Royd Moor Wind Farm in the United Kingdom

Bangui Wind Farm in the Philippines.

Lillgrund Wind Farm in Sweden

A wind farm is a group of wind turbines in the same location used to produce electric power. A large wind farm may consist of several hundred individual wind turbines, and cover an extended area of hundreds of square miles, but the land between the turbines may be used for agricultural or other purposes. A wind farm may also be located offshore.

Many of the largest operational onshore wind farms are located in the USA and China. The Gansu Wind Farm in China has over 5,000 MW installed with a goal of 20,000 MW by 2020. China has several other "wind power bases" of similar size. The Alta Wind Energy Center in California is the largest onshore wind farm outside of China, with a capacity of 1020 MW of power^[1]. As of February 2012, the Walney Wind Farm in United Kingdom is the largest offshore wind farm in the world at 367 MW, followed by Thanet Offshore Wind Project (300 MW), also in the UK.

There are many large wind farms under construction and these include Anholt Offshore Wind Farm (400 MW), BARD Offshore 1 (400 MW), Clyde Wind Farm (548 MW), Greater Gabbard wind farm (500 MW), Lincs Wind Farm (270 MW), London Array (1000 MW), Lower Snake River Wind Project (343 MW), Macarthur Wind Farm (420 MW), Shepherds Flat Wind Farm (845 MW), and Sheringham Shoal (317 MW).

Design[link]

A large wind farm may consist of several hundred individual wind turbines, and cover an extended area of hundreds of square miles, but the land between the turbines may be used for agricultural or other purposes. A wind farm may be located offshore to take advantage of strong winds blowing over the surface of an ocean or lake.

Map of available wind power over the United States. Color codes indicate wind power density class

As a general rule, economic wind generators require windspeed of 10 mph (16 km/h) or greater. An ideal location would have a near constant flow of non-turbulent wind throughout the year, with a minimum likelihood of sudden powerful bursts of wind. An important factor of turbine siting is also access to local demand or transmission capacity.

Usually sites are screened on the basis of a wind atlas, and validated with wind measurements. Meteorological wind data alone is usually not sufficient for accurate siting of a large wind power project. Collection of site specific data for wind speed and direction is crucial to determining site potential^[2][3] in order to finance the project.^[4] Local winds are often monitored for a year or more, and detailed wind maps constructed before wind generators are installed.

The wind blows faster at higher altitudes because of the reduced influence of drag. The increase in velocity with altitude is most dramatic near the surface and is affected by topography, surface roughness, and upwind obstacles such as trees or buildings. Typically, the increase of wind speeds with increasing height follows a wind profile power law, which predicts that wind speed rises proportionally to the seventh root of altitude. Doubling the altitude of a turbine, then, increases the expected wind speeds by 10% and the expected power by 34%.

Individual turbines are interconnected with a medium voltage (usually 34.5 kV) power collection system and communications network. At a substation, this medium-voltage electrical current is increased in voltage with a transformer for connection to the high voltage transmission system. Construction of a land-based wind farm requires installation of the collector system and substation, and possibly access roads to each turbine site.

Onshore installations[link]

Part of the Biglow Canyon Wind Farm, with a turbine under construction

The world's first wind farm – consisting of 20 wind turbines rated at 30 kilowatts each – was installed on the shoulder of Crotched Mountain in southern New Hampshire in December, 1980.^[5][6]

Falling costs mean that the average onshore wind farm will be competitive with natural gas-fired power generation by 2016.^[7]

Onshore turbine installations in hilly or mountainous regions tend to be on ridgelines generally three kilometers or more inland from the nearest shoreline. This is done to exploit the topographic acceleration as the wind accelerates over a ridge. The additional wind speeds gained in this way can increase energy produced because more wind goes through the turbines. The exact position of each turbines matters, because a difference of 30m could potentially double output. This careful placement is referred to as 'micro-siting'.

