclones
Tropical cyclones require low values of vertical wind shear so that their warm core can remain stacked above their surface circulation center, and further development as a warm-core cyclone can continue. Strongly sheared tropical cyclones tend to either level in intensity or dissipate due to the breakdown of their internal heat engine. [19]
Strong wind shear in the high troposphere forms the anvil-shaped top characteristic of the mature cumulonimbus cloud. The anvil may stretch several kilometers downwind in the direction of the shear.[20]
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 of time by separating the storm's inflow from its rain-cooled outflow. Thunderstorms in an atmosphere with virtually no vertical wind shear weaken as soon as they send out an outflow boundary in all directions, which quickly cuts off its inflow of relatively warm, moist air and subsequently kills the thunderstorm.[21]
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 gradients of 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
:,
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 f is small or zero there, the equation reduces to stating that is small.
See also
★ Air safety
★ Convergence zone
★ Hodograph
★ Low level windshear alert system
★ Microburst
★ Refraction
★ Tropical cyclone
★ Thunderstorm
★ Turbulence
References
1. National Center for Atmospheric Research. T-REX: Catching the Sierra’s waves and rotors Retrieved on 2006-10-21.
2. Glossary of Meteorology. E. Retrieved on 2007-06-03.
3. Glider Flying Handbook, , , , U.S. Federal Aviation Administration, 2003, FAA-8083-13_GFH
4. Glider Basics from First Flight to Solo, , Thomas, Knauff, Thomas Knauff, 1984,
5. Principles of Animal Locomotion, , R., Alexander, Princeton University Press, 2002,
6. Bird Migration, , Thomas, Alerstam, Cambridge University Press, 1990,
7. National Weather Service Forecast Office, Riverton, Wyoming. http://www.crh.noaa.gov/riw/severe/microburst.php Downburst.] Retrieved on 2006-10-22.
8. NASA Langley Air Force Base. Making the Skies Safer From Windshear. Retrieved on 2006-10-22.
9. The Symmetry of Sailing, , Ross, Garrett, Sheridan House, 1996,
10.
11. C.Michael Hogan, '' Analysis of highway noise'', Journal of Water, Air, & Soil Pollution, Volume 2, Number 3, Biomedical and Life Sciences and Earth and Environmental Science Issue, Pages 387-392, September, 1973, Springer Verlag, Netherlands ISSN 0049-6979
12. The Master Handbook of Acoustics, , F., Everest, McGraw-Hill, 2001,
13. Lightning, , Martin, Uman, Dover Publications, 1984,
14. Handbook of Atmospheric Electrodynamics, , Hans, Volland, CRC Press, 1995,
15. Noise Pollution and Control Strategy, , S., Singal, Alpha Science International, Ltd, 2005,
16. Grant as Military Commander, , Sir, Cornwall, Barnes & Noble Inc, 1996,
17. The Darkest Days of the War: the Battles of Iuka and Corinth, , Peter, Cozzens, The University of North Carolina Press, 2006,
18. Large Wind Turbines, , Robert, Harrison, John Wiley & Sons, 2001,
19. University of Illinois. Hurricanes. Retrieved 2006-10-21.
20. Fundamentals of Weather and Climate, , J., Mcilveen, Chapman & Hall, 1992,
21. University of Illinois. Vertical Wind Shear Retrieved on 2006-10-21.
External links
★ National Science Digital Library - Wind shear
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