Wind speed
Encyclopedia
Wind speed, or wind velocity
, is a fundamental atmospheric
rate.
Wind speed affects weather forecasting
, aircraft and maritime operations, construction projects, growth and metabolism rate of many plant species, and countless other implications.
Wind
speed is now commonly measured with an anemometer
but can also be classified using the older Beaufort scale
which is based on people's observation of specifically defined wind effects.
, notably with the pressure gradient and surfaces over which the air is found.
Pressure gradient is a term to describe the difference in air pressure between two points in the atmosphere or on the surface of the Earth. It is vital to wind speed, because the greater the difference in pressure, the faster the wind flows (from the high to low pressure) to balance out the variation. The pressure gradient, when combined with the Coriolis Effect
and friction
, also influences wind direction
.
Rossby wave
s are strong winds in the upper troposphere
. These operate on a global scale and move from West to East (hence being known as Westerlies
). The Rossby waves are themselves a different wind speed from what we experience in the lower troposphere
.
Local weather conditions play a key role in influencing wind speed, as the formation of hurricanes, monsoon
s and cyclones as freak weather conditions can drastically affect the velocity of the wind.
on 10 April 1996, an automatic weather station
on Barrow Island
, Australia
, registered a maximum wind gust of 408 km/h (220 kn; 253 mph). The wind gust was evaluated by the WMO Evaluation Panel who found that the anemometer was mechanically sound and the gust was within statistical probability and ratified the measurement in 2010. The anemometer was mounted 10 m above ground level and so 64 m above sea level. During the cyclone, several extreme gusts of greater than 300 km/h (160 kt) were recorded, with a maximum 5-minute mean speed of 176 km/h (95 kt), the extreme gust factor was in the order of 2.27–2.75 times the mean wind speed. The pattern and scales of the gusts suggests that a mesovortex was embedded in the already strong eyewall of the cyclone.
The second-highest surface wind speed ever officially recorded is 372 km/h (231 mph) at the Mount Washington (New Hampshire)
Observatory in the US on 12 April 1934, using a heated anemometer
. The anemometer, specifically designed for use on Mount Washington, was later tested by the US National Weather Bureau
and confirmed to be accurate. The highest surface wind speed ever officially recorded in Asia was recorded in Afghanistan on 14 August 2008: 328 km/h (204 mph) in Ab-Paran, Ghowr.
Wind speeds within certain atmospheric phenomena (such as tornado
es) may greatly exceed these values but have never been accurately measured. The figure of (302 mph) during the F5 tornado in Bridge Creek, Oklahoma
on May 3, 1999 is often quoted as the highest surface wind speed.
In 1991, a chase team from the University of Oklahoma
chased a tornado in Red Rock, Oklahoma
and used a portable Doppler weather radar
to measure a wind speed of 460 km/h (286 mph).
According to Alan F. Arbogast ("Discovering Physical Geography") wind direction and speed are affected by three main factors:
All three of these combined result in the spiral motion of air in both high and low pressure systems.
Wind speed is a common factor in the design of structures and buildings around the world. The wind speed is often the governing factor in the "lateral" design of a structure and is used by professional engineers and designers.
In the United States, the wind speed used in design is often referred to as a "3-second gust" which is the highest sustained gust over a 3 second period having a probability of being exceeded per year of 1 in 50 (ASCE 7-latest edition). Windspeedbyzip maps out the design wind speed as suggested by ASCE 7-05 for the United States. This design wind speed is accepted by most building codes in the United States and often times governs the lateral design of buildings and structures.
In Canada, reference wind pressures are used in design and are based on the "mean hourly" wind speed having a probability of being exceeded per year of 1 in 50. The reference wind pressure (q) is calculated in Pascals using the following equation (ref: NBC 2005 Structural Commentaries - Part 4 of Div. B, Comm. I): q=(1/2)pV**2
where p is the air density in kg/m**3 and V is wind speed in m/s.
Historically, wind speeds have been reported with a variety of averaging times (fastest mile, 3-second gust, 1-minute and mean hourly for example) which designers may have to take into account. To convert wind speeds from one averaging time to another, the Durst Curve (Ref: ASCE 7-05 commentary Figure C6-4) was developed which defines the relation between probable maximum wind speed averaged over t seconds, V(t), and mean wind speed over one hour V(3600).
