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The westerlies (blue) and trade winds (yellow and brown) Map prevailing winds on earth.png
The westerlies (blue) and trade winds (yellow and brown)

The westerlies, anti-trades, [1] or prevailing westerlies, are prevailing winds from the west toward the east in the middle latitudes between 30 and 60 degrees latitude. They originate from the high-pressure areas in the horse latitudes and trend towards the poles and steer extratropical cyclones in this general manner. [2] Tropical cyclones which cross the subtropical ridge axis into the westerlies recurve due to the increased westerly flow. The winds are predominantly from the southwest in the Northern Hemisphere and from the northwest in the Southern Hemisphere.

Prevailing winds

Prevailing winds are winds that blow predominantly from an individual direction over a particular point on the Earth's surface. The dominant winds are the trends in direction of wind with the highest speed over a particular point on the Earth's surface. A region's prevailing and dominant winds are enacted by global patterns of movement in the Earth's atmosphere. In general, easterly flow occurs at low and medium latitudes globally. In the mid-latitudes, westerly winds are dominant, and their strength is largely determined by the polar cyclone. In areas where winds tend to be light, the sea breeze/land breeze cycle is the most important to the prevailing wind; in areas which have variable terrain, mountain and valley breezes dominate the wind pattern. Highly elevated surfaces can induce a thermal low, which then augments the environmental wind flow.

Middle latitudes

The middle latitudes of Earth lie between 23°26'22" and 66°33'39" north, and between 23°26'22" and 66°33'39" south. They include Earth's subtropical temperate zones, which lie between the tropics and the polar circles. Weather fronts and extratropical cyclones are usually found in this area, as well as occasional tropical cyclones, which have traveled from their areas of formation closer to the Equator.

Latitude The angle between zenith at a point and the plane of the equator

In geography, latitude is a geographic coordinate that specifies the north–south position of a point on the Earth's surface. Latitude is an angle which ranges from 0° at the Equator to 90° at the poles. Lines of constant latitude, or parallels, run east–west as circles parallel to the equator. Latitude is used together with longitude to specify the precise location of features on the surface of the Earth. On its own, the term latitude should be taken to be the geodetic latitude as defined below. Briefly, geodetic latitude at a point is the angle formed by the vector perpendicular to the ellipsoidal surface from that point, and the equatorial plane. Also defined are six auxiliary latitudes which are used in special applications.


The westerlies are strongest in the winter hemisphere and times when the pressure is lower over the poles, while they are weakest in the summer hemisphere and when pressures are higher over the poles. The westerlies are particularly strong, especially in the Southern Hemisphere, in areas where land is absent, because land amplifies the flow pattern, making the current more north-south oriented, slowing the westerlies. The strongest westerly winds in the middle latitudes can come in the roaring forties, between 40 and 50 degrees latitude. The westerlies play an important role in carrying the warm, equatorial waters and winds to the western coasts of continents, especially in the southern hemisphere because of its vast oceanic expanse.


If the Earth was tidally locked to the Sun, solar heating would cause winds across the mid-latitudes to blow in a poleward direction, away from the subtropical ridge. However, the Coriolis effect caused by the rotation of Earth causes winds to steer to the right of what would otherwise be expected across the Northern Hemisphere, and left of what would be expected in the Southern Hemisphere. [3] This is why winds across the Northern Hemisphere tend to blow from the southwest, but they tend to be from the northwest in the Southern Hemisphere. [4] When pressures are lower over the poles, the strength of the westerlies increases, which has the effect of warming the mid-latitudes. This occurs when the Arctic oscillation is positive, and during winter low pressure near the poles is stronger than it would be during the summer. When it is negative and pressures are higher over the poles, the flow is more meridional, blowing from the direction of the pole towards the Equator, which brings cold air into the mid-latitudes. [5]

Earth Third planet from the Sun in the Solar System

Earth is the third planet from the Sun, and the only astronomical object known to harbor life. According to radiometric dating and other sources of evidence, Earth formed over 4.5 billion years ago. Earth's gravity interacts with other objects in space, especially the Sun and the Moon, Earth's only natural satellite. Earth revolves around the Sun in 365.26 days, a period known as an Earth year. During this time, Earth rotates about its axis about 366.26 times.

