All about the wind.

Wind is a stream of air that moves near the earth's surface. On Earth, the wind is a stream of air moving mainly in a horizontal direction, on other planets it is a stream of atmospheric gases characteristic of them. In the solar system, the strongest winds are observed on Neptune and Saturn. The solar wind is a stream of rarefied gases from a star, and the planetary wind is a stream of gases responsible for the degassing of the planetary atmosphere into outer space. Winds are usually classified according to their scale, speed, types of forces that cause them, places of distribution and impact on the environment.

First of all, winds are classified according to their strength, duration and direction. So, gusts are considered to be short-term (several seconds) and strong movements of air. Strong winds of medium duration (about 1 minute) are called squalls. The names of longer winds depend on the strength, for example, such names are breeze, storm, storm, hurricane, typhoon. Wind duration also varies greatly, with some thunderstorms lasting several minutes; the breeze, depending on the features of the relief, namely, on the difference in heating of its elements, is several hours; the duration of global winds caused by seasonal temperature changes - monsoons - is several months, while global winds caused by differences in temperature at different latitudes and the Coriolis force - trade winds - blow constantly. Monsoons and trade winds are the winds that make up the general and local circulation of the atmosphere.

Winds have always influenced human civilization. They gave rise to mythological representations, to a certain extent determined some [what?] historical activities, the range of trade, cultural development and wars, supplied energy for various energy production mechanisms, created opportunities for a number of forms of recreation. Thanks to sailing ships that moved due to the wind, people got the opportunity to travel long distances across the seas and oceans. Balloons, also propelled by the power of the wind, were the first to allow air travel, and modern aircraft use the wind to increase lift and save fuel. However, winds are also unsafe: for example, their gradient fluctuations can cause loss of control over the aircraft; fast winds, as well as the large waves they cause on large bodies of water, often lead to the destruction of artificial structures, and in some cases the winds increase the scale of the fire.


Eolian pillars (Bryce Canyon Park, Utah) - an example of the work of the wind
Winds also affect the formation of relief, causing the accumulation of eolian deposits that form various types of soils. They can carry sand and dust from deserts over long distances. The winds disperse plant seeds and aid the movement of flying animals, resulting in an expansion of species diversity in the new area. Wind-related phenomena affect wildlife in a variety of ways.

Wind occurs as a result of an uneven distribution of atmospheric pressure, it is directed from a high pressure zone to a low pressure zone. Due to the continuous change in pressure in time and space, the speed and direction of the wind is also constantly changing. With height, the wind speed changes due to a decrease in the friction force.

The Beaufort scale is used to visually estimate wind speed. In meteorology, the direction of the wind is indicated by the azimuth of the point from which the wind is blowing, while in air navigation[1] by the azimuth of the point into which it blows; thus the values ​​differ by 180°. Based on the results of long-term observations of the direction and strength of the wind, a graph is drawn up, depicted in the form of a so-called wind rose, which displays the wind regime in a particular area.

In some cases, it is not the direction of the wind that is important, but the position of the object relative to it. So, when hunting an animal with a sharp scent, they approach it from the leeward side [2] - in order to avoid the spread of smell from the hunter towards the animal.

The vertical movement of air is called updraft or downdraft.

General patterns

Wind is caused by the difference in pressure between two different air areas. If there is a non-zero baric gradient, then the wind moves with acceleration from the high pressure zone to the low pressure zone. On a planet that is rotating, the Coriolis force is added to this gradient. Thus, the main factors that form the circulation of the atmosphere on a global scale are the difference in air heating between the equatorial and polar regions (which causes a difference in temperature and, accordingly, the density of air flows, and hence the difference in pressure) and the Coriolis force. As a result of these factors, the movement of air in the middle latitudes in the near-surface region leads to the formation of a geostrophic wind directed almost parallel to the isobars [3].

An important factor that speaks about the movement of air is its friction against the surface, which delays this movement and forces the air to move towards low pressure zones [4]. In addition, local barriers and local surface temperature gradients can create local winds. The difference between real and geostrophic wind is called ageostrophic wind. It is responsible for creating chaotic vortex processes such as cyclones and anticyclones[5]. While the direction of surface winds in tropical and polar regions is determined mainly by the effects of global atmospheric circulation, which are usually weak in temperate latitudes, cyclones, together with anticyclones, replace each other and change their direction every few days.