Velocity
In physics, velocity is speed in a given direction. Speed describes only how fast an object is moving, whereas velocity gives both the speed and direction of the object's motion. To have a constant velocity, an object must have a constant speed and motion in a constant direction. Constant ...
, is a fundamental atmospheric
Atmosphere
An atmosphere is a layer of gases that may surround a material body of sufficient mass, and that is held in place by the gravity of the body. An atmosphere may be retained for a longer duration, if the gravity is high and the atmosphere's temperature is low...
rate.
Wind speed affects weather forecasting
Weather forecasting
Weather forecasting is the application of science and technology to predict the state of the atmosphere for a given location. Human beings have attempted to predict the weather informally for millennia, and formally since the nineteenth century...
, aircraft and maritime operations, construction projects, growth and metabolism rate of many plant species, and countless other implications.
Wind
Wind
Wind is the flow of gases on a large scale. On Earth, wind consists of the bulk movement of air. In outer space, solar wind is the movement of gases or charged particles from the sun through space, while planetary wind is the outgassing of light chemical elements from a planet's atmosphere into space...
speed is now commonly measured with an anemometer
Anemometer
An anemometer is a device for measuring wind speed, and is a common weather station instrument. The term is derived from the Greek word anemos, meaning wind, and is used to describe any airspeed measurement instrument used in meteorology or aerodynamics...
but can also be classified using the older Beaufort scale
Beaufort scale
The Beaufort Scale is an empirical measure that relates wind speed to observed conditions at sea or on land. Its full name is the Beaufort Wind Force Scale.-History:...
which is based on people's observation of specifically defined wind effects.
Factors affecting wind speed
Wind speed is affected by a number of factors and situations, operating on varying scales (from micro to macro scales). These include the pressure gradient, Rossby waves and jet streams, and local weather conditions. There are also links to be found between wind speed and wind directionWind direction
Wind direction is reported by the direction from which it originates. For example, a northerly wind blows from the north to the south. Wind direction is usually reported in cardinal directions or in azimuth degrees...
, notably with the pressure gradient and surfaces over which the air is found.
Pressure gradient is a term to describe the difference in air pressure between two points in the atmosphere or on the surface of the Earth. It is vital to wind speed, because the greater the difference in pressure, the faster the wind flows (from the high to low pressure) to balance out the variation. The pressure gradient, when combined with the Coriolis Effect
Coriolis effect
In physics, the Coriolis effect is a deflection of moving objects when they are viewed in a rotating reference frame. In a reference frame with clockwise rotation, the deflection is to the left of the motion of the object; in one with counter-clockwise rotation, the deflection is to the right...
and friction
Friction
Friction is the force resisting the relative motion of solid surfaces, fluid layers, and/or material elements sliding against each other. There are several types of friction:...
, also influences wind direction
Wind direction
Wind direction is reported by the direction from which it originates. For example, a northerly wind blows from the north to the south. Wind direction is usually reported in cardinal directions or in azimuth degrees...
.
Rossby wave
Rossby wave
Atmospheric Rossby waves are giant meanders in high-altitude winds that are a major influence on weather.They are not to be confused with oceanic Rossby waves, which move along the thermocline: that is, the boundary between the warm upper layer of the ocean and the cold deeper part of the...
s are strong winds in the upper troposphere
Troposphere
The troposphere is the lowest portion of Earth's atmosphere. It contains approximately 80% of the atmosphere's mass and 99% of its water vapor and aerosols....
. These operate on a global scale and move from West to East (hence being known as Westerlies
Westerlies
The Westerlies, anti-trades, or Prevailing Westerlies, are the prevailing winds in the middle latitudes between 30 and 60 degrees latitude, blowing from the high pressure area in the horse latitudes towards the poles. These prevailing winds blow from the west to the east, and steer extratropical...
). The Rossby waves are themselves a different wind speed from what we experience in the lower troposphere
Troposphere
The troposphere is the lowest portion of Earth's atmosphere. It contains approximately 80% of the atmosphere's mass and 99% of its water vapor and aerosols....
.