Offshore installations[link]

Offshore wind turbines near Copenhagen

Europe is the leader in offshore wind energy, with the first offshore wind farm being installed in Denmark in 1991. As of 2010, there are 39 offshore wind farms in waters off Belgium, Denmark, Finland, Germany, Ireland, the Netherlands, Norway, Sweden and the United Kingdom, with a combined operating capacity of 2,396 MW. More than 100 GW (or 100, 000 MW) of offshore projects are proposed or under development in Europe. The European Wind Energy Association has set a target of 40 GW installed by 2020 and 150 GW by 2030.^[14]

As of February 2012, the Walney Wind Farm in United Kingdom is the largest offshore wind farm in the world at 367 MW, followed by Thanet Offshore Wind Project (300 MW), also in the UK. The London Array (630 MW) is the largest project under construction.

World's largest offshore wind farms

Wind farm	Capacity (MW)	Country	Turbines and model	Commissioned	References
Walney	367	United Kingdom	102 × Siemens SWT-3.6	2012	[15][16]
Thanet	300	United Kingdom	100 × Vestas V90-3MW	2010	[17][18]
Horns Rev II	209	Denmark	91 × Siemens 2.3-93	2009	[19]
Rødsand II	207	Denmark	90 × Siemens 2.3-93	2010	[20]
Lynn and Inner Dowsing	194	United Kingdom	54 × Siemens 3.6-107	2008	[21][22][23][24]
Robin Rigg (Solway Firth)	180	United Kingdom	60 × Vestas V90-3MW	2010	[25][26]
Gunfleet Sands	172	United Kingdom	48 × Siemens 3.6-107	2010	[26][27]
Nysted (Rødsand I)	166	Denmark	72 × Siemens 2.3	2003	[21][28][29]
Ormonde	150	United Kingdom	30 × REpower 5.0	2011	[26][30]

Offshore wind turbines are less obtrusive than turbines on land, as their apparent size and noise is mitigated by distance. Because water has less surface roughness than land (especially deeper water), the average wind speed is usually considerably higher over open water. Capacity factors (utilisation rates) are considerably higher than for onshore locations.^[31]

The province of Ontario in Canada is pursuing several proposed locations in the Great Lakes, including the suspended^[32] Trillium Power Wind 1 approximately 20 km from shore and over 400 MW in size.^[33] Other Canadian projects include one on the Pacific west coast.^[34]

As of 2010, there are no offshore wind farms in the United States. However, projects are under development in wind-rich areas of the East Coast, Great Lakes, and Pacific coast.^[14]

Installation and service / maintenance of off-shore wind farms are a specific challenge for technology and economic operation of a wind farm. Service vessels have to be operated nearly 24/7 (availability higher than 80 % of time) to get sufficient amortisation from the wind turbines.^{[citation needed]} Therefore special fast service vehicles for installation (like Wind Turbine Shuttle) as well as for maintenance (including heave compensation and heave compensated working platforms to allow the service staff to enter the wind turbine also at difficult weather conditions) are required. So-called inertial and optical based Ship Stabilization and Motion Control systems (iSSMC) are used for that.

By region[link]

Australia[link]

Canada[link]

Image of Pubnico Wind Farm taken from Beach Point, Lower East Pubnico, Nova Scotia

Large wind farms in Canada^[38]

Name	Capacity (MW)	Location	Province
Anse-à-Valleau Wind Farm	100	49°03′36″N 64°33′18″W / 49.06°N 64.555°W / 49.06; -64.555 (Anse-à-Valleau Wind Farm)	Quebec
Caribou Wind Park	99	70 km west of Bathurst	New Brunswick
Bear Mountain Wind Park	120	Dawson Creek	British Columbia
Centennial Wind Power Facility	150	Swift Current	Saskatchewan
Enbridge Ontario Wind Farm	181	Kincardine	Ontario
Erie Shores Wind Farm	99	Port Burwell	Ontario
Jardin d'Eole Wind Farm	127	Saint-Ulric	Quebec
Kent Hills Wind Farm	96	Riverside-Albert	New Brunswick
Melancthon EcoPower Centre	199	Melancthon	Ontario
Port Alma Wind Farm	101	Chatham-Kent	Ontario
Chatham Wind Farm	101	Chatham-Kent	Ontario
Prince Township Wind Farm	189	Sault Ste. Marie	Ontario
St. Joseph Wind Farm	138	Montcalm	Manitoba
St. Leon Wind Farm	99	St. Leon	Manitoba
Wolfe Island Wind Project	197	Frontenac Islands	Ontario

China[link]

