A high-pressure area, high, or anticyclone, is a region where the atmospheric pressure at the surface of the planet is greater than its surrounding environment. Winds within high-pressure areas flow outward from the higher pressure areas near their centers towards the lower pressure areas further from their centers. Gravity adds to the forces causing this general movement, because the higher pressure compresses the column of air near the center of the area into greater density – and so greater weight compared to lower pressure, lower density, and lower weight of the air outside the center. However, because the planet is rotating, the air flow from center to periphery is not direct, but is twisted due to the Coriolis effect.
Introduction
High-pressure area
High-pressure area | Introduction
A high-pressure area, high ,or anticyclone. High-pressure systems rotate clockwise in the northern Hemisphere. Sorry. I cannot find the answer based on the context. Referred as anticyclones
the general surface wind circulation associated with the high-pressure center (h) is most likely
user
Wind flows from areas of high pressure to areas of low pressure. This is due to density differences between the two air masses. Since stronger high-pressure systems contain cooler or drier air, the air mass is more dense and flows towards areas that are warm or moist, which are in the vicinity of low pressure areas in advance of their associated cold fronts. The stronger the pressure difference, or pressure gradient, between a high-pressure system and a low-pressure system, the stronger the wind. The coriolis force caused by the Earth's rotation is what gives winds within high-pressure systems their clockwise circulation in the northern hemisphere (as the wind moves outward and is deflected right from the center of high pressure) and counterclockwise circulation in the southern hemisphere (as the wind moves outward and is deflected left from the center of high pressure).
High-pressure area | Connection to wind
Wind is initially accelerated from areas of high pressure to areas of low pressure. This is due to density (or temperature and moisture) differences between two air masses. Since stronger high-pressure systems contain cooler or drier air, the air mass is denser and flows towards areas that are warm or moist, which are in the vicinity of low-pressure areas in advance of their associated cold fronts. The stronger the pressure difference, or pressure gradient, between a high-pressure system and a low-pressure system, the stronger the wind. Thus, stronger areas of low pressure are associated with stronger winds. The Coriolis force caused by the Earth's rotation is what gives winds around low-pressure areas (such as in hurricanes, cyclones, and typhoons) their counter-clockwise (anticlockwise) circulation in the northern hemisphere (as the wind moves inward and is deflected right from the center of high pressure) and clockwise circulation in the southern hemisphere (as the wind moves inward and is deflected left from the center of high pressure).
Low-pressure area | Associated weather
A high-pressure area, high, or anticyclone, is a region where the atmospheric pressure at the surface of the planet is greater than its surrounding environment. Winds within high-pressure areas flow outward from the higher pressure areas near their centers towards the lower pressure areas further from their centers. Gravity adds to the forces causing this general movement, because the higher pressure compresses the column of air near the center of the area into greater density – and so greater weight compared to lower pressure, lower density, and lower weight of the air outside the center. However, because the planet is rotating, the air flow from center to periphery is not direct, but is twisted due to the Coriolis effect.
High-pressure area | Introduction
High-pressure areas form due to downward motion through the troposphere, the atmospheric layer where weather occurs. Preferred areas within a synoptic flow pattern in higher levels of the troposphere are beneath the western side of troughs. On weather maps, these areas show converging winds (isotachs), also known as convergence, near or above the level of non-divergence, which is near the 500 hPa pressure surface about midway up through the troposphere, and about half the atmospheric pressure at the surface. High-pressure systems are alternatively referred to as anticyclones. On English-language weather maps, high-pressure centers are identified by the letter H in English, within the isobar with the highest pressure value.
High-pressure area | Formation
This is an apparent, but fictitious, force that arises when an observer is in a rotating reference frame, due to the conservation of angular momentum of the air as it moves towards or away from the Earth's axis of rotation. Viewed from above this twist in wind direction is in the opposite direction as the rotation of the planet.
High-pressure area | Introduction
High-pressure systems form due to downward motion through the troposphere, the atmospheric layer where weather occurs. Preferred areas within a synoptic flow pattern in higher levels of the troposphere are beneath the western side of troughs. On weather maps, these areas show converging winds (isotachs), also known as confluence, or converging height lines near or above the level of non-divergence, which is near the 500 hPa pressure surface about midway up through the troposphere. On weather maps, high-pressure centers are associated with the letter H placed within the isobar with the highest pressure value. On constant pressure upper level charts, it is located within the highest height line contour.
Anticyclogenesis | Process
Friction with land slows down the wind flowing out of high-pressure systems and causes wind to flow more outward than would be the case in the absence of friction. This is known as a geostrophic wind.
High-pressure area | Connection to wind
In terms of climatology, high pressure forms at the horse latitudes, or torrid zone, between the latitudes of 20 and 40 degrees from the equator, as a result of air that has been uplifted at the equator. As the hot air rises it cools, losing moisture; it is then transported poleward where it descends, creating the high-pressure area. This is part of the Hadley cell circulation and is known as the subtropical ridge or subtropical high, and is strongest in the summer. The subtropical ridge is a warm core high-pressure system, meaning it strengthens with height. Many of the world's deserts are caused by these climatological high-pressure systems.
High-pressure area | In climatology
The direction of wind flow around an atmospheric high-pressure area and a low-pressure area, as seen from above, depends on the hemisphere. High-pressure systems rotate clockwise in the northern Hemisphere; low-pressure systems rotate clockwise in the southern hemisphere. The scientific terms in English used to describe the weather systems generated by highs and lows were introduced in the mid-1800s, mostly by the British. The scientific theories which explain the general phenomena originated about two centuries earlier. The term cyclone was coined by Henry Piddington of the British East India Company to describe the devastating storm of December 1789 in Coringa, India.
High-pressure area | Wind circulation in the northern and southern hemispheres
High pressures that build to the north and extend southwards will often bring clear weather. This is due to being cooled at the base (as opposed to warmed) which helps prevent clouds from forming. On weather maps, these areas show converging winds (isotachs), also known as confluence, or converging height lines near or above the level of non-divergence, which is near the 500 hPa pressure surface about midway up through the troposphere. High-pressure systems are alternatively referred to as anticyclones. On weather maps, high-pressure centers are associated with the letter H in English, or A in Spanish, because alta is the Spanish word for high, within the isobar with the highest pressure value.