Global effects of wind formation

Most regions of the Earth are dominated by winds that blow in a particular direction. East winds usually dominate near the poles, westerly winds usually dominate in temperate latitudes, while east winds again dominate in the tropics. On the borders between these belts - the polar front and the subtropical ridge - there are zones of calm, where the prevailing winds are practically absent. In these zones, the movement of air is predominantly vertical, which causes zones of high humidity (near the polar front) or deserts (near the subtropical ridge) [6].

Tropical winds
Trade winds and monsoons

Earth circulation processes that lead to wind formation.

The trade winds are called the near-surface part of the Hadley cell - the prevailing near-surface winds blowing in the tropical regions of the Earth in a westerly direction, approaching the equator [7], that is, northeast winds in the Northern Hemisphere and southeast winds in the South [8]. The constant movement of the trade winds leads to the mixing of the Earth's air masses, which can manifest itself on a very large scale: for example, the trade winds blowing over the Atlantic Ocean are capable of carrying dust from the African deserts to the West Indies and some regions of North America [9].

Monsoons are the prevailing seasonal winds that blow for several months each year in tropical regions. The term originated in British India and surrounding countries as the name of seasonal winds that blow from the Indian Ocean and the Arabian Sea to the northeast, bringing a significant amount of precipitation to the region [10]. Their movement towards the poles is caused by the formation of areas of low pressure as a result of the heating of tropical regions in the summer months, that is, in Asia, Africa and North America from May to July, and in Australia in December[11][12].

Trade winds and monsoons are the main factors that lead to the formation of tropical cyclones over the oceans of the Earth[13].

Temperate westerly winds

In temperate latitudes, that is, between 35 and 65 degrees north and south latitude, westerly winds predominate[14][15], the near-surface part of the Ferrell cell, these are southwest winds in the Northern Hemisphere and northwest winds in the Southern Hemisphere[8]. These are the strongest winds in winter, when the pressure at the poles is lowest, and the weakest in summer[16].

Together with the trade winds, the prevailing westerly winds allow sailing ships to cross the oceans. In addition, due to the strengthening of these winds near the western coasts of the oceans of both hemispheres, strong ocean currents[17][18][19] are formed in these areas, carrying warm tropical waters towards the poles. The prevailing westerly winds are generally stronger in the Southern Hemisphere, where there is less land to block the wind, and are particularly strong in the Roaring Forties (between 40 and 50 degrees south latitude)[20].


Map of the Gulf Stream by Benjamin Franklin.
Eastern winds of the polar regions.
The eastern winds of the polar regions, the near-surface part of the polar cells, are predominantly dry winds blowing from near-polar high-pressure zones to low-pressure regions along the polar front. These winds are usually weaker and less regular than the mid-latitude westerly winds[21]. Due to the low amount of solar heat, the air in the polar regions cools and sinks, forming areas of high pressure and pushing the polar air towards lower latitudes[22]. This air, as a result of the Coriolis force, is deflected to the west, forming northeasterly winds in the Northern Hemisphere and southeasterly winds in the Southern Hemisphere.

Local effects of wind formation

The most important local winds on Earth
Local winds and prevailing winds
Local effects of wind formation arise depending on the presence of local geographical objects. One such effect is the temperature difference between not very distant areas, which can be caused by different absorption coefficients of sunlight or different heat capacities of the surface. The latter effect is strongest between land and water and causes a breeze. Another important local factor is the presence of mountains, which act as a barrier to the winds.

Sea and continental breeze

A: sea breeze (occurs during the daytime),
B: continental breeze (occurs at night)
Breeze
Important effects of the formation of prevailing winds in coastal areas are the sea and continental breeze. The sea (or other large body of water) heats up more slowly than land due to the greater effective heat capacity of water [23]. Warm (and therefore light) air above the land rises, forming a zone of low pressure. The result is a pressure difference between land and sea, typically around 0.002 atm. As a result of this pressure difference, cool air over the sea moves towards land, creating a cool sea breeze on the coast. In the absence of strong winds, the speed of the sea breeze is proportional to the temperature difference. In the presence of wind from the land with a speed of more than 4 m/s, a sea breeze usually does not form.