Wind farm in Xinjiang, China

In just five years, China leapfrogged the rest of the world in wind energy production, going from 2,599 MW of capacity in 2006 to 62,733 MW at the end of 2011. ^[39][40][41]. However, the rapid growth outpaced China's infrastructure and new construction slowed significantly in 2012.^[42]

At the end of 2009, wind power in China accounted for 25.1 gigawatts (GW) of electricity generating capacity,^[43] and China has identified wind power as a key growth component of the country's economy.^[44] With its large land mass and long coastline, China has exceptional wind resources.^[45] Researchers from Harvard and Tsinghua University have found that China could meet all of their electricity demands from wind power through 2030.^[46]

By the end of 2008, at least 15 Chinese companies were commercially producing wind turbines and several dozen more were producing components.^[47] Turbine sizes of 1.5 MW to 3 MW became common. Leading wind power companies in China were Goldwind, Dongfang Electric, and Sinovel^[48] along with most major foreign wind turbine manufacturers.^[49] China also increased production of small-scale wind turbines to about 80,000 turbines (80 MW) in 2008. Through all these developments, the Chinese wind industry appeared unaffected by the global financial crisis, according to industry observers.^[48]

According to the Global Wind Energy Council, the development of wind energy in China, in terms of scale and rhythm, is absolutely unparalleled in the world. The National People's Congress permanent committee passed a law that requires the Chinese energy companies to purchase all the electricity produced by the renewable energy sector.^[50]

European Union[link]

Parts of this article (those related to section) are outdated. Please update this article to reflect recent events or newly available information. Please see the talk page for more information. (December 2010)

A wind farm in a mountainous area in Galicia, Spain

Wind farm in Lower Saxony, Germany

The European Union has a total installed wind capacity of 74,767MW. Germany has the second largest number of wind farms in the world after the United States. Its installed capacity was 25,777 MW at the End of 2009. The fourth country in capacity was Spain with 19,149 MW. Italy was in the sixth position, with 4,850 MW.^[51]

Whitelee Wind Farm near Glasgow, Scotland is Europe's largest windfarm with a total capacity of 322 MW. In 2012 Europe's largest windfarm will be Fântânele-Cogealac Wind Farm near Tulcea, Romania with 600 MW (300 MW already operational) ^[52] ).

In 2006, the British government gave planning consent for the world's largest offshore wind farm, the 'London Array'. It is currently being built 12 miles off of the Kent coast and will include 341 turbines when finished.

An important limiting factor of wind power is variable power generated by wind farms. In most locations the wind blows only part of the time, which means that there has to be back-up capacity of conventional generating capacity to cover periods that the wind is not blowing. To address this issue it has been proposed to create a "supergrid" to connect national grids together^[53] across western Europe, ranging from Denmark across the southern North Sea to England and the Celtic Sea to Ireland, and further south to France and Spain especially in Higueruela which was considered for some time the biggest wind farm in the world.^[54] The idea is that by the time a low pressure area has moved away from Denmark to the Baltic Sea the next low appears off the coast of Ireland. Therefore, while it is true that the wind is not blowing everywhere all of the time, it will always be blowing somewhere.

South Africa[link]

Turbines at the Darling Wind Farm, South Africa.

There are currently no large scale wind farms operational in South Africa, though a number are in the initial planning stages. Most of these are earmarked for locations along the Eastern Cape coastline.^[55][56][57] Eskom has constructed one small scale prototype windfarm at Klipheuwel in the Western Cape and another demonstrator site is near Darling with phase 1 completed. The first commercial wind farm, Coega Wind Farm in Port Elisabeth, was developed by the Belgian company Electrawinds.

