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For other uses, see Wind (disambiguation).
Wind, from the Tacuinum Sanitatis

Wind is the flow of air or other gases that compose an atmosphere (including, but not limited to, that of the planet Earth). On Earth, wind consists of air molecules in motion. 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 elements from a planet's atmosphere into space.

Differences in density between two air masses lead to wind. Winds are commonly classified by their spatial scale, their speed, the types of forces that cause them, the geographic regions in which they occur, and their effect. Winds are plotted on surface weather analyses indicating the direction the wind is blowing from as well as its strength, and over much of the globe wind speed are measured over a ten-minute time frame, with the United States and India using different averaging intervals.

Long-duration wind speeds have various names associated with their average strength, such as breeze, gale, storm, hurricane, and typhoon. Shorter duration winds, such as wind gusts, exceed the minimum value over the observed time frame and can cause substantial damage to power lines and suspension bridges. Winds with an intermediate duration, which sharply increase and last for a minute are termed squalls. While wind is often a standalone weather phenomenon, it can also occur as part of a storm system, most notably in a cyclone.

Winds can shape landforms, via a variety of aeolian processes such as the formation of fertile soils, such as loess, and by erosion. Differential heating between the poles and the equator lead to the development of the jet stream and the associated climatological mid-latitude westerlies, polar easterlies, and the trade winds.

Wind occurs on a range of scales, from local breezes generated by heating of land surfaces and lasting tens of minutes, to global winds resulting from the difference in absorption of solar energy between the climate zones on Earth. The two major driving factors of large scale atmospheric circulation are the differential heating between the equator and the poles, which causes the jet stream and the associated climatological mid-latitude westerlies, polar easterlies, and the trade winds, and the rotation of the planet (Coriolis effect), which causes the circular motion of air around areas of high and low pressure.

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 has been used to steer sailing ships across vast oceans. By air, hot air balloons use the wind to take short trips. Airships have historically been used for longer trips, but nowadays are used for a variety of monitoring efforts such as during public sporting events and drug trafficking efforts. Wind can be dangerous, as areas of wind shear caused by various weather phenomena can lead to dangerous situations for airplanes. When winds become strong, trees and man-made structures are damaged or destroyed. Nature uses wind to help disperse seeds from various plants, in order to enable the survival of those plant species. Dust from large deserts can be moved large distances from their source region by the prevailing winds. In Modern Western Nations, wind has the detrimental effect of ruining long or elaborate [hairstyles], leading some individuals to dislike or even [hate] the wind.

Cause

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

The first known scientific description of wind was from the seventeenth-century Italian physicist Evangelista Torricelli,

... winds are produced by differences of air temperature, and hence density, between two regions of the earth.[1]

Other forces which drive wind or affect it are the pressure gradient force, the coriolis force, buoyancy forces, and friction forces. When a difference in density exists between two adjacent air masses, the air tends to flow from the regions of higher to lower pressure. On a rotating planet, this air flow will be acted upon by the Coriolis force, in regions sufficiently far from the equator and sufficiently high above the surface. Surface friction with land causes winds to blow more inward into low pressure areas.[2]

Globally, the two major driving factors of large scale winds are the differential heating between the equator and the poles (difference in absorption of solar energy between these climate zones), and the rotation of the planet. It is the differential heating between the poles and the equator that lead to the development of the jet stream.[3]

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 if frictional effects are low.[4] The thermal wind is a wind difference between two levels which only exists in an atmosphere with horizontal temperature gradients, or baroclinicity.[5] The ageostrophic wind component is the difference between actual and geostrophic wind which is responsible for air "filling up" cyclones over time.[6] The gradient wind is similar to the geostrophic wind but also includes centrifugal force (or centripetal acceleration).[7]

Measurement

A windmill style of anemometer

Wind direction is reported by the direction from which it originates with the use of weather vanes.[8] At airports, windsocks are primarily used to indicate wind direction, but can also be used to estimate wind speed by its angle of hang.[9] For example, a northerly wind blows from the north to the south.[10] Wind speed is measured by anemometers, either directly with rotating cups, or indirectly via pressure differences or the propagation speed of ultrasound signals.[11] Another type of anemometer uses pitot tubes which take advantage of the pressure differential between an inner tube and an outer tube which is exposed to the wind to determine the dynamic pressure, which is then used to compute the wind speed.[12]