Synoptic scale meteorology | Surface high-pressure systems
The wind belts girdling the planet are organised into three cells in each hemisphere—the Hadley cell, the Ferrel cell, and the polar cell. Those cells exist in both the northern and southern hemispheres. The vast bulk of the atmospheric motion occurs in the Hadley cell. The high pressure systems acting on the Earth's surface are balanced by the low pressure systems elsewhere. As a result, there is a balance of forces acting on the Earth's surface. The horse latitudes are an area of high pressure at about 30° to 35° latitude (north or south) where winds diverge into the adjacent zones of Hadley or Ferrel cells, and which typically have light winds, sunny skies, and little precipitation.
Atmospheric circulation | Latitudinal circulation features
"warm-core"). From hydrostatic balance, the warm core translates to lower pressure at the center at all altitudes, with the maximum pressure drop located at the surface.
Tropical cyclone | Primary circulation: rotating winds
Along its southerly periphery, the clockwise circulation often impels easterly waves, and tropical cyclones that develop from them, across the ocean towards landmasses in the western portion of ocean basins during the hurricane season. The highest barometric pressure ever recorded on Earth was measured in Tosontsengel,_Zavkhan, Mongolia on 19 December 2001.
High-pressure area | In climatology
High-pressure systems, also known as anticyclones, rotate outward at the surface and clockwise in the northern hemisphere as opposed to outward and counterclockwise in the southern hemisphere. Under surface highs, sinking of the atmosphere slightly warms the air by compression, leading to clearer skies, winds that are lighter, and a reduced chance of precipitation. The descending air is dry, hence less energy is required to raise its temperature. If high pressure persists, air pollution will build up due to pollutants trapped near the surface caused by the subsiding motion associated with the high.
Surface weather analysis | High pressure
As a result, the wind direction near ground level is partly parallel to the isobars in the region, and partly across the isobars in the direction from higher pressure to lower pressure. As a result of the slower wind speed at the earth's surface, in a region of low pressure the barometric pressure is usually significantly higher at the surface than would be expected, given the barometric pressure at mid altitudes, due to Bernoulli's principle. Hence, the secondary flow toward the center of a region of low pressure is also drawn upward by the significantly lower pressure at mid altitudes.
Secondary flow | Wind near ground level
Diverging winds aloft ahead of these troughs cause atmospheric lift within the troposphere below, which lowers surface pressures as upward motion partially counteracts the force of gravity. A low-pressure area is commonly associated with inclement weather, while a high-pressure area is associated with light winds and fair skies. Thermal lows form due to localized heating caused by greater sunshine over deserts and other land masses. Since localized areas of warm air are less dense than their surroundings, this warmer air rises, which lowers atmospheric pressure near that portion of the Earth's surface. Large-scale thermal lows over continents help drive monsoon circulations.
Low-pressure area | Introduction
In the absence of rotation, the wind tends to blow from areas of high pressure to areas of low pressure. The stronger the pressure difference (pressure gradient) between a high-pressure system and a low-pressure system, the stronger the wind. The coriolis force caused by Earth's rotation gives winds within high-pressure systems their clockwise circulation in the northern hemisphere (as the wind moves outward and is deflected right from the center of high pressure) and anticlockwise circulation in the southern hemisphere (as the wind moves outward and is deflected left from the center of high pressure). Friction with land slows down the wind flowing out of high-pressure systems and causes wind to flow more outward (more ageostrophically) from the center.
Anticyclone | Structure
On the weather timescale, positive wind curls are related to low-pressure systems of a subsynoptic scale in space, energized by surface baroclinicity and latent heat release along the STF front. The SST front also anchors a meridional maximum in column-integrated water vapor, indicating a deep structure of the atmosphere response.
Subtropical Countercurrent | Introduction
A cyclone forms around a low-pressure area. Anticyclone, the term for the kind of weather around a high-pressure area, was coined in 1877 by Francis Galton to indicate an area whose winds revolved in the opposite direction of a cyclone. In British English, the opposite direction of clockwise is referred to as anticlockwise, making the label "anticyclones" a logical extension.
High-pressure area | Wind circulation in the northern and southern hemispheres
Highs are frequently associated with light winds at the surface and subsidence through the lower portion of the troposphere. In general, subsidence will dry out an air mass by adiabatic, or compressional, heating. Thus, high pressure typically brings clear skies. During the day, since no clouds are present to reflect sunlight, there is more incoming shortwave solar radiation and temperatures rise. At night, the absence of clouds means that outgoing longwave radiation (i.e. heat energy from the surface) is not absorbed, giving cooler diurnal low temperatures in all seasons. When surface winds become light, the subsidence produced directly under a high-pressure system can lead to a buildup of particulates in urban areas under the ridge, leading to widespread haze.
High-pressure area | Typical conditions
This is illustrated in panel (a) of the figure below. The balance that develops between these two forces results in a flow that parallels the horizontal pressure difference, or pressure gradient. In addition, when forces acting in the vertical dimension are dominated by the vertical pressure-gradient force and the gravitational force, hydrostatic balance occurs. In a barotropic atmosphere, where density is a function only of pressure, a horizontal pressure gradient will drive a geostrophic wind that is constant with height. However, if a horizontal temperature gradient exists along isobars, the isobars will also vary with the temperature. In the mid-latitudes there often is a positive coupling between pressure and temperature.
Thermal wind | Physical explanation
The atmospheric circulation pattern that George Hadley described was an attempt to explain the trade winds. The Hadley cell is a closed circulation loop which begins at the equator. There, moist air is warmed by the Earth's surface, decreases in density and rises. A similar air mass rising on the other side of the equator forces those rising air masses to move poleward. The rising air creates a low pressure zone near the equator. As the air moves poleward, it cools, becomes denser, and descends at about the 30th parallel, creating a high-pressure area. The descended air then travels toward the equator along the surface, replacing the air that rose from the equatorial zone, closing the loop of the Hadley cell.
Atmospheric circulation | Hadley cell
Cold cyclones are stronger aloft than at the Earth's surface, or stronger in areas of the troposphere with lower pressures, per the thermal wind relationship and the hypsometric equation. The hypsometric equation dictates that colder atmospheres have less room between pressure surfaces, which corresponds to the concept of lower atmospheric thickness, and the thermal wind relation indicates that in this situation, the wind increases with height. It also means that an isolated pool of cold air aloft is associated with the feature. Because of its cold center, the center shows an area of thickness minimum. Since the temperature at any given radius is similar and parallel to the orientation of the pressure gradient, and therefore the wind, it is barotropic in nature.
Cold-core low | Characteristics
Many of the ocean's largest currents circulate around warm, high-pressure areas called gyres. Though the circulation is not as significant as that in the air, the deflection caused by the Coriolis effect is what creates the spiralling pattern in these gyres. The spiralling wind pattern helps the hurricane form. The stronger the force from the Coriolis effect, the faster the wind spins and picks up additional energy, increasing the strength of the hurricane. Air within high-pressure systems rotates in a direction such that the Coriolis force is directed radially inwards, and nearly balanced by the outwardly radial pressure gradient. As a result, air travels clockwise around high pressure in the Northern Hemisphere and anticlockwise in the Southern Hemisphere.
Coriolis force | Meteorology
High-pressure systems are frequently associated with light winds at the surface and subsidence through the lower portion of the troposphere. Subsidence will generally dry out an air mass by adiabatic, or compressional, heating. Thus, high pressure typically brings clear skies. During the day, since no clouds are present to reflect sunlight, there is more incoming shortwave solar radiation and temperatures rise. At night, the absence of clouds means that outgoing longwave radiation (i.e. heat energy from the surface) is not absorbed, giving cooler diurnal low temperatures in all seasons. When surface winds become light, the subsidence produced directly under a high-pressure system can lead to a buildup of particulates in urban areas under the ridge, leading to widespread haze.
Synoptic scale meteorology | Surface high-pressure systems
High-pressure systems are frequently associated with light winds at the surface and subsidence through the lower portion of the troposphere. In general, subsidence will dry out an air mass by adiabatic or compressional heating. Thus, high pressure typically brings clear skies. During the day, since no clouds are present to reflect sunlight, there is more incoming shortwave solar radiation and temperatures rise. At night, the absence of clouds means that outgoing longwave radiation (i.e. heat energy from the surface) is not absorbed, giving cooler diurnal low temperatures in all seasons. When surface winds become light, the subsidence produced directly under a high-pressure system can lead to a buildup of particulates in urban areas under the ridge, leading to widespread haze.
Pressure system | High-pressure system
On constant pressure upper level charts, it is located within the highest height line contour.
High-pressure area | Formation
In general when the wind blows on a building then the air pressure, P varies all over the building surface (Fig 7). where Pa reference pressure (Pa)
Dynamic insulation | Influence of the wind
The thermal wind relation results from hydrostatic balance and geostrophic balance in the presence of a temperature gradient along constant pressure surfaces, or isobars.
Thermal wind | Introduction
On weather maps, these areas show converging winds (isotachs), also known as confluence, or converging height lines near or above the level of non-divergence, which is near the 500 hPa pressure surface about midway up the troposphere. Because they weaken with height, these high-pressure systems are cold.
Anticyclone | History
A streamline analysis is a series of arrows oriented parallel to wind, showing wind motion within a certain geographic area. Cs depict cyclonic flow or likely areas of low pressure, while As depict anticyclonic flow or likely positions of high-pressure areas. An area of confluent streamlines shows the location of shearlines within the tropics and subtropics.
Pressure system | Surface weather maps
On weather maps, high-pressure centers are associated with the letter H in English, within the isobar with the highest pressure value. On constant-pressure upper-level charts, anticyclones are located within the highest height line contour.
Anticyclone | Depiction on weather maps
As part of the Hadley cell, surface air flows toward the equator while the flow aloft is towards the poles. A low-pressure area of calm, light variable winds near the equator is known as the doldrums, near-equatorial trough, intertropical front, or the Intertropical Convergence Zone. When located within a monsoon region, this zone of low pressure and wind convergence is also known as the monsoon trough. Around 30° in both hemispheres, air begins to descend toward the surface in subtropical high-pressure belts known as subtropical ridges. The subsident (sinking) air is relatively dry because as it descends, the temperature increases, but the moisture content remains constant, which lowers the relative humidity of the air mass.
Trade winds | Cause
In the region called the Horse latitudes, between 30 and 35 degrees of latitude North or South, there is a series of semi-permanent anticyclones on the downward side of the Hadley cell of the general atmospheric circulation. The thermal contrasts over oceanic waters leads to the formation of these centers of action as a direct convective circulation is established between the equator and higher latitudes in a very weak Coriolis force zone. The Azores/Bermuda High is found in this zone. Similarly, low pressure areas are formed in the oceanic circulation near the Polar regions were sea water is much warmer than the land.
Centers of action | Causes
In general, winds are strongest immediately under the tropopause (except locally, during tornadoes, tropical cyclones or other anomalous situations). If two air masses of different temperatures or densities meet, the resulting pressure difference caused by the density difference (which ultimately causes wind) is highest within the transition zone. The wind does not flow directly from the hot to the cold area, but is deflected by the Coriolis effect and flows along the boundary of the two air masses. All these facts are consequences of the thermal wind relation. The balance of forces acting on an atmospheric air parcel in the vertical direction is primarily between the gravitational force acting on the mass of the parcel and the buoyancy force, or the difference in pressure between the top and bottom surfaces of the parcel.
Jet stream | Cause
The center of a subtropical gyre is a high pressure zone. Circulation around the high pressure is clockwise in the northern hemisphere and counterclockwise in the southern hemisphere, due to the Coriolis effect. The high pressure in the center is due to the westerly winds on the northern side of the gyre and easterly trade winds on the southern side. These cause frictional surface currents towards the latitude at the center of the gyre. This build-up of water in the center creates flow towards the equator in the upper of the ocean, through rather complex dynamics. This flow is returned towards the pole in an intensified western boundary current.
Ocean gyre | Subtropical gyres
Even though the wind speed near the center of a tropical cyclone is very fast, at any point on the Earth's surface it is not as fast as it is above that point away from the retarding influence of the Earth's surface. The slower speed of the air at the earth's surface prevents the barometric pressure from falling as low as would be expected from the barometric pressure at mid altitudes. This is compatible with Bernoulli's principle. The secondary flow at the Earth's surface is toward the center of the cyclone but is then drawn upward by the significantly lower pressure at mid and high altitudes.
Secondary flow | Tropical cyclones
Once aloft, air flows away from the storm's center, producing a shield of cirrus clouds. The previously mentioned processes result in a nearly axisymmetric wind field: Wind speeds are low at the center, increase rapidly moving outwards to the radius of maximum winds, and then decay more gradually with radius to large radii. However, the wind field often exhibits additional spatial and temporal variability due to the effects of localized processes, such as thunderstorm activity and horizontal flow instabilities. In the vertical direction, winds are strongest near the surface and decay with height within the troposphere.
Tropical cyclone | Wind field
High-pressure systems are frequently associated with light winds at the surface and subsidence of air from higher portions of the troposphere. Subsidence will generally warm an air mass by adiabatic (compressional) heating. Thus, high pressure typically brings clear skies. Because no clouds are present to reflect sunlight during the day, there is more incoming solar radiation and temperatures rise rapidly near the surface. At night, the absence of clouds means that outgoing longwave radiation (i.e. heat energy from the surface) is not blocked, giving cooler diurnal low temperatures in all seasons. When surface winds become light, the subsidence produced directly under a high-pressure system can lead to a buildup of particulates in urban areas under the high pressure, leading to widespread haze.
Anticyclone | Surface-based systems
The existence of upper-level (altitude) high pressure allows upper level divergence which leads to surface convergence. If a capping mid-level ridge does not exist, this leads to free convection and the development of showers and thunderstorms if the lower atmosphere is humid. Because a positive feedback loop develops between the convective tropical cyclone and the upper level high, the two system are strengthened. This loop stops once ocean temperatures cool to below , reducing the thunderstorm activity, which then weakens the upper level high pressure system.
Anticyclone | Upper tropospheric systems
On constant pressure upper level charts, it is located within the highest height line contour.
Synoptic scale meteorology | Surface high-pressure systems
Low pressure also creates surface winds deriving from high pressure zones. Various symbols are used not just for frontal zones and other surface boundaries on weather maps, but also to depict the present weather at various locations on the weather map. In addition, areas of precipitation help determine the frontal type and location.
Weather front | Surface weather analysis
The windfield of an extratropical cyclone constricts with distance in relation to surface level pressure, with the lowest pressure being found near the center, and the highest winds typically just on the cold/poleward side of warm fronts, occlusions, and cold fronts, where the pressure gradient force is highest. The area poleward and west of the cold and warm fronts connected to extratropical cyclones is known as the cold sector, while the area equatorward and east of its associated cold and warm fronts is known as the warm sector. The wind flow around an extratropical cyclone is counterclockwise in the northern hemisphere, and clockwise in the southern hemisphere, due to the Coriolis effect (this manner of rotation is generally referred to as "cyclonic").
Extratropical cyclone | Surface pressure and wind distribution
A useful heuristic is to imagine air starting from rest, experiencing a force directed from areas of high pressure toward areas of low pressure, called the pressure gradient force. If the air began to move in response to that force, however, the Coriolis "force" would deflect it, to the right of the motion in the northern hemisphere or to the left in the southern hemisphere. As the air accelerated, the deflection would increase until the Coriolis force's strength and direction balanced the pressure gradient force, a state called geostrophic balance. At this point, the flow is no longer moving from high to low pressure, but instead moves along isobars.
Geostrophic wind | Origin
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.
Wind | Introduction
Centers of surface high- and low-pressure areas that are found within closed isobars on a surface weather analysis are the absolute maxima and minima in the pressure field, and can tell a user in a glance what the general weather is in their vicinity. Weather maps in English-speaking countries will depict their highs as Hs and lows as Ls, while Spanish-speaking countries will depict their highs as As and lows as Bs.
Surface weather analysis | Pressure centers
The region below this stagnation point is called the interaction region, since all streamlines in this region are deflected downward into the street canyon. The characteristics of the vortex flow patterns inside the canyon strongly depend the height ratio of buildings on either side of the canyon. For a downwind building height H to upwind building height H ratio of 3, a single primary vortex has been observed. But for H/H=1.67, counter-rotating vortexes can occupy the whole depth of the canyon. Other factors that have an effect on the strength of this re-circulation flow are traffic induced turbulence and roof shapes of the buildings.
Urban canyon | Wind perpendicular to canyon
Low amounts of wind shear are needed, as high shear is disruptive to the storm's circulation. Lastly, a formative tropical cyclone needs a pre-existing system of disturbed weather, although without a circulation no cyclonic development will take place. Mesocyclones form as warm core cyclones over land, and can lead to tornado formation. Waterspouts can also form from mesocyclones, but more often develop from environments of high instability and low vertical wind shear. In deserts, lack of ground and plant moisture that would normally provide evaporative cooling can lead to intense, rapid solar heating of the lower layers of air. The hot air is less dense than surrounding cooler air.
Low-pressure area | Formation
A strong upper-level high pressure area generated moderate shear, though the National Hurricane Center noted that it would not interfere with intensification. However, the depression initially failed to strengthen significantly. It remained disorganized, with the circulation elongated northeastward to southwestward and deep convection displaced to the west. Early on July 23, a burst of deep convection occurred southwest of the center of circulation, while satellite imagery indicated that development of banding features. Until late on July 23, it was predicted that the depression would strengthen significantly and peak with winds of 70 mph (110 km/h), just slightly under hurricane intensity.
Tropical Storm Dalila (2007) | Meteorological history
In the Horse latitudes, generally in the region between 30 and 35 degrees of north and south latitude, there are more or less permanent highs. This is the subsiding part of Hadley cells. Indeed, near the equator, where the Coriolis force is quite weak, a direct circulation of air is established. In the lower levels of the atmosphere, the difference in temperature between the equator and the northerly warmer regions gives rise to the intertropical convergence zone where the warmer air rises because of convergence and relatively lower density. Subsequently, this air spreads north and south and cools at higher altitude and descends further north and south.
South Atlantic High | Formation
Near this center, the pressure gradient force (from the pressure at the center of the cyclone compared to the pressure outside the cyclone) and the Coriolis force must be in an approximate balance for the cyclone to avoid collapsing in on itself as a result of the difference in pressure. The central pressure of the cyclone will lower with increasing maturity, while outside of the cyclone, the sea-level pressure is about average. In most extratropical cyclones, the part of the cold front ahead of the cyclone will develop into a warm front, giving the frontal zone (as drawn on surface weather maps) a wave-like shape.
Extratropical cyclone | Surface pressure and wind distribution
This mid-level circulation is referred to as a Mesoscale Convective Vortex. The upper-levels contain an anti-cyclonic (clockwise in the Northern Hemisphere) rotating high pressure which is a sign of divergence of air. This high pressure is colder relative to its surrounding environment. This divergence at upper-levels and convergence of air at the surface along the cool pool's outflow boundary results in rising motion which aids maintenance of the MCC.
Mesoscale convective complex | Structure
Wind patterns push air across the surface mixing it horizontally. At higher altitudes, other atmospheric circulation regimes exist, such as the Brewer-Dobson circulation in the terrestrial stratosphere, which mixes the air. In Earth's mesophere, atmospheric waves become unstable and dissipate, creating turbulent mixing of this region.
Homosphere | Introduction
The barotropic vorticity equation assumes the atmosphere is nearly barotropic, which means that the direction and speed of the geostrophic wind are independent of height. In other words, there is no vertical wind shear of the geostrophic wind. It also implies that thickness contours (a proxy for temperature) are parallel to upper level height contours. In this type of atmosphere, high and low pressure areas are centers of warm and cold temperature anomalies. Warm-core highs (such as the subtropical ridge and the Bermuda-Azores high) and cold-core lows have strengthening winds with height, with the reverse true for cold-core highs (shallow Arctic highs) and warm-core lows (such as tropical cyclones).
Barotropic vorticity equation | Introduction
This slow, widespread ascent of the air in a region of low pressure can cause widespread cloud and rain if the air is of sufficiently high relative humidity.
Secondary flow | Wind near ground level
The wind-induced surface heat exchange (WISHE) is a positive feedback mechanism between the ocean and atmosphere in which a stronger ocean-to-atmosphere heat flux results in a stronger atmospheric circulation, which results in a strong heat flux. It has been hypothesized that this is the mechanism by which low pressure areas in the tropics develop into tropical cyclones.
Wind-induced surface heat exchange | Introduction
By manipulating the outflow and direction of theses fans the facility will be capable of producing time-dependent, straight, sheared or swirl winds of variable directionality. Therefore, a large variety of wind fields such as boundary layers, portions of hurricanes, tornados, downbursts, low level currents or gust fronts will be physically simulated. An active topographic capability will generate a wide diversity of surface topographies at unprecedented scales allowing wind simulations over areas of the order of 10 km2 . The same system will be used to locally seed for the Particle Image Velocimetry (PIV) system that will measure the wind field over extended areas.
WindEEE Dome | WindEEE Dome
Over elevated surfaces, heating of the ground exceeds the heating of the surrounding air at the same altitude above sea level, creating an associated thermal low over the terrain and enhancing any lows which would have otherwise existed, and changing the wind circulation of the region. In areas where there is rugged topography that significantly interrupts the environmental wind flow, the wind can change direction and accelerate parallel to the wind obstruction. This barrier jet can increase the low level wind by 45%. In mountainous areas, local distortion of the airflow is more severe. Jagged terrain combines to produce unpredictable flow patterns and turbulence, such as rotors.
Prevailing winds | Circulation in elevated regions
The Hadley cell, named after George Hadley, is a global scale tropical atmospheric circulation that features air rising near the Equator, flowing poleward at a height of 10 to 15 kilometers above the earth's surface, descending in the subtropics, and then returning equatorward near the surface. This circulation creates the trade winds, tropical rain-belts and hurricanes, subtropical deserts and the jet streams. Hadley cells are the low-altitude overtuning circulation that have air sinking at roughly zero to 30 degree latitude.
Hadley cell | Introduction
In anticyclones (air circulation around pressure highs), the Coriolis force is always inward (and positive), and the pressure force outward (and negative) irrespective of the hemisphere. The cross-trajectory momentum equation is formula_26. Dividing both sides by |"f"|"V", we obtain
Balanced flow | Pressure highs and anticyclones
At high altitudes, outward-spreading air rotates in the opposite direction. Cyclones rarely form along the equator due to the weak Coriolis effect present in this region.
Coriolis force | Flow around a low-pressure area
The basic principles of physics and the Coriolis effect satisfactorily explain that the direction of the wind in the atmosphere is parallel to the isobars. Measurements of wind speed and direction at heights well above ground level confirm that the speed of the wind matches that predicted by considerations of gradient flow, and the direction of the wind is indeed parallel to the isobars in the region. However, from ground level up to heights where the influence of the earth's surface can be neglected, the wind speed is less than predicted by the barometric pressure gradient, and the wind direction is partly across the isobars rather than parallel to them.
Secondary flow | Wind near ground level
Inversely, sometimes collapsed frontal systems will degenerate into troughs. Sometimes the region between two high pressure centers may also assume the character of a trough when there is a detectable wind shift noted at the surface. In the absence of a wind shift, the region is designated a col, akin to a geographic saddle between two mountain peaks.
Trough (meteorology) | Description
The poleward movement of the air in the upper part of the troposphere deviates toward the east, caused by the coriolis acceleration (a manifestation of conservation of angular momentum). At the ground level, however, the movement of the air toward the equator in the lower troposphere deviates toward the west, producing a wind from the east. The winds that flow to the west (from the east, easterly wind) at the ground level in the Hadley cell are called the Trade Winds. Though the Hadley cell is described as located at the equator, in the northern hemisphere it shifts to higher latitudes in June and July and toward lower latitudes in December and January, which is the result of the Sun's heating of the surface.
Atmospheric circulation | Hadley cell
This, combined with the rising of the hot air, results in a low-pressure area called a thermal low. Monsoon circulations are caused by thermal lows which form over large areas of land and their strength is driven by how land heats more quickly than the surrounding nearby ocean. This generates a steady wind blowing toward the land, bringing the moist near-surface air over the oceans with it. Similar rainfall is caused by the moist ocean-air being lifted upwards by mountains, surface heating, convergence at the surface, divergence aloft, or from storm-produced outflows at the surface. However the lifting occurs, the air cools due to expansion in lower pressure, which in turn produces condensation.
Low-pressure area | Formation
A low-pressure area, or "low", is a region where the atmospheric pressure at sea level is below that of surrounding locations. Low-pressure systems form under areas of wind divergence that occur in upper levels of the troposphere. The formation process of a low-pressure area is known as cyclogenesis. Within the field of atmospheric dynamics, areas of wind divergence aloft occur in two areas: Diverging winds aloft ahead of these troughs cause atmospheric lift within the troposphere below, which lowers surface pressures as upward motion partially counteracts the force of gravity. Thermal lows form due to localized heating caused by greater sunshine over deserts and other land masses.
Pressure system | Low-pressure system
A weak surface area of low pressure detached from a frontal system on August 8 while 1,150 miles (1,850 km) to the east of Bermuda. It moved southwestward, and on August 9, it generated convection as it passed beneath a cold-core upper-level low. The surface low and the upper-level low turned westward as it revolved around a common center, and by August 11, the surface low developed into a trough while 440 miles (700 km) south of Bermuda. As the system rapidly continued westward, much of the convection remained near the center of the upper-level low, preventing development of a closed surface circulation.
Hurricane Erika (2003) | Meteorological history
The balanced-flow approach identifies typical trajectories and steady-state wind speeds derived from balance-giving pressure patterns. In reality, pressure patterns and the motion of air masses are tied together, since accumulation (or density increase) of air mass somewhere increases the pressure on the ground and vice versa. Any new pressure gradient will cause a new displacement of air, and thus a continuous rearrangement. As weather itself demonstrates, steady-state conditions are exceptional. Since friction, pressure gradient and Coriolis forces do not necessarily balance out, air masses actually accelerate and decelerate, so the actual speed depends on its past values too. As seen next, the neat arrangement of pressure fields and flow trajectories, either parallel or at a right angle, in balanced-flow follows from the assumption of steady flow.
Balanced flow | Unsteadiness
Some climatological high-pressure areas acquire regionally based names. The land-based Siberian High often remains quasi-stationary for more than a month during the most frigid time of the year, making it unique in that regard. It is also a bit larger and more persistent than its counterpart in North America. Surface winds accelerating down valleys down the western Pacific ocean coastline, causing the winter monsoon. Arctic high-pressure systems such as the Siberian High are cold core, meaning that they weaken with height. The influence of the Azores High, also known as the Bermuda High, brings fair weather over much of the North Atlantic Ocean and mid to late summer heat waves in western Europe.
High-pressure area | In climatology
The primary flow around a tropical cyclone is parallel to the isobars – and hence circular. The closer to the center of the cyclone, the faster is the wind speed. In accordance with Bernoulli's principle where the wind speed is fastest the barometric pressure is lowest. Consequently, near the center of the cyclone the barometric pressure is very low. There is a strong pressure gradient across the isobars toward the center of the cyclone. This pressure gradient provides the centripetal force necessary for the circular motion of each parcel of air. This strong gradient, coupled with the slower speed of the air near the Earth's surface, causes a secondary flow at surface level toward the center of the cyclone, rather than a wholly circular flow.
Secondary flow | Tropical cyclones
Since localized areas of warm air are less dense than their surroundings, this warmer air rises, which lowers atmospheric pressure near that portion of the Earth's surface. Large-scale thermal lows over continents help create pressure gradients that drive monsoon circulations. Low-pressure areas can also form due to organized thunderstorm activity over warm water. When this occurs over the tropics in concert with the Intertropical Convergence Zone, it is known as a monsoon trough. Monsoon troughs reach their northerly extent in August and their southerly extent in February. When a convective low acquires a well-defined circulation in the tropics it is termed a tropical cyclone.
Pressure system | Low-pressure system
There are two wind flow bands that envelope Earth. One is in the southern hemisphere at the latitude of Patagonia, while the other is in the northern hemisphere over Europe. The flow height ranges from 800 meters up to 10,000 meters, while the width is 4,000 or 5,000 km. The average power of the wind is about 2 kW per square meter. High-altitude wind is much more powerful and constant than that at earth level, which is intense in very few places, and at full speed for only about 1,700-1,800 hours per year, which limits the annual production of energy.
KiteGen | High-altitude wind
The 500 hPa pressure surface can be used as a rough guide for the motion of many tropical cyclones. Shallower tropical cyclones, which have experienced vertical wind shear, tend to be steered by winds at the 700 hPa level. Use of the 300 and 200 hPa constant pressure charts can indicate the strength of systems in the lower troposphere, as stronger systems near the Earth's surface are reflected as stronger features at these levels of the atmosphere. Isotachs are drawn at these levels, which a lines of equal wind speed. They are helpful in finding maxima and minima in the wind pattern.
Weather map | Constant pressure charts
The thermal wind is the vector difference between the geostrophic wind at upper altitudes minus that at lower altitudes in the atmosphere. It is the hypothetical vertical wind shear that would exist if the winds obey geostrophic balance in the horizontal, while pressure obeys hydrostatic balance in the vertical. The combination of these two force balances is called "thermal wind balance", a term generalizable also to more complicated horizontal flow balances such as gradient wind balance"." Since the geostrophic wind at a given pressure level flows along geopotential height contours on a map, and the geopotential thickness of a pressure layer is proportional to virtual temperature, it follows that the thermal wind flows along thickness or temperature contours.
Thermal wind | Introduction
RFDs may present themselves as a clear slot wrapping itself at least two-thirds of the way around the tornado, but the clear slot is not always evident in cases where an RFD is present. Many documents indicate that surface pressure excesses up to a few millibars exist within RFDs. Some findings showed that within the RFDs equivalent potential temperature (θe) is cold with respect to the inflow. Moreover, the lowest wet-bulb potential temperature (θw) values observed at the surface were within the RFD. There are, however, also observations of warm, high-θe air within RFDs.
Rear flank downdraft | Thermodynamic characteristics
The thermal wind relation does not explain why the winds are organized into tight jets, rather than distributed more broadly over the hemisphere. One factor that contributes to the creation of a concentrated polar jet is the undercutting of sub-tropical air masses by the more dense polar air masses at the polar front. This causes surface low pressure and higher pressure at altitude. At high altitudes, lack of friction allows air to respond freely to the steep pressure gradient with low pressure at high altitude over the pole. This results in the formation of planetary wind circulations that experience a strong Coriolis deflection and thus can be considered 'quasi-geostrophic'.
Jet stream | Polar jet stream
A difference in air pressure causes an air displacement and generates the wind. The Coriolis Force deflects the air movement to the right in the northern hemisphere and the left in the southern one, which makes the winds parallel to the isobars on an elevation in pressure card. It's called the geostrophic wind. Pressure differences depend, in turn, the average temperature in the air column. As the sun does not heat the Earth evenly, there is a temperature difference between the poles and the equator, creating air masses with more or less homogeneous temperature with latitude. Differences in atmospheric pressure are also at the origin of the general atmospheric circulation while the air masses are separated by ribbons where temperature changes rapidly.
Air current | Horizontal currents
The parent high-pressure system is centered over the upper Midwest beneath the 250-mb jet entrance region, setting up conditions for CAD east of the Rocky Mountains. For dry onset classical events, the 250-mb jet is weaker and centered farther east relative to the diabatically enhanced classical events. The jet also does not extend as far southwest compared to diabatically enhanced classical CAD events. The center of the high-pressure system is farther east, so ridging extends southward into the south-central eastern United States. Although both types of classical events begin differently, their results are very similar.
Cold-air damming | Classical
In Canada, reference wind pressures are used in design and are based on the "mean hourly" wind speed having a probability of being exceeded per year of 1 in 50. The reference wind pressure (q) is calculated in Pascals using the following equation: q=(1/2)pV² where p is the air density in kg/m³ and V is wind speed in m/s.
Wind speed | Design of structures
C. Meneveau, “Big wind power: Seven questions for turbulence research” (2019), J. Turbulence 20, 2-20. Z. Wu, J. Lee, C. Meneveau and T. Zaki, “Application of a self-organizing map to identify the turbulent-boundary-layer interface in a transitional flow” (2019), Phys. Rev. Fluids 4, 023902. L.A. Martínez-Tossas and C. Meneveau, "Filtered lifting line theory and application to the actuator line model" (2019), J. Fluid Mech. 863, 269-292.
Charles Meneveau | Selected Journal Publications
1), the inertia of the geostrophic wind field (that is, its propensity to carry on forward) is creating a demand for decreasing thickness formula_48 in order for thermal wind balance to continue to hold. For instance, when there is an approaching upper-level cyclone or trough above the level in question, the part of formula_5 attributable to the first term in Eq. 1 is upward motion needed to create the increasingly cool air column that is required hypsometrically under the falling heights. That adiabatic reasoning must be supplemented by an appreciation of feedbacks from flow-dependent heating, such as latent heat release.
Omega equation | Interpretation
So, there will be a built of pressure, entering the gap, which will lead to higher wind loads on the sides of buildings. When wind blows over the face of a high rise building, a vortex is created by the downward flow on the front face (as shown in figure-1). The wind speed in the reverse direction near the ground level may have 140 percent of the reference wind speed. So, if any building exist in such region, then that may be subjected to damage (especially the roof of building may get severe damage). Such damage to buildings can be prohibited successfully, if the effects of wind loading are considered in the early stage of construction of a building.
CFD in buildings | Orientation, site and location selection
There is a pressure gradient toward the center of the vortex. This gradient, coupled with the slower speed of the air near the earth's surface, causes a "secondary flow" toward the center of the tornado or dust devil, rather than in a purely circular pattern. The slower speed of the air at the surface prevents the air pressure from falling as low as would normally be expected from the air pressure at greater heights. This is compatible with Bernoulli's principle. The secondary flow is toward the center of the tornado or dust devil, and is then drawn upward by the significantly lower pressure several thousands of feet above the surface in the case of a tornado, or several hundred feet in the case of a dust devil.
Secondary flow | Tornadoes and dust devils
Strong convection increased mainly to its east during the next day, and by 2300, UTC data from a reconnaissance aircraft confirmed the presence of a closed circulation center. At this point, the system was considered sufficiently organized to be upgraded to a tropical depression, with its center located 255 mi (360 km) to the southwest of the mouth of the Mississippi River. Situated within an area of weak steering currents, the depression remained nearly stationary during the initial stages of its existence. It exhibited a poor organization at the time; the circulation remained elongated, with a broad center of light winds removed from the sheared convective mass.
Tropical Storm Lee (2011) | Meteorological history
The near-surface wind field of a tropical cyclone is characterized by air rotating rapidly around a center of circulation while also flowing radially inwards. At the outer edge of the storm, air may be nearly calm; however, due to the Earth's rotation, the air has non-zero absolute angular momentum. As air flows radially inward, it begins to rotate cyclonically (counter-clockwise in the Northern Hemisphere, and clockwise in the Southern Hemisphere) to conserve angular momentum. At an inner radius, air begins to ascend to the top of the troposphere. This radius is typically coincident with the inner radius of the eyewall, and has the strongest near-surface winds of the storm; consequently, it is known as the "radius of maximum winds".
Tropical cyclone | Wind field
After being named Beni executed a small clockwise loop for two days to the southeast of Rennell, partially as a result of a strong surface ridge of high pressure to the south of the low level circulation center. Environmental conditions surrounding the system also fluctuated during 25 January, as a result of Beni's position to the north of the strongly diffluent flow on the northern side of the upper-level ridge axis. As a result, the systems low level circulation center became partially exposed with deep convection located to the west of the system before Beni's low level circulation slipped back under the convection during 26 January, with spiral bands wrapping tightly around the centre.
Cyclone Beni | Meteorological history
Usually, an anticyclone in the upper layers of the troposphere above the storm must be present as well—for extremely low surface pressures to develop. This is because air must be converging towards the low pressure at the surface which then forces the air to rise very rapidly in the eyewall of the storm, and due to conservation of mass requires a divergence of wind at the top of the troposphere. This process is aided by an upper-level anticyclone which helps channel this air away from the cyclone efficiently. Hot towers have been implicated in tropical cyclone rapid intensification, though they have diagnostically seen varied impacts across basins.
Rapid intensification | Internal
The static pressure of air is the pressure in a free-flowing air stream and is depicted by isobars in weather maps. Differences in static pressure arise from global and microclimate thermal phenomena and create the air flow we call wind. Dynamic pressure is the pressure exerted when the wind comes into contact with an object such as a hill or a building and it is described by the following equation: where (using SI units): The impact of wind on a building affects the ventilation and infiltration rates through it and the associated heat losses or heat gains. Wind speed increases with height and is lower towards the ground due to frictional drag.
Passive ventilation | Process
This system greatly affects sailing because the associated winds are generally not strong and sailing vessels have to go far north or south of the centre of the high pressure region depending on the direction of travel to find winds that are favourable in strength and direction.
South Atlantic High | Associated weather
Convective cells may develop in the vicinity of troughs and give birth to a tropical cyclone. Some tropical or subtropical regions such as the Philippines or south China are greatly affected by convection cells along a trough. In the mid-latitude westerlies, upper level troughs and ridges often alternate in a high-amplitude pattern. For a trough in the westerlies, the region just west of the trough axis is typically an area of convergent winds and descending air – and hence high pressure –, while the region just east of the trough axis is an area of fast, divergent winds and low pressure.
Trough (meteorology) | Weather associated
A mesohigh (sometimes called a "bubble high") is a mesoscale high-pressure area that forms beneath thunderstorms. While not always the case, it is usually associated with a mesoscale convective system. In the early stages of research on the subject, the mesohigh was often referred to as a "thunderstorm high".
Mesohigh | Introduction
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. 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. 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.
Wind | Station model
For instance, the thermal wind associated with pole-to-equator temperature gradients is the primary physical explanation for the jet stream in the upper half of the troposphere, which is the atmospheric layer extending from the surface of the planet up to altitudes of about 12-15 km. Mathematically, the thermal wind relation defines a vertical wind shear – a variation in wind speed or direction with height. The wind shear in this case is a function of a horizontal temperature gradient, which is a variation in temperature over some horizontal distance. Also called baroclinic flow, the thermal wind varies with height in proportion to the horizontal temperature gradient.
Thermal wind | Introduction
It can also be the focus of locally heavy precipitation, with thunderstorms possible if the atmosphere along the trowal is unstable enough for convection.
Extratropical cyclone | Surface pressure and wind distribution
Sea surface height (SSH) either wholly negative or wholly positive within such contours indicates cyclonic or anticyclonic polarity respectively. The centre of the eddy is defined to be the maximum or minimum SSH area within the W contour and the eddy diameter is defined as that of the circle with area equal to that enclosed by the W contour. The method may track noise-induced artifacts as eddies in certain cases, to avoid this, the W field is smoothed with half power filter cutoffs, for example of 1.5°×1.5° and only cases for which the W contour of at least 0.25° pixels or equivalent to an area about 50 km² can be considered.
Okubo-Weiss parameter | Introduction
An area of low pressure soon formed within the system just off the coast of South Carolina. Generally favorable environmental conditions fostered further organization of the low, though convection remained minimal for several days. During the latter half of June 30, a United States Air Force weather reconnaissance mission into the system confirmed the presence of a well-organized circulation; however, displacement of showers and thunderstorms from its center delayed its classification. Subsequent organization of convection into a banding feature along the system's southern flank, as depicted by radar imagery, prompted the NHC to designate the low as Tropical Depression One at 00:00 UTC on July 1.
Hurricane Arthur | Meteorological history
A high enthalpy wind tunnel is intended to study flow of air around objects moving at speeds much faster than the local speed of sound (hypersonic speeds). "Enthalpy" is the total energy of a gas stream, composed of internal energy due to temperature, the product of pressure and volume, and the velocity of flow. Duplication of the conditions of hypersonic flight requires large volumes of high-pressure, heated air; large pressurized hot reservoirs, and electric arcs, are two techniques used.
Wind tunnel | High enthalpy
In the real atmosphere, highs and lows have rarely the same strength and thus equal wind flow pattern around them. Thus the advection of warm or cold air is rarely equal. Furthermore, their shape and position are rarely as perfect as in the illustration quadrants either. There are thus 3 types of saddles or cols: the symmetrical, the cyclonic and anticyclonic. In the symmetrical saddle area, cyclonic and anticyclonic flows are in balance. This type is the least prevalent, since usually one of these two influences prevails. In the anticyclonic col, the curvature of the high pressure area isobars is greater than that of the cyclonic flow.
Col (meteorology) | Types
Wind flow modeling methods calculate very high-resolution maps of wind flow, often at horizontal resolution finer than 100-m. When doing fine resolution modeling, to avoid exceeding available computing resource, the typical model domains used by these small-scale models have a few kilometers in the horizontal direction and several hundred meters in the vertical direction. Models with such a small domain are not capable of capturing meso-scale atmospheric phenomena that often drive wind patterns. To over come this limitation nested modeling is sometimes used.
Wind resource assessment | Atmospheric simulation modeling
For regular cyclones (air circulation around pressure lows), the pressure force is inward (positive term) and the Coriolis force outward (negative term) irrespective of the hemisphere. The cross-trajectory momentum equation is formula_23. Dividing both sides by |"f"|"V", one recognizes that
Balanced flow | Pressure lows and cyclones