At night, due to the lower heat capacity, the land cools faster than the sea, and the sea breeze stops. When the temperature of the land falls below the temperature of the surface of the reservoir, then a reverse pressure drop occurs, causing (in the absence of a strong wind from the sea) a continental breeze blowing from the land to the sea[24].

Mountain influence

Schematic representation of lee waves. The wind that blows in the direction of the mountain forms the first oscillation (A), which repeats after passing the mountain (B). Lenticular (lenticular) clouds form at the highest points.
Mountains have a very diverse influence on the wind, they either cause wind formation or act as a barrier to its passage. Above the hills, the air warms up more strongly than the air at the same height above the lowlands; this creates low pressure zones over the mountains[25][26] and leads to wind formation. This effect often leads to the formation of mountain-valley winds - the prevailing winds in areas with rugged terrain. An increase in friction near the surface of the valleys leads to the deviation of the wind blowing parallel to the valley from the surface to the height of the surrounding mountains, which leads to the formation of a high-altitude jet stream. The high-altitude jet stream can exceed the ambient wind in speed by up to 45% [27]. Bypassing mountains can also change the direction of the wind[28].

The difference in the height of the mountains significantly affects the movement of the wind. So, if there is a pass in the mountain range that the wind overcomes, the wind passes it with an increase in speed as a result of the Bernoulli effect. Even small differences in altitude cause fluctuations in wind speed. As a result of a significant gradient in the speed of movement, the air becomes turbulent and remains so at a certain distance even on the plain behind the mountain. Such effects are important, for example, for aircraft taking off or landing on mountain airfields[28]. The fast, cold winds blowing through the mountain passes have been given a variety of local names. In Central America, these are the papagayo near Lake Nicaragua, the Panamanian wind on the Isthmus of Panama, and the tehuano on the isthmus of Tehuantepec. Similar winds in Europe are known as bora, tramontana and mistral.

Another effect associated with the passage of wind over mountains is lee waves (standing waves of air movement that occur behind a high mountain), which often lead to the formation of lenticular clouds. As a result of this and other effects of the passage of wind through obstacles, numerous vertical currents and eddies arise over rough terrain. In addition, heavy precipitation falls on the windward slopes of the mountains, due to the adiabatic cooling of the air rising up and the condensation of moisture in it. On the leeward side, on the contrary, the air becomes dry, which causes the formation of a rainy dusk. As a result, in areas where the prevailing winds overcome the mountains, a humid climate dominates on the windward side, and arid on the leeward[29]. The winds blowing from the mountains to the lower regions are called downwinds. These winds are warm and dry. They also have numerous local names. Thus, the downward winds descending from the Alps in Europe are known as foehn, a term sometimes extended to other areas. Downward winds are known as halny in Poland and Slovakia, probes in Argentina, koembang in Java, and Nor'west arch in New Zealand. On the Great Plains in the USA they are known as Chinook, and in California they are known as Santa Ana and Sundowner. The downwind wind speed can exceed 45 m/s[31].

Short-term processes of wind formation

Tropical Cyclone Katharina over the South Atlantic Ocean
Short-term processes also lead to the formation of winds, which, unlike the prevailing winds, are not regular, but occur chaotically, often during a certain season. Such processes are the formation of cyclones, anticyclones and similar phenomena of a smaller scale, in particular thunderstorms.

Cyclones and anticyclones are called areas of low or, respectively, high atmospheric pressure, usually those that occur over a space larger than several kilometers. On Earth, they form over most of the surface and are characterized by their typical circulation structure. Due to the influence of the Coriolis force in the Northern Hemisphere, the movement of air around the cyclone rotates counterclockwise, and around the anticyclone - clockwise. In the Southern Hemisphere, the direction of movement is reversed. In the presence of friction on the surface, a component of movement towards the center or away from the center appears, as a result, the air moves in a spiral towards the area of ​​low pressure or away from the area of ​​high pressure.

Extratropical cyclone.

Cyclones that form outside the tropics are known as extratropics. Of the two types of large-scale cyclones, they are the larger (classified as synoptic cyclones), the most common, and occur over most of the earth's surface. It is this class of cyclones that is most responsible for day-to-day weather changes, and their prediction is the main goal of modern weather forecasts.

According to the classical (or Norwegian) model of the Bergen School, extratropical cyclones form mainly near the polar front in areas of especially strong high-altitude jet stream and receive energy due to a significant temperature gradient in this region. During the formation of a cyclone, the stationary atmospheric front breaks into sections of warm and cold fronts moving towards each other with the formation of an occlusion front and the swirling of the cyclone. A similar picture arises in the later Shapiro-Keiser model based on the observation of oceanic cyclones, except for the long movement of the warm front perpendicular to the cold one without the formation of an occlusion front.

After the formation of a cyclone, it usually exists for several days. During this time, it manages to advance a distance of several hundred to several thousand kilometers, causing sharp changes in winds and precipitation in some areas of its structure.

Although large extratropical cyclones are usually associated with fronts, smaller cyclones can form within a relatively homogeneous air mass. A typical example is cyclones that form in polar air currents at the beginning of the formation of a frontal cyclone. These small cyclones are called polar cyclones and often occur over the polar regions of the oceans. Other small cyclones arise on the lee side of mountains under the influence of westerly winds of temperate latitudes[32].

Tropical cyclones

Tropical cyclone diagram[33]
Main article: Tropical cyclone
Cyclones that form in the tropics are somewhat smaller than extratropical ones (they are classified as mesocyclones) and have a different mechanism of origin. These cyclones are powered by the upwelling of warm, moist air and can exist exclusively over warm regions of the oceans, which are why they are called warm-core cyclones (as opposed to cold-core extratropical cyclones). Tropical cyclones are characterized by very strong winds and significant rainfall. They develop and gain strength over the surface of the water, but quickly lose it over land, which is why their destructive effect usually manifests itself only on the coast (up to 40 km inland).

For the formation of a tropical cyclone, a section of a very warm water surface is required, the heating of the air above which leads to a decrease in atmospheric pressure by at least 2.5 mm Hg. Art. Humid warm air rises, but due to its adiabatic cooling, a significant amount of retained moisture condenses at high altitudes and falls as rain. Dryer and therefore denser air, just freed from moisture, sinks down, forming zones of high pressure around the cyclone core. This process has a positive feedback, whereby, as long as the cyclone is above a fairly warm water surface, which supports convection, it continues to intensify. Although tropical cyclones most often form in the tropics, sometimes other types of cyclones develop later in their existence, as is the case with subtropical cyclones.

Anticyclones
Main article: Anticyclone
Unlike cyclones, anticyclones are usually larger than cyclones and are characterized by low meteorological activity and weak winds. Most often, anticyclones form in zones of cold air behind a passing cyclone. Such anticyclones are called cold, but as they grow, air from higher layers of the atmosphere (2-5 km) descends to the cyclone, which leads to an increase in temperature and the formation of a warm anticyclone. Anticyclones move rather slowly, often gathering in the anticyclone band near the subtropical ridge, although many of them remain in the zone of westerly winds of temperate latitudes. Such anticyclones usually block winds and are therefore called blocking anticyclones[32].

Measurements

propeller anemometer

Radar wind profiler
Wind direction in meteorology is defined as the direction from which the wind is blowing[34], while in air navigation[1] it is where it blows: thus the values ​​differ by 180°. The simplest device for determining the direction of the wind is a weather vane[35]. Windsocks installed at airports are capable, in addition to the direction, of approximately showing the wind speed, depending on which the inclination of the device changes [36].

Typical instruments intended directly for measuring wind speed are a variety of anemometers that use rotatable bowls or propellers. For measurements with greater accuracy, in particular for scientific research, measurements of the speed of sound or measurements of the cooling rate of a heated wire or membrane under the influence of wind are used [37]. Another common type of anemometer is the pitot tube: it measures the difference in dynamic pressure between two concentric tubes under the influence of wind; are widely used in aviation technology [38].

Wind speed at meteorological stations in most countries of the world is usually measured at a height of 10 m and averaged over 10 minutes. The exceptions are the United States, where the speed is averaged over 1 minute[39], and India, where it is averaged over 3 minutes[40]. The averaging period is important because, for example, a constant wind speed measured over 1 minute is typically 14% higher than that measured over 10 minutes[41]. Short periods of fast wind are investigated separately, and periods during which the wind speed exceeds the 10-minute average speed by at least 10 knots (5.14 m/s) are called gusts. A squall is a doubling of wind speed above a certain threshold that lasts a minute or more.

Sondes are used to measure wind speeds at many locations, with speeds determined using GLONASS or GPS, radio navigation, or tracking the sonde using radar[42] or theodolite[43]. In addition, sodars, Doppler lidars, and radars can be used to measure the Doppler shift of electromagnetic radiation reflected or scattered by aerosol particles or even air molecules. In addition, radiometers and radars are used to measure the roughness of the water surface, which is a good reflection of the near-surface wind speed over the ocean. With the help of shooting the movement of clouds from geostationary satellites, it is possible to establish the wind speed at high altitudes.

Wind speed
Average wind speeds and their images
Wind atlases and charts are a typical way of presenting wind data. These atlases are usually compiled for climatological studies and may contain information on both the mean speed and the relative frequency of the winds of each speed in a region. Typically, an atlas contains hourly averages of data measured at a height of 10 m and averaged over decades. Other wind mapping standards are used for individual needs. So, for the needs of wind energy, measurements are carried out at a height of more than 10 m, usually 30-100 m, and data are given in the form of an average specific power of the wind flow.

Maximum wind speed
The highest wind gust speed on Earth (at a standard height of 10 m) was recorded by an automatic weather station on the Australian island of Barrow during Cyclone Olivia [en] on April 10, 1996. It was 113 m/s (408 km/h)[44]. The second highest gust speed is 103 m/s (371 km/h). It was recorded on April 12, 1934 at Mount Washington Observatory in New Hampshire[45][46]. The fastest constant winds blow over the Commonwealth Sea - 320 km / h. Velocities can be high during events such as tornadoes, but they are difficult to measure accurately and reliable data do not exist. The Fujita Scale is used to classify tornadoes and tornadoes according to wind speed and destructive power. The record for wind speed on flat terrain was recorded on March 8, 1972 at the US Air Force Base in Tula, Greenland - 333 km / h. The strongest winds blowing at a constant speed were observed on Adélie land, Antarctica. Speed ​​- about 87 m / s. It was registered by Belarusian polar explorer Aleksey Gaydashov.

Wind speed gradient
Main article: Wind gradient

Hodographic plot of the wind speed vector at different heights, which is used to determine the wind gradient.
The wind gradient is the difference in wind speed on a small scale, most often in the direction perpendicular to its movement [47]. The wind gradient is divided into vertical and horizontal components, of which the horizontal one has markedly non-zero values ​​along atmospheric fronts and near the coast[48], and the vertical one in the boundary layer near the earth's surface[49], although zones of significant wind gradient of different directions also occur in high layers of the atmosphere along high-altitude current flows [50]. The wind gradient is a micrometeorological phenomenon that is only significant over short distances, but it can be related to meso- and synoptic meteorological weather phenomena such as squall lines or atmospheric fronts. Significant wind gradients are often observed in thunderstorm-driven microbursts[51], in areas of strong local surface winds—low-level jets, near mountains[52], buildings[53], wind turbines[54] and ships[55].

The wind gradient has a significant impact on the landing and takeoff of aircraft: on the one hand, it can help reduce the takeoff distance of the aircraft, and on the other hand, it complicates the control of the aircraft[56]. The wind gradient is the cause of a significant number of aircraft accidents[51].

The wind gradient also affects the propagation of sound waves in the air, which can bounce off atmospheric fronts and reach places they would not otherwise reach (or vice versa)[57]. Strong wind gradients prevent the development of tropical cyclones[58] but increase the duration of individual thunderstorms[59]. A special form of wind gradient - thermal wind - leads to the formation of high-altitude jet streams [60].

Classification by wind strength
Main articles: Beaufort scale and Tropical cyclone scales
Since the influence of wind on a person depends on the speed of the air flow, this characteristic was the basis of the first wind classifications. The most common of these classifications is the Beaufort Wind Scale, which is an empirical description of wind strength as a function of observed sea conditions. At first the scale had 13 levels, but beginning in the 1940s it was expanded to 18 levels[61]. To describe each level, this scale originally used colloquial English terms such as breeze, gale, storm, hurricane[62], which were also replaced by colloquial terms from other languages ​​such as "calm", "storm" and "hurricane". in Russian. So, on the Beaufort scale, a storm corresponds to a wind speed (averaged over 10 minutes and rounded to an integer number of knots) from 41 to 63 knots (20.8-32.7 m/s), while this range is divided into three subcategories using adjectives "strong" and "violent".