United States[link]

Brazos Wind Farm in the plains of West Texas

U.S. wind power installed capacity now exceeds 46,919 MW and supplies 3% of the nation's electricity.^[65][66]

New installations place the U.S. on a trajectory to generate 20% of the nation’s electricity by 2030 from wind energy.^[67] Growth in 2008 channeled some $17 billion into the economy, positioning wind power as one of the leading sources of new power generation in the country, along with natural gas. Wind projects completed in 2008 accounted for about 42% of the entire new power-producing capacity added in the U.S. during the year.^[68]

At the end of 2008, about 85,000 people were employed in the U.S. wind industry,^[69] and GE Energy was the largest domestic wind turbine manufacturer.^[70] Wind projects boosted local tax bases and revitalized the economy of rural communities by providing a steady income stream to farmers with wind turbines on their land.^[70] Wind power in the U.S. provides enough electricity to power the equivalent of nearly 9 million homes, avoiding the emissions of 57 million tons of carbon each year and reducing expected carbon emissions from the electricity sector by 2.5%.^[68]

Texas, with 10,394 MW of capacity, has the most installed wind power capacity of any U.S. state, followed by Iowa with 3,917 MW.^[71] The Alta Wind Energy Center (1,020 MW) in California is the nation's largest wind farm in terms of capacity.^[72] Altamont Pass Wind Farm is the largest wind farm in the U.S. in terms of the number of individual turbines.^[73]

Impact[link]

Environmental and aesthetic impact[link]

Livestock ignore wind turbines,^[74] and continue to graze as they did before wind turbines were installed.

Compared to the environmental impact of traditional energy sources, the environmental impact of wind power is relatively minor. Wind power consumes no fuel, and emits no air pollution, unlike fossil fuel power sources. The energy consumed to manufacture and transport the materials used to build a wind power plant is equal to the new energy produced by the plant within a few months. While a wind farm may cover a large area of land, many land uses such as agriculture are compatible, with only small areas of turbine foundations and infrastructure made unavailable for use.^[75]

There are reports of bird and bat mortality at wind turbines as there are around other artificial structures. The scale of the ecological impact may^[76] or may not be significant, depending on specific circumstances. Prevention and mitigation of wildlife fatalities, and protection of peat bogs, affect the siting and operation of wind turbines.

There are anecdotal reports of negative effects from noise on people who live very close to wind turbines. Peer-reviewed research has generally not supported these statements.^[77]

Effect on power grid[link]

Utility-scale wind farms must have access to transmission lines to transport energy. The wind farm developer may be obligated to install extra equipment or control systems in the wind farm to meet the technical standards set by the operator of a transmission line. The company or person that develops the wind farm can then sell the power on the grid through the transmission lines and ultimately chooses whether to hold on to the rights or sell the farm or parts of it to big business like GE, for example.

Ground radar interference[link]

Wind farm interference (in yellow circle) on radar map

Wind farms can interfere with ground radar systems used for defense, weather and air traffic control. The large, rapidly moving blades of the turbines can return signals to the radar that can be mistaken as an aircraft or weather pattern.^[78] Actual aircraft and weather patterns around wind farms can be accurately detected, as there is no fundamental physical constraint preventing that. But aging radar infrastructure is significantly challenged with the task.^[79][80] The US military is using wind turbines on some bases, including Barstow near the radar test facility.^[81][82]

Effects[link]

The level of interference is a function of the signal processors used within the radar, the speed of the aircraft and the relative orientation of wind turbines/aircraft with respect to the radar. An aircraft flying above the wind farm's turning blades could become impossible to detect because the blade tips can be moving at nearly aircraft velocity. Studies are currently being performed to determine the level of this interference and will be used in future site planning.^[83] Issues include masking (shadowing), clutter (noise), and signal alteration.^[84] Radar issues have stalled as much as 10,000 MW of projects in USA.^[85]

Some very long range radars are not affected by wind farms.^[86]

Mitigation[link]

Permanent problem solving include Non-Initiation Window to hide the turbines while still tracking aircraft over the wind farm, and a similar method mitigates the false returns.^[87] England's Newcastle Airport is using a short-term mitigation; to "blank" the turbines on the radar map with a software patch.^[88] Wind turbine blades using stealth technology are being developed to mitigate radar reflection problems for aviation.^{[89][90][91][92]} As well as stealth windfarms, the future development of infill radar systems could filter out the turbine interference.

In early 2011, the U.S. government awarded a program to build a radar/wind turbine analysis tool. This tool will allow developers to predict the impact of a wind farm on a radar system before construction, thus allowing rearrangement of the turbines or even the entire wind farm to avoid negative impacts on the radar system.^[83][93]

A mobile radar system, the Lockheed Martin TPS-77, has shown in recent tests that it can distinguish between aircraft and wind turbines, and more than 170 TPS-77 radars are in use around the world. In Britain, the Lockheed Martin TPS-77 will be delivered and installed in November 2011 at Trimingham in Norfolk, removing military objections to a series of offshore wind farms in the North Sea. A second TPS-77 is to be installed in the Scottish Borders, overcoming objections to a 48-turbine wind farm at Fallago.^[94]

Agriculture[link]

The professor of atmospheric science Somnath Baidya Roy of the University of Illinois, in a study published in October 2010 in the scientific journal PNAS^[95] shows that in the immediate vicinity of wind farms, the climate is cooler during the day and slightly warmer during the night than the surrounding areas. According to Roy, the effect is due to the turbulence generated by the blades.

In another study conducted by Gene Takle and Julie Lundquist University of Colorado, presented at San Francisco conference of the American Geophysical Union Fall Meeting (December 13–18, 2010), the analysis carried out on corn and soybean crops in the central areas of the United States has noted that the microclimate generated by wind turbines improves crops as it prevents the spring and autumn frosts, and it reduces the action of pathogenic fungi that grow on the leaves. Even at the height of summer heat, the lowering of 2.5-3 degrees above the crops due to turbulence caused by the blades, can make a difference for the cultivation of maize.^[96]

See also[link]

	Renewable energy portal
	Sustainable development portal

• Floating wind turbine
• Renewable energy commercialization
• Sea breeze
• Wave farm
• Wind energy software
• Wind turbines on public display

References[link]

Further reading[link]

• Righter, Robert W. Windfall: Wind Energy in America Today (University of Oklahoma Press; 2011) 219 pages; looks at the land-use decisions involved in setting up a wind farm.

External links[link]

Wikimedia Commons has media related to: Wind farm

• World Wind Energy Association
• Wind Power in the United States: Technology, Economic, and Policy Issues (53p), Congressional Research Service, June 2008
• Database of projects throughout the World
• Database of offshore wind projects in North America
• Wind Project Community Organizing
• World Wind Energy Association
• 4C Offshore's Global Wind Farm Interactive Map and Database
• The Largest Windparks — Index of planned and operational wind farms at RE-Database

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Wind, from the Tacuinum Sanitatis

A breeze lifts a veil

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. Winds are commonly classified by their spatial scale, their speed, the types of forces that cause them, the regions in which they occur, and their effect. The strongest observed winds on a planet in our solar system occur on Neptune and Saturn.

In meteorology, winds are often referred to according to their strength, and the direction from which the wind is blowing. Short bursts of high speed wind are termed gusts. Strong winds of intermediate duration (around one minute) are termed squalls. Long-duration winds have various names associated with their average strength, such as breeze, gale, storm, hurricane, and typhoon. Wind occurs on a range of scales, from thunderstorm flows lasting tens of minutes, to local breezes generated by heating of land surfaces and lasting a few hours, to global winds resulting from the difference in absorption of solar energy between the climate zones on Earth. The two main causes of large-scale atmospheric circulation are the differential heating between the equator and the poles, and the rotation of the planet (Coriolis effect). Within the tropics, thermal low circulations over terrain and high plateaus can drive monsoon circulations. In coastal areas the sea breeze/land breeze cycle can define local winds; in areas that have variable terrain, mountain and valley breezes can dominate local winds.

In human civilization, wind has inspired mythology, influenced the events of history, expanded the range of transport and warfare, and provided a power source for mechanical work, electricity and recreation. Wind powers the voyages of sailing ships across Earth's oceans. Hot air balloons use the wind to take short trips, and powered flight uses it to increase lift and reduce fuel consumption. Areas of wind shear caused by various weather phenomena can lead to dangerous situations for aircraft. When winds become strong, trees and man-made structures are damaged or destroyed.

Winds can shape landforms, via a variety of aeolian processes such as the formation of fertile soils, such as loess, and by erosion. Dust from large deserts can be moved great distances from its source region by the prevailing winds; winds that are accelerated by rough topography and associated with dust outbreaks have been assigned regional names in various parts of the world because of their significant effects on those regions. Wind affects the spread of wildfires. Winds disperse seeds from various plants, enabling the survival and dispersal of those plant species, as well as flying insect populations. When combined with cold temperatures, wind has a negative impact on livestock. Wind affects animals' food stores, as well as their hunting and defensive strategies.

Cause[link]

Surface analysis of Great Blizzard of 1888. Areas with greater isobaric packing indicate higher winds.

Wind is caused by differences in pressure. When a difference in pressure exists, the air is accelerated from higher to lower pressure. On a rotating planet, the air will be deflected by the Coriolis effect, except exactly on the equator. Globally, the two major driving factors of large-scale winds (the atmospheric circulation) are the differential heating between the equator and the poles (difference in absorption of solar energy leading to buoyancy forces) and the rotation of the planet. Outside the tropics and aloft from frictional effects of the surface, the large-scale winds tend to approach geostrophic balance. Near the Earth's surface, friction causes the wind to be slower than it would be otherwise. Surface friction also causes winds to blow more inward into low pressure areas.^[1]

Winds defined by an equilibrium of physical forces are used in the decomposition and analysis of wind profiles. They are useful for simplifying the atmospheric equations of motion and for making qualitative arguments about the horizontal and vertical distribution of winds. The geostrophic wind component is the result of the balance between Coriolis force and pressure gradient force. It flows parallel to isobars and approximates the flow above the atmospheric boundary layer in the midlatitudes.^[2] The thermal wind is the difference in the geostrophic wind between two levels in the atmosphere. It exists only in an atmosphere with horizontal temperature gradients.^[3] The ageostrophic wind component is the difference between actual and geostrophic wind, which is responsible for air "filling up" cyclones over time.^[4] The gradient wind is similar to the geostrophic wind but also includes centrifugal force (or centripetal acceleration).^[5]

Measurement[link]

A windmill style of anemometer

An occluded mesocyclone tornado (Oklahoma, May 1999)

Wind direction is reported by the direction from which it originates. For example, a northerly wind blows from the north to the south.^[6] Weather vanes pivot to indicate the direction of the wind.^[7] At airports, windsocks are primarily used to indicate wind direction, but can also be used to estimate wind speed by its angle of hang.^[8] Wind speed is measured by anemometers, most commonly using rotating cups or propellers. When a high measurement frequency is needed (such as in research applications), wind can be measured by the propagation speed of ultrasound signals or by the effect of ventilation on the resistance of a heated wire.^[9] Another type of anemometer uses pitot tubes that take advantage of the pressure differential between an inner tube and an outer tube that is exposed to the wind to determine the dynamic pressure, which is then used to compute the wind speed.^[10]

Sustained wind speeds are reported globally at a 10 meters (33 ft) height and are averaged over a 10 minute time frame. The United States reports winds over a 1 minute average for tropical cyclones,^[11] and a 2 minute average within weather observations.^[12] India typically reports winds over a 3 minute average.^[13] Knowing the wind sampling average is important, as the value of a one-minute sustained wind is typically 14% greater than a ten-minute sustained wind.^[14] A short burst of high speed wind is termed a wind gust, one technical definition of a wind gust is: the maxima that exceed the lowest wind speed measured during a ten minute time interval by 10 knots (19 km/h). A squall is a doubling of the wind speed above a certain threshold, which lasts for a minute or more.

To determine winds aloft, rawinsondes determine wind speed by GPS, radio navigation, or radar tracking of the probe.^[15] Alternatively, movement of the parent weather balloon position can be tracked from the ground visually using theodolites.^[16] Remote sensing techniques for wind include SODAR, Doppler LIDARs and RADARs, which can measure the Doppler shift of electromagnetic radiation scattered or reflected off suspended aerosols or molecules, and radiometers and radars can be used to measure the surface roughness of the ocean from space or airplanes. Ocean roughness can be used to estimate wind velocity close to the sea surface over oceans. Geostationary satellite imagery can be used to estimate the winds throughout the atmosphere based upon how far clouds move from one image to the next. Wind Engineering describes the study of the effects of the wind on the built environment, including buildings, bridges and other man-made objects.

Wind force scale[link]

Historically, the Beaufort wind force scale provides an empirical description of wind speed based on observed sea conditions. Originally it was a 13-level scale, but during the 1940s, the scale was expanded to 17 levels.^[17] There are general terms that differentiate winds of different average speeds such as a breeze, a gale, a storm, tornado, or a hurricane. Within the Beaufort scale, gale-force winds lie between 28 knots (52 km/h) and 55 knots (102 km/h) with preceding adjectives such as moderate, fresh, strong, and whole used to differentiate the wind's strength within the gale category.^[18] A storm has winds of 56 knots (104 km/h) to 63 knots (117 km/h).^[19] The terminology for tropical cyclones differs from one region to another globally. Most ocean basins use the average wind speed to determine the tropical cyclone's category. Below is a summary of the classifications used by Regional Specialized Meteorological Centers worldwide:

Fujita scale[link]

The Enhanced Fujita Scale (EF Scale) rates the strength of tornadoes in the United States based on the damage they cause. Below is that scale.

Scale	Wind speed		Relative frequency	Potential damage
	mph	km/h
EF0	65–85	105–137	53.5%	Minor or no damage. Peels surface off some roofs; some damage to gutters or siding; branches broken off trees; shallow-rooted trees pushed over. Confirmed tornadoes with no reported damage (i.e., those that remain in open fields) are always rated EF0.
EF1	86–110	138–178	31.6%	Moderate damage. Roofs severely stripped; mobile homes overturned or badly damaged; loss of exterior doors; windows and other glass broken.
EF2	111–135	179–218	10.7%	Considerable damage. Roofs torn off well-constructed houses; foundations of frame homes shifted; mobile homes completely destroyed; large trees snapped or uprooted; light-object missiles generated; cars lifted off ground.
EF3	136–165	219–266	3.4%	Severe damage. Entire stories of well-constructed houses destroyed; severe damage to large buildings such as shopping malls; trains overturned; trees debarked; heavy cars lifted off the ground and thrown; structures with weak foundations are badly damaged.
EF4	166–200	267–322	0.7%	Extreme damage. Well-constructed and whole frame houses completely leveled; cars and other large objects thrown and small missiles generated.
EF5	>200	>322	<0.1%	Total Destruction. Strong-framed, well-built houses leveled off and foundations swept away; steel-reinforced concrete structures are critically damaged; tall buildings collapse or have severe structural deformations.

Station model[link]

Wind plotting within a station model

The station model plotted on surface weather maps uses a wind barb to show both wind direction and speed. The wind barb shows the speed using "flags" on the end.

• Each half of a flag depicts 5 knots (9.3 km/h) of wind.
• Each full flag depicts 10 knots (19 km/h) of wind.
• Each pennant (filled triangle) depicts 50 knots (93 km/h) of wind.^[21]

Winds are depicted as blowing from the direction the barb is facing. Therefore, a northeast wind will be depicted with a line extending from the cloud circle to the northeast, with flags indicating wind speed on the northeast end of this line.^[22] Once plotted on a map, an analysis of isotachs (lines of equal wind speeds) can be accomplished. Isotachs are particularly useful in diagnosing the location of the jet stream on upper level constant pressure charts, and are usually located at or above the 300 hPa level.^[23]

Wind energy[link]

Wind energy is the kinetic energy of the air in motion. Total wind energy flowing through an imaginary area A during the time t is:

E = A·v·t·ρ·½ v²,

where v is the wind velocity and ρ is the air density. The formula presented is structured in two parts: (A·v·t) is the volume of air passing through A, which is considered perpendicular to the wind velocity; (ρ·½ v²) is the kinetic energy of the moving air per unit volume.

Total wind power is:

P = E/t = A·ρ·½ v³

Wind power is thus proportional to the third power of the wind velocity.

Theoretical power captured by a wind turbine[link]

Total wind power could be captured only if the wind velocity is reduced to zero. In a realistic wind turbine this is impossible, as the captured air must also leave the turbine. A relation between the input and output wind velocity must be considered. Using the concept of stream tube, the maximal achievable extraction of wind power by a wind turbine is 59% of the total theoretical wind power^[24] (see: Betz' law).

Practical wind turbine power[link]

Further insufficiencies, such as rotor blade friction and drag, gearbox losses, generator and converter losses, reduce the power delivered by a wind turbine. The basic relation that the turbine power is (approximately) proportional to the third power of velocity remains.

Global climatology[link]

The westerlies and trade winds

Winds are part of Earth's atmospheric circulation.

Easterly winds, on average, dominate the flow pattern across the poles, westerly winds blow across the mid-latitudes of the earth, to the north of the subtropical ridge, while easterlies again dominate the tropics.

Directly under the subtropical ridge are the doldrums, or horse latitudes, where winds are lighter. Many of the Earth's deserts lie near the average latitude of the subtropical ridge, where descent reduces the relative humidity of the air mass.^[25] The strongest winds are in the mid-latitudes where cold Arctic air meets warm air from the tropics.

Tropics[link]

The trade winds (also called trades) are the prevailing pattern of easterly surface winds found in the tropics towards the Earth's equator.^[26] The trade winds blow predominantly from the northeast in the Northern Hemisphere and from the southeast in the Southern Hemisphere.^[27] The trade winds act as the steering flow for tropical cyclones that form over world's oceans.^[28] Trade winds also steer African dust westward across the Atlantic Ocean into the Caribbean Sea, as well as portions of southeast North America.^[29]

A monsoon is a seasonal prevailing wind that lasts for several months within tropical regions. The term was first used in English in India, Bangladesh, Pakistan, and neighboring countries to refer to the big seasonal winds blowing from the Indian Ocean and Arabian Sea in the southwest bringing heavy rainfall to the area.^[30] Its poleward progression is accelerated by the development off a heat low over the Asian, African, and North American continents during May through July, and over Australia in December.^[31][32][33]

Westerlies and their impact[link]

Benjamin Franklin's map of the Gulf Stream

The Westerlies or the Prevailing Westerlies are the prevailing winds in the middle latitudes between 35 and 65 degrees latitude. These prevailing winds blow from the west to the east to the north of the subtropical ridge,^[34][35] and steer extratropical cyclones in this general manner. The winds are predominantly from the southwest in the Northern Hemisphere and from the northwest in the Southern Hemisphere.^[27] They are strongest in the winter when the pressure is lower over the poles, and weakest during the summer and when pressures are higher over the poles.^[36]

Together with the trade winds, the westerlies enabled a round-trip trade route for sailing ships crossing the Atlantic and Pacific Oceans, as the westerlies lead to the development of strong ocean currents on the western sides of oceans in both hemispheres through the process of western intensification.^[37] These western ocean currents transport warm, sub tropical water polewards toward the polar regions. The westerlies can be particularly strong, especially in the southern hemisphere, where there is less land in the middle latitudes to cause the flow pattern to amplify, which slows the winds down. The strongest westerly winds in the middle latitudes are within a band known as the Roaring Forties, between 40 and 50 degrees latitude south of the equator.^[38] The Westerlies play an important role in carrying the warm, equatorial waters and winds to the western coasts of continents,^[39][40] especially in the southern hemisphere because of its vast oceanic expanse.

Polar easterlies[link]

The polar easterlies, also known as Polar Hadley cells, are dry, cold prevailing winds that blow from the high-pressure areas of the polar highs at the north and south poles towards the low-pressure areas within the Westerlies at high latitudes. Unlike the Westerlies, these prevailing winds blow from the east to the west, and are often weak and irregular.^[41] Because of the low sun angle, cold air builds up and subsides at the pole creating surface high-pressure areas, forcing an equatorward outflow of air;^[42] that outflow is deflected eastward by the Coriolis effect.

Local considerations[link]

Local winds around the world. These winds are formed through the heating of land (from mountains or flat terrain)

Sea and land breezes[link]

A: Sea breeze (occurs at daytime), B: Land breeze (occurs at night)

In coastal regions, sea breezes and land breezes can be important factors in a location's prevailing winds. The sea is warmed by the sun more slowly because of water's greater specific heat compared to land.^[43] As the temperature of the surface of the land rises, the land heats the air above it by conduction. The warm air is less dense than the surrounding environment and so it rises. This causes a pressure gradient of about 2 millibars from the ocean to the land. The cooler air above the sea, now with higher sea level pressure, flows inland into the lower pressure, creating a cooler breeze near the coast. When large-scale winds are calm, the strength of the sea breeze is directly proportional to the temperature difference between the land mass and the sea. If an offshore wind of 8 knots (15 km/h) exists, the sea breeze is not likely to develop.