Tidal locking situation in which an astronomical objects orbital period matches its rotational period

Tidal locking occurs when the long-term interaction between a pair of co-orbiting astronomical bodies drives the rotation rate of at least one of them into the state where there is no more net transfer of angular momentum between this body and its orbit around the second body ; this condition of "no net transfer" must be satisfied over the course of one orbit around the second body. This does not mean that the rotation and spin rates are always perfectly synchronized throughout an orbit, as there can be some back and forth transfer over the course of an orbit. This effect arises from the gravitational gradient between the co-orbiting bodies, acting over a sufficiently long period of time.

Arctic oscillation

The Arctic oscillation (AO) or Northern Annular Mode/Northern Hemisphere Annular Mode (NAM) is a weather phenomenon at the Arctic and Antarctic poles north of 20 degrees latitude. The index varies over time with no particular periodicity, and is characterized by non-seasonal sea-level pressure anomalies of one sign in the Arctic, balanced by anomalies of opposite sign centered at about 37–45N.

Throughout the year, the westerlies vary in strength with the polar cyclone. As the cyclone reaches its maximum intensity in winter, the westerlies increase in strength. As the cyclone reaches its weakest intensity in summer, the Westerlies weaken. [6] An example of the impact of the westerlies is when dust plumes, originating in the Gobi desert combine with pollutants and spread large distances downwind, or eastward, into North America. [7] The westerlies can be particularly strong, especially in the Southern Hemisphere, where there is less land in the middle latitudes to cause the progression of west to east winds to slow down. In the Southern hemisphere, because of the stormy and cloudy conditions, it is usual to refer to the westerlies as the roaring forties, furious fifties, or shrieking sixties according to the varying degrees of latitude. [8]

Winter one of the Earths four temperate seasons, occurring between autumn and spring

Winter is the coldest season of the year in polar and temperate zones. It occurs after autumn and before spring in each year. Winter is caused by the axis of the Earth in that hemisphere being oriented away from the Sun. Different cultures define different dates as the start of winter, and some use a definition based on weather. When it is winter in the Northern Hemisphere, it is summer in the Southern Hemisphere, and vice versa. In many regions, winter is associated with snow and freezing temperatures. The moment of winter solstice is when the Sun's elevation with respect to the North or South Pole is at its most negative value. The day on which this occurs has the shortest day and the longest night, with day length increasing and night length decreasing as the season progresses after the solstice. The earliest sunset and latest sunrise dates outside the polar regions differ from the date of the winter solstice, however, and these depend on latitude, due to the variation in the solar day throughout the year caused by the Earth's elliptical orbit.

Summer one of the Earths four temperate seasons, occurring between spring and autumn

Summer is the hottest of the four temperate seasons, falling after spring and before autumn. At the summer solstice, the days are longest and the nights are shortest, with day length decreasing as the season progresses after the solstice. The date of the beginning of summer varies according to climate, tradition, and culture. When it is summer in the Northern Hemisphere, it is winter in the Southern Hemisphere, and vice versa.

North America Continent entirely within the Northern Hemisphere and almost all within the Western Hemisphere

North America is a continent entirely within the Northern Hemisphere and almost all within the Western Hemisphere; it is also considered by some to be a northern subcontinent of the Americas. It is bordered to the north by the Arctic Ocean, to the east by the Atlantic Ocean, to the west and south by the Pacific Ocean, and to the southeast by South America and the Caribbean Sea.