Sustained wind speeds are reported globally at a 10 metres (33 ft) height and are averaged over a 10 minute time frame. The United States reports winds over a 2 minute average,[13] while India typically reports winds over a 3 minute average.[14] Knowing the wind sampling average is important, as the value of a one-minute sustained wind is 14 percent greater than a ten-minute sustained wind.[15] Shorter bursts of higher winds, known as wind gusts, are defined as maxima which 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 or radar tracking of the probe.[16] Alternatively, movement of the parent weather balloon position can be tracked from the ground visually via a theodolite, with the wind profile is computed from drift rate and the theoretical speed of ascent.[17] 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. Wind Engineering describes the study of the effects of the wind on the built environment, including buildings, bridges and other man-made objects.

Plotting on surface weather maps

Wind plotting within a station model
See also: Surface weather analysis and 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.[18]

More than a century ago, winds were initially plotted as arrows facing downwind, with feathers on both sides of the staff to indicate wind direction.[19] In the United States, the change to the modern convention of flags shown on one side of the staff to indicate wind speed took effect on August 1, 1941.[20][21]

Winds are depicted as blowing from the direction the flags are 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.[10] 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.[22]

Scales

See also: Tropical cyclone scales

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.[23] There are general terms which differentiate winds of different average speeds such as a breeze, a gale, a storm, 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.[24] A storm has winds of 56 knots (104 km/h) to 63 knots (117 km/h).[25] 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:

General Wind Classifications Tropical Cyclone Classifications (all winds are 10-minute averages)
Beaufort scale[23] 10-minute sustained winds (knots) General term[26] N Indian Ocean
IMD 		SW Indian Ocean
MF 		Australia
BOM 		SW Pacific
FMS 		NW Pacific
JMA 		NW Pacific
JTWC 		NE Pacific &
N Atlantic
NHC & CPHC
0 <1 Calm Depression Tropical Disturbance Tropical Low Tropical Depression Tropical Depression Tropical Depression Tropical Depression
1 1-3 Light air
2 4-6 Light breeze
3 7-10 Gentle breeze
4 11-16 Moderate breeze
5 17-21 Fresh breeze
6 22-27 Strong breeze
7 28-29 Moderate gale Deep Depression Tropical Depression
30-33
8 34–40 Fresh gale Cyclonic Storm Moderate Tropical Storm Tropical Cyclone (1) Tropical Cyclone (1) Tropical Storm Tropical Storm Tropical Storm
9 41-47 Strong gale
10 48–55 Whole gale Severe Cyclonic Storm Severe Tropical Storm Tropical Cyclone (2) Tropical Cyclone (2) Severe Tropical Storm
11 56–63 Storm
12 64–72 Hurricane Very Severe Cyclonic Storm Tropical Cyclone Severe Tropical Cyclone (3) Severe Tropical Cyclone (3) Typhoon Typhoon Hurricane (1)
13 73–85 Hurricane (2)
14 86–89 Severe Tropical Cyclone (4) Severe Tropical Cyclone (4) Major Hurricane (3)
15 90–99 Intense Tropical Cyclone
16 100–106 Major Hurricane (4)
17 107-114 Severe Tropical Cyclone (5) Severe Tropical Cyclone (5)
115–119 Very Intense Tropical Cyclone Super Typhoon
>120 Super Cyclonic Storm Major Hurricane (5)

Global climatology

Main article: Prevailing winds
The westerlies and trade winds are part of the Earth's atmospheric circulation

Easterly winds, on average, dominate the flow pattern across the poles and tropics, while westerly winds blow across the mid-latitudes of the earth, to the north of the subtropical ridge. 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.[27]

Trades and their impact

Main article: Trade wind

The trade winds (also called trades) are the prevailing pattern of easterly surface winds found in the tropics towards the Earth's equator.[28] The trade winds blow predominantly from the northeast in the Northern Hemisphere and from the southeast in the Southern Hemisphere.[29] The trade winds act as the steering flow for tropical cyclones that form over world's oceans.[30] Trade winds also steer African dust westward across the Atlantic ocean into the Caribbean sea, as well as portions of southeast North America.[31]

Westerlies and their impact

Benjamin Franklin's map of the Gulf Stream
Main article: Westerlies

The Westerlies or the Prevailing Westerlies are the prevailing winds in the middle latitudes between 35 and 65 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,[32] 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.[29] They are strongest in the wind when the pressure is lower over the poles, and weakest during the summer and when pressures are higher over the poles.[33]

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.[34] These western ocean currents transport warm, 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.[35] The Westerlies play an important role in carrying the warm, equatorial waters and winds to the western coasts of continents,[36][37] especially in the southern hemisphere because of its vast oceanic expanse.

Polar easterlies

Main article: Polar easterlies

The polar easterlies, also known as Polar Hadley cells, are dry, cold prevailing winds which 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 trade winds and the Westerlies, these prevailing winds blow from the east to the west, and are often weak and irregular.[38] Due to the low sun angle, cold air builds up and subsides at the pole creating surface high-pressure areas, forcing an equatorward outflow of air;[39] that outflow is deflected eastward by the Coriolis effect.

Shear

Hodograph plot of wind vectors at various heights in the troposphere, which is used to diagnose vertical wind shear
Main article: Wind shear

Wind shear, sometimes referred to as windshear or wind gradient, is a difference in wind speed and direction over a relatively short distance in the Earth's atmosphere.[40] Wind shear can be broken down into vertical and horizontal components, with horizontal wind shear seen across weather fronts and near the coast,[41] and vertical shear typically near the surface,[42] though also at higher levels in the atmosphere near upper level jets and frontal zones aloft.[43]

Wind shear itself is a microscale meteorological phenomenon occurring over a very small distance, but it can be associated with mesoscale or synoptic scale weather features such as squall lines and cold fronts. It is commonly observed near microbursts and downbursts caused by thunderstorms,[44] weather fronts, areas of locally higher low level winds referred to as low level jets, near mountains,[45] radiation inversions that occur due to clear skies and calm winds, buildings,[46] wind turbines,[47] and sailboats.[48] Wind shear has a significant effect during take-off and landing of aircraft due to their effects on control of the aircraft,[49] and was a significant cause of aircraft accidents involving large loss of life within the United States.[44]

Sound movement through the atmosphere is affected by wind shear, which can bend the wave front, causing sounds to be heard where they normally would not, or vice versa.[50] Strong vertical wind shear within the troposphere also inhibits tropical cyclone development,[51] but helps to organize individual thunderstorms into living longer life cycles which can then produce severe weather.[52] The thermal wind concept explains how differences in wind speed with height are dependent on horizontal temperature differences, and explains the existence of the jet stream.[53]

In civilization

History

Further information: Wind god

Ancient religions

As a natural force, the wind was often personified as one or more wind gods or as an expression of the supernatural in many cultures. Vayu is the Hindu God of Wind.[54] The Greek wind gods include Boreas, Notus, Eurus, and Zephyrus. Aeolus, in varying interpretations the ruler or keeper of the four winds, has also been described as Astraeus, the god of dusk who fathered the four winds with Eos, goddess of dawn. The Ancient Greeks also observed the seasonal change of the winds, as evidenced by the Tower of the Winds in Athens.[55] Venti are the Roman gods of the winds.[56] Fūjin, the Japanese wind god and one of the eldest Shinto gods. According to legend, he was present at the creation of the world and first let the winds out of his bag to clear the world of mist.[57] In Norse mythology, Njord is the god of the wind. There are also four dvärgar (Norse dwarves), named Norðri, Suðri, Austri and Vestri, and probably the four stags of Yggdrasil, personify the four winds, and parallel the four Greek wind gods.[58] Stribog is the name of the Slavic god of winds, sky and air. He is said to be the ancestor (grandfather) of the winds of the eight directions.[59]

Bible

The winds are discussed in the Bible:

Winds - blowing from the four quarters of heaven (Jer. 49:36; Ezek.

37:9; Dan. 8:8; Zech. 2:6). The east wind was parching (Ezek. 17:10; 19:12), and is sometimes mentioned as simply denoting a strong wind (Job 27:21; Isa. 27:8). This wind prevails in Israel from February to June, as the west wind (Luke 12:54) does from November to February. The south was a hot wind (Job 37:17; Luke 12:55). It swept over the Arabian peninsula. The rush of invaders is figuratively spoken of as a whirlwind (Isa. 21:1); a commotion among the nations of the world as a striving of the four winds (Dan. 7:2). The winds are subject to the divine power (Ps. 18:10; 135:7).

[60]

Events where its influence changed history

Kamikaze (神風) is a Japanese word, usually translated as divine wind, believed to be a gift from the gods. The term is first known to have been used as the name of a pair or series of typhoons that are said to have saved Japan from two Mongol fleets under Kublai Khan that attacked Japan in 1274 and again in 1281.[61] Protestant Wind is a name for the storm that deterred the Spanish Armada from an invasion of England in 1588,[62] or the favorable winds that enabled William of Orange to invade England in 1688.[63]

Transportation

RAF Exeter airfield on 20 May 1944, showing the layout of the runways which allow aircraft to take off and land into the wind

There are many different types of sailing ships, but they all have certain basic things in common. Every sailing ship has a hull, rigging and at least one mast to hold up the sails that use the wind to power the ship.[64] Ocean journeys by sailing ship can take many months,[65] and a common hazard is becoming becalmed because of lack of wind,[66] or being blown off course by severe storms or winds that do not allow progress in the desired direction.[67] A severe storm could lead to shipwreck, and the loss of all hands.[68] Sailing ships can only carry a certain quantity of supplies in their hold, so they have to plan long voyages carefully to include appropriate provisions, including fresh water.[69]

While aircraft usually travel under an internal power source, tail winds affect groundspeed,[70] and in the case of hot air balloons and other lighter-than-air vehicles, wind may play a significant role in their movement and ground track.[71] In addition, the direction of wind plays a role in the takeoff and landing of fixed-wing aircraft and airfield runways are usually aligned to take the direction of wind into account. Of all factors affecting the direction of flight operations at an airport, wind direction is considered the primary governing factor. While taking off with a tailwind may be permissible under certain circumstances, it is generally considered the least desirable choice due to performance and safety considerations, with a headwind the desirable choice. A tailwind will increase takeoff distance and decrease climb gradient such that runway length and obstacle clearance may become limiting factors.[72] An airship, or dirigible, is a lighter-than-air aircraft that can be steered and propelled through the air using rudders and propellers or other thrust.[73] Unlike other aerodynamic aircraft such as fixed-wing aircraft and helicopters, which produce lift by moving a wing, or airfoil, through the air, aerostatic aircraft, such as airships and hot air balloons, stay aloft by filling a large cavity, such as a balloon, with a lifting gas.[74] The main types of airship are non-rigid (or blimps), semi-rigid and rigid. Blimps are small airships without internal skeletons. Semi-rigid airships are slightly larger and have some form of internal support such as a fixed keel. Rigid airships with full skeletons, such as the massive Zeppelin transoceanic models,[75] all but disappeared after several high-profile catastrophic accidents during the mid-20th century.[76]

Power

This wind turbine produces wind power.
Further information: Wind power

A yardstick used to determine the best locations for wind energy development is referred to as Wind Power Density (WPD.) It is a calculation relating to the effective force of the wind at a particular location, frequently expressed in terms of the elevation above ground level over a period of time. It takes into account wind velocity and mass. Color coded maps are prepared for a particular area are described as, for example, "Mean Annual Power Density at 50 Meters." The results of the above calculation are included in an index developed by the National Renewable Energy Lab and referred to as "NREL CLASS." The larger the WPD calculation, the higher it is rated by class.[77] At the end of 2008, worldwide nameplate capacity of wind-powered generators was 120.8 gigawatts.[78] Although wind produces only about 1.5 percent of worldwide electricity use,[78] it is growing rapidly, having doubled in the three years between 2005 and 2008. In several countries it has achieved relatively high levels of penetration, accounting for approximately 19 percent of electricity production in Denmark, 10 percent in Spain and Portugal, and 7 percent in Germany and the Republic of Ireland in 2008. One study indicates that an entirely renewable energy supply based on 70 percent wind is attainable at today's power prices by linking wind farms with an HVDC supergrid.[79]

Recreation

Otto Lilienthal in flight
See also: Wind shear and Wind gradient

Wind figures prominently in several popular sports, including recreational hang gliding, hot air ballooning, kite flying, kiteboarding, paragliding, sailing, and windsurfing. In gliding, wind gradients just above the surface affect the takeoff and landing phases of flight of a glider. Wind gradient can have a noticeable effect on ground launches, also known as winch launches or wire launches. If the wind gradient is significant or sudden, or both, and the pilot maintains the same pitch attitude, the indicated airspeed will increase, possibly exceeding the maximum ground launch tow speed. The pilot must adjust the airspeed to deal with the effect of the gradient.[80] 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.[81]

Role in the natural world

In arid climates, the main source of erosion is wind.[82] The general wind circulation moves small particulates such as dust across wide oceans thousands of kilometers downwind of their point of origin,[83] which is known as deflation. Westerly winds in the mid-latitudes of the planet drive the movement of ocean currents from west to east across the world's oceans. Wind has a very important role in aiding plants and other immobile organisms in dispersal of seeds, spores, pollen, etc. Although wind is not the primary form of seed dispersal in plants, it provides dispersal for a large percentage of the biomass of land plants.

Erosion

A rock formation in the Altiplano, Bolivia sculpted by wind erosion.
Main article: Aeolian processes

Erosion can be the result of material movement by the wind. There are two main effects. First, wind causes small particles to be lifted and therefore moved to another region. This is called deflation. Second, these suspended particles may impact on solid objects causing erosion by abrasion (ecological succession). Wind erosion generally occurs in areas with little or no vegetation, often in areas where there is insufficient rainfall to support vegetation. An example is the formation of sand dunes, on a beach or in a desert.[84] Loess is a homogeneous, typically nonstratified, porous, friable, slightly coherent, often calcareous, fine-grained, silty, pale yellow or buff, windblown (aeolian) sediment.[85] It generally occurs as a widespread blanket deposit that covers areas of hundreds of square kilometers and tens of meters thick. Loess often stands in either steep or vertical faces.[86] Loess tends to develop into highly rich soils. Under appropriate climatic conditions, areas with loess are among the most agriculturally productive in the world.[87] Loess deposits are geologically unstable by nature, and will erode very readily. Therefore, windbreaks (such as big trees and bushes) are often planted by farmers to reduce the wind erosion of loess.[82]

Desert dust migration

During mid-summer (July), the westward-moving trade winds south of the northward-moving subtropical ridge expand northwestward from the Caribbean sea into southeastern North America. When dust from the Sahara moving around the southern periphery of the ridge within the belt of trade winds moves over land, rainfall is suppressed and the sky changes from a blue to a white appearance which leads to an increase in red sunsets. Its presence negatively impacts air quality by adding to the count of airborne particulates.[31] Over 50 percent of the African dust that reaches the United States affects Florida.[88] Since 1970, dust outbreaks have worsened due to periods of drought in Africa. There is a large variability in the dust transport to the Caribbean and Florida from year to year.[89] Dust events have been linked to a decline in the health of coral reefs across the Caribbean and Florida, primarily since the 1970s.[90] Similar dust plumes originate in the Gobi desert, which combined with pollutants, spread large distances downwind, or eastward, into North America.[83]

Seed dispersal

Tumbleweed blown against a fence
Main article: Seed dispersal

Wind dispersal, or anemochory, is one of the more primitive means of dispersal. Wind dispersal can take on one of two primary forms: seeds can float on the breeze or alternatively, they can flutter to the ground.[91] The classic examples of these dispersal mechanisms include dandelions (Taraxacum spp., Asteraceae), which have a feathery pappus attached to their seeds and can be dispersed long distances, and maples (Acer spp., Sapindaceae), which have winged seeds and flutter to the ground. An important constraint on wind dispersal is the need for abundant seed production to maximize the likelihood of a seed landing in a site suitable for germination. There are also strong evolutionary constraints on this dispersal mechanism. For instance, species in the Asteraceae on islands tended to have reduced dispersal capabilities (i.e., larger seed mass and smaller pappus) relative to the same species on the mainland.[92] Reliance upon wind dispersal is common among many weedy or ruderal species. Unusual mechanisms of wind dispersal include tumbleweeds.

Effect on flying animals

See also: Prevailing winds

Insects, also known as arthropods, are swept along by the prevailing winds, while birds follow their own course.[93] As such, fine line patterns within weather radar imagery, associated with converging winds, are dominated by insect returns.[94] Bird migration, which tends to occur overnight within the lowest 7,000 feet (2,100 m) of the Earth's atmosphere, contaminates wind profiles gathered by weather radar, particularly the WSR-88D, by increasing the environmental wind returns by 15 knots (28 km/h) to 30 knots (56 km/h).[95]

Damage from Hurricane Andrew

High winds are known to cause damage, depending upon their strength. Infrequent wind gusts can cause poorly-designed suspension bridges to sway. When wind gusts are at a similar frequency to the swaying of the bridge, the bridge can be destroyed easier, such as what occurred with the Tacoma Narrows Bridge in 1940.[96] Wind speeds as low as 23 knots (43 km/h) can lead to power outages due to tree branches disrupting the flow of energy through power lines.[97] While no species of tree is guaranteed to stand up to hurricane-force winds, those with shallow roots are more prone to uproot, and brittle trees such as eucalyptus, sea hibiscus, and avocado are more prone to damage.[98] Hurricane-force winds cause substantial damage to mobile homes, and begin to structurally damage homes with foundations. Once winds exceed 135 knots (250 km/h), homes completely collapse, and significant damage is done to larger buildings. Total destruction to man-made structures occurs when winds reach 175 knots (324 km/h). The Saffir-Simpson scale and Enhanced Fujita scale were designed to help estimate wind speed from the damage caused by high winds related to tropical cyclones and tornadoes, and vice versa.[99][100]

Outgasing from objects in space

The solar wind is quite different than a terrestrial wind, in that its origin is the sun, and it is composed of charged particles which have escaped the sun's atmosphere. Similar to the solar wind, the planetary wind is composed of light gases which escape planetary atmospheres. Over long periods of time, the planetary wind can radically change the composition of planetary atmospheres.

Planetary

Possible future for Earth due to the planetary wind: Venus
Main article: Atmospheric escape

The hydrodynamic wind allows light elements such as Hydrogen to move up through the atmosphere of a planet to the exobase, the lower limit of the exosphere, where the gases can then reach escape velocity, entering outer space without impacting other particles of gas. This type of gas loss from the planet is known as planetary wind.[101] Such a process over geologic time causes water-rich planets such as the Earth to evolve into planets such as Venus over billions of years.[102] Planets with hot lower atmospheres could result in humid upper atmospheres which accelerate the loss of hydrogen.[103]

Solar

Main article: Solar wind
The plasma in the solar wind meeting the heliopause

Rather than air, the solar wind is a stream of charged particles—a plasma—ejected from the upper atmosphere of the sun at a rate of 400 kilometres per second (890,000 mph). It consists mostly of electrons and protons with energies of about 1 keV. The stream of particles varies in temperature and speed with the passage of time. These particles are able to escape the sun's gravity, in part because of the high temperature of the corona,[104] but also because of high kinetic energy that particles gain through a process that is not well-understood. The solar wind creates the Heliosphere, a vast bubble in the interstellar medium surrounding the solar system.[105] Planets require large magnetic fields in order to reduce the ionization of their upper atmosphere by the solar wind.[103] Other phenomena include geomagnetic storms that can knock out power grids on Earth,[106] the aurorae such as the Northern Lights,[107] and the plasma tails of comets that always point away from the sun.[108]

See also

References

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