Local weather conditions play a key role in influencing wind speed, as the formation of hurricanes, monsoon
Monsoon
Monsoon is traditionally defined as a seasonal reversing wind accompanied by corresponding changes in precipitation, but is now used to describe seasonal changes in atmospheric circulation and precipitation associated with the asymmetric heating of land and sea...
s and cyclones as freak weather conditions can drastically affect the velocity of the wind.
Highest speed
During the passage of Tropical Cyclone OliviaCyclone Olivia
Severe Tropical Cyclone Olivia was a powerful Category 4 cyclone that produced the highest non-tornadic winds on record, 408 km/h .-Meteorological history:...
on 10 April 1996, an automatic weather station
Automatic weather station
An automatic weather station is an automated version of the traditional weather station, either to save human labour or to enable measurements from remote areas...
on Barrow Island
Barrow Island (Western Australia)
Barrow Island is a island located northwest off the coast of Western Australia. The island is the second largest in Western Australia after Dirk Hartog Island.-Discovery and early history:...
, Australia
Australia
Australia , officially the Commonwealth of Australia, is a country in the Southern Hemisphere comprising the mainland of the Australian continent, the island of Tasmania, and numerous smaller islands in the Indian and Pacific Oceans. It is the world's sixth-largest country by total area...
, registered a maximum wind gust of 408 km/h (220 kn; 253 mph). The wind gust was evaluated by the WMO Evaluation Panel who found that the anemometer was mechanically sound and the gust was within statistical probability and ratified the measurement in 2010. The anemometer was mounted 10 m above ground level and so 64 m above sea level. During the cyclone, several extreme gusts of greater than 300 km/h (160 kt) were recorded, with a maximum 5-minute mean speed of 176 km/h (95 kt), the extreme gust factor was in the order of 2.27–2.75 times the mean wind speed. The pattern and scales of the gusts suggests that a mesovortex was embedded in the already strong eyewall of the cyclone.
The second-highest surface wind speed ever officially recorded is 372 km/h (231 mph) at the Mount Washington (New Hampshire)
Mount Washington (New Hampshire)
Mount Washington is the highest peak in the Northeastern United States at , famous for dangerously erratic weather. For 76 years, a weather observatory on the summit held the record for the highest wind gust directly measured at the Earth's surface, , on the afternoon of April 12, 1934...
Observatory in the US on 12 April 1934, using a heated anemometer
Anemometer
An anemometer is a device for measuring wind speed, and is a common weather station instrument. The term is derived from the Greek word anemos, meaning wind, and is used to describe any airspeed measurement instrument used in meteorology or aerodynamics...
. The anemometer, specifically designed for use on Mount Washington, was later tested by the US National Weather Bureau
National Weather Service
The National Weather Service , once known as the Weather Bureau, is one of the six scientific agencies that make up the National Oceanic and Atmospheric Administration of the United States government...
and confirmed to be accurate. The highest surface wind speed ever officially recorded in Asia was recorded in Afghanistan on 14 August 2008: 328 km/h (204 mph) in Ab-Paran, Ghowr.
Wind speeds within certain atmospheric phenomena (such as tornado
Tornado
A tornado is a violent, dangerous, rotating column of air that is in contact with both the surface of the earth and a cumulonimbus cloud or, in rare cases, the base of a cumulus cloud. They are often referred to as a twister or a cyclone, although the word cyclone is used in meteorology in a wider...
es) may greatly exceed these values but have never been accurately measured. The figure of (302 mph) during the F5 tornado in Bridge Creek, Oklahoma
Bridge Creek, Oklahoma
Bridge Creek is a town in Grady County, Oklahoma, United States. As of the 2006 US Census estimate, the town is estimated to have a total population of 340.-Geography:Bridge Creek is located at ....
on May 3, 1999 is often quoted as the highest surface wind speed.
In 1991, a chase team from the University of Oklahoma
University of Oklahoma
The University of Oklahoma is a coeducational public research university located in Norman, Oklahoma. Founded in 1890, it existed in Oklahoma Territory near Indian Territory for 17 years before the two became the state of Oklahoma. the university had 29,931 students enrolled, most located at its...
chased a tornado in Red Rock, Oklahoma
Red Rock, Oklahoma
Red Rock is a town in Noble County, Oklahoma, United States. The population was 293 at the 2000 census...
and used a portable Doppler weather radar
Weather radar
Weather radar, also called weather surveillance radar and Doppler weather radar, is a type of radar used to locate precipitation, calculate its motion, estimate its type . Modern weather radars are mostly pulse-Doppler radars, capable of detecting the motion of rain droplets in addition to the...
to measure a wind speed of 460 km/h (286 mph).
According to Alan F. Arbogast ("Discovering Physical Geography") wind direction and speed are affected by three main factors:
- Pressure gradient - the difference in barometric pressure between adjacent zones of high and low pressure.
- Frictional forces - features on the Earth's surface which oppose the wind; e.g.: mountains, trees, buildings, etc.
- Coriolis effect - the Earth's rotation causes winds to be deflected to the right in the Northern Hemisphere, and in the Southern Hemisphere to the left.
All three of these combined result in the spiral motion of air in both high and low pressure systems.
Design of structures
Main article: Wind EngineeringWind engineering
Wind engineering analyzes effects of wind in the natural and the built environment and studies the possible damage, inconvenience or benefits which may result from wind. In the field of structural engineering it includes strong winds, which may cause discomfort, as well as extreme winds, such as in...
Wind speed is a common factor in the design of structures and buildings around the world. The wind speed is often the governing factor in the "lateral" design of a structure and is used by professional engineers and designers.
In the United States, the wind speed used in design is often referred to as a "3-second gust" which is the highest sustained gust over a 3 second period having a probability of being exceeded per year of 1 in 50 (ASCE 7-latest edition). Windspeedbyzip maps out the design wind speed as suggested by ASCE 7-05 for the United States. This design wind speed is accepted by most building codes in the United States and often times governs the lateral design of buildings and structures.
In Canada, reference wind pressures are used in design and are based on the "mean hourly" wind speed having a probability of being exceeded per year of 1 in 50. The reference wind pressure (q) is calculated in Pascals using the following equation (ref: NBC 2005 Structural Commentaries - Part 4 of Div. B, Comm. I): q=(1/2)pV**2
where p is the air density in kg/m**3 and V is wind speed in m/s.
Historically, wind speeds have been reported with a variety of averaging times (fastest mile, 3-second gust, 1-minute and mean hourly for example) which designers may have to take into account. To convert wind speeds from one averaging time to another, the Durst Curve (Ref: ASCE 7-05 commentary Figure C6-4) was developed which defines the relation between probable maximum wind speed averaged over t seconds, V(t), and mean wind speed over one hour V(3600).
See also
- Beaufort scaleBeaufort scaleThe Beaufort Scale is an empirical measure that relates wind speed to observed conditions at sea or on land. Its full name is the Beaufort Wind Force Scale.-History:...
- Fujita scaleFujita scaleThe Fujita scale , or Fujita-Pearson scale, is a scale for rating tornado intensity, based primarily on the damage tornadoes inflict on human-built structures and vegetation...
and Enhanced Fujita ScaleEnhanced Fujita ScaleThe Enhanced Fujita Scale rates the strength of tornadoes in the United States based on the damage they cause.Implemented in place of the Fujita scale introduced in 1971 by Ted Fujita, it began operational use on February 1, 2007. The scale has the same basic design as the original Fujita scale:... - Prevailing wind
- Saffir-Simpson Hurricane ScaleSaffir-Simpson Hurricane ScaleThe Saffir–Simpson Hurricane Scale , or the Saffir–Simpson Hurricane Wind Scale , classifies hurricanes — Western Hemisphere tropical cyclones that exceed the intensities of tropical depressions and tropical storms — into five categories distinguished by the intensities of their sustained winds...
- TORRO scaleTORRO scaleThe TORRO tornado intensity scale is a scale measuring tornado intensity between T0 and T11. It was developed by Terence Meaden of the Tornado and Storm Research Organisation , a meteorological organisation in the United Kingdom, as an extension of the Beaufort scale.- History and derivation from...
- Wind directionWind directionWind direction is reported by the direction from which it originates. For example, a northerly wind blows from the north to the south. Wind direction is usually reported in cardinal directions or in azimuth degrees...
- Knot (unit)