Impact on ocean currents

Benjamin Franklin's map of the Gulf Stream Franklingulfstream.jpg
Benjamin Franklin's map of the Gulf Stream

Due to persistent winds from west to east on the poleward sides of the subtropical ridges located in the Atlantic and Pacific oceans, ocean currents are driven in a similar manner in both hemispheres. The currents in the Northern Hemisphere are weaker than those in the Southern Hemisphere due to the differences in strength between the westerlies of each hemisphere. [9] The process of western intensification causes currents on the western boundary of an ocean basin to be stronger than those on the eastern boundary of an ocean. [10] These western ocean currents transport warm, tropical water polewards toward the polar regions. Ships crossing both oceans have taken advantage of the ocean currents for centuries.

Ocean current Directional mass flow of oceanic water generated by external or internal forces

An ocean current is a continuous, directed movement of sea water generated by a number of forces acting upon the water, including wind, the Coriolis effect, breaking waves, cabbeling, and temperature and salinity differences. Depth contours, shoreline configurations, and interactions with other currents influence a current's direction and strength. Ocean currents are primarily horizontal water movements.

The Antarctic Circumpolar Current (ACC), or the West Wind Drift, is an ocean current that flows from west to east around Antarctica. The ACC is the dominant circulation feature of the Southern Ocean and, at approximately 125 Sverdrups, the largest ocean current. [11] In the northern hemisphere, the Gulf Stream, part of the North Atlantic Subtropical Gyre, has led to the development of strong cyclones of all types at the base of the Westerlies, both within the atmosphere and within the ocean. [12] [13] [14] The Kuroshio (Japanese for "Black Tide") is a strong western boundary current in the western north Pacific Ocean, similar to the Gulf Stream, which has also contributed to the depth of ocean storms in that region.

The Antarctic Circumpolar Current (ACC) is an ocean current that flows clockwise from west to east around Antarctica. An alternative name for the ACC is the West Wind Drift. The ACC is the dominant circulation feature of the Southern Ocean and has a mean transport estimated at 100-150 Sverdrups, or possibly even higher, making it the largest ocean current. The current is circumpolar due to the lack of any landmass connecting with Antarctica and this keeps warm ocean waters away from Antarctica, enabling that continent to maintain its huge ice sheet.

Antarctica Polar continent in the Earths southern hemisphere

Antarctica is Earth's southernmost continent. It contains the geographic South Pole and is situated in the Antarctic region of the Southern Hemisphere, almost entirely south of the Antarctic Circle, and is surrounded by the Southern Ocean. At 14,200,000 square kilometres, it is the fifth-largest continent. For comparison, Antarctica is nearly twice the size of Australia. At 0.00008 people per square kilometre, it is by far the least densely populated continent. About 98% of Antarctica is covered by ice that averages 1.9 km in thickness, which extends to all but the northernmost reaches of the Antarctic Peninsula.

Southern Ocean The ocean around Antarctica

The Southern Ocean, also known as the Antarctic Ocean or the Austral Ocean, and the "Southern Icy Ocean".</ref> comprises the southernmost waters of the World Ocean, generally taken to be south of 60° S latitude and encircling Antarctica. As such, it is regarded as the fourth largest of the five principal oceanic divisions: smaller than the Pacific, Atlantic, and Indian Oceans but larger than the Arctic Ocean. This ocean zone is where cold, northward flowing waters from the Antarctic mix with warmer subantarctic waters.

Extratropical cyclones

A fictitious synoptic chart of an extratropical cyclone affecting the UK and Ireland. The blue arrows between isobars indicate the direction of the wind, while the "L" symbol denotes the centre of the "low". Note the occluded, cold and warm frontal boundaries. UK-Cyclone.gif
A fictitious synoptic chart of an extratropical cyclone affecting the UK and Ireland. The blue arrows between isobars indicate the direction of the wind, while the "L" symbol denotes the centre of the "low". Note the occluded, cold and warm frontal boundaries.

An extratropical cyclone is a synoptic scale low pressure weather system that has neither tropical nor polar characteristics, being connected with fronts and horizontal gradients in temperature and dew point otherwise known as "baroclinic zones". [15]

The descriptor "extratropical" refers to the fact that this type of cyclone generally occurs outside of the tropics, in the middle latitudes of the planet, where the Westerlies steer the system generally from west to east. These systems may also be described as "mid-latitude cyclones" due to their area of formation, or "post-tropical cyclones" where extratropical transition has occurred, [15] [16] and are often described as "depressions" or "lows" by weather forecasters and the general public. These are the everyday phenomena which along with anti-cyclones, drive the weather over much of the Earth.

Although extratropical cyclones are almost always classified as baroclinic since they form along zones of temperature and dewpoint gradient, they can sometimes become barotropic late in their life cycle when the temperature distribution around the cyclone becomes fairly uniform along the radius from the center of low pressure. [17] An extratropical cyclone can transform into a subtropical storm, and from there into a tropical cyclone, if it dwells over warm waters and develops central convection, which warms its core and causes temperature and dewpoint gradients near their centers to fade. [18]

Interaction with tropical cyclones

Storm track of Hurricane Bill (2009), showing a classic recurvature off the American coast in 2009 Bill 2009 track.png
Storm track of Hurricane Bill (2009), showing a classic recurvature off the American coast in 2009

When a tropical cyclone crosses the subtropical ridge axis, normally through a break in the high-pressure area caused by a system traversing the Westerlies, its general track around the high-pressure area is deflected significantly by winds moving towards the general low-pressure area to its north. When the cyclone track becomes strongly poleward with an easterly component, the cyclone has begun recurvature, entering the Westerlies. [19] A typhoon moving through the Pacific Ocean towards Asia, for example, will recurve offshore of Japan to the north, and then to the northeast, if the typhoon encounters southwesterly winds (blowing northeastward) around a low-pressure system passing over China or Siberia. Many tropical cyclones are eventually forced toward the northeast by extratropical cyclones in this manner, which move from west to east to the north of the subtropical ridge. An example of a tropical cyclone in recurvature was Typhoon Ioke in 2006, which took a similar trajectory. [20]

See also

Related Research Articles

Cyclone large scale air mass that rotates around a strong center of low pressure

In meteorology, a cyclone is a large scale air mass that rotates around a strong center of low atmospheric pressure. Cyclones are characterized by inward spiraling winds that rotate about a zone of low pressure. The largest low-pressure systems are polar vortices and extratropical cyclones of the largest scale. Warm-core cyclones such as tropical cyclones and subtropical cyclones also lie within the synoptic scale. Mesocyclones, tornadoes and dust devils lie within smaller mesoscale. Upper level cyclones can exist without the presence of a surface low, and can pinch off from the base of the tropical upper tropospheric trough during the summer months in the Northern Hemisphere. Cyclones have also been seen on extraterrestrial planets, such as Mars and Neptune. Cyclogenesis is the process of cyclone formation and intensification. Extratropical cyclones begin as waves in large regions of enhanced mid-latitude temperature contrasts called baroclinic zones. These zones contract and form weather fronts as the cyclonic circulation closes and intensifies. Later in their life cycle, extratropical cyclones occlude as cold air masses undercut the warmer air and become cold core systems. A cyclone's track is guided over the course of its 2 to 6 day life cycle by the steering flow of the subtropical jet stream.

Horse latitudes A lattitude of 25 deg to 35 degree of both hemisphere is called hourse lattitude.

Horse latitudes, subtropical ridges or subtropical highs are the subtropical latitudes between 30 and 35 degrees both north and south where Earth's atmosphere is dominated by the subtropical high, an area of high pressure, which suppresses precipitation and cloud formation, and has variable winds mixed with calm winds.

Subtropical cyclone

A subtropical cyclone is a weather system that has some characteristics of a tropical and an extratropical cyclone.

Anticyclone opposite to a cyclone

An anticyclone is a weather phenomenon defined by the United States National Weather Service's glossary as "a large-scale circulation of winds around a central region of high atmospheric pressure, clockwise in the Northern Hemisphere, counterclockwise in the Southern Hemisphere". Effects of surface-based anticyclones include clearing skies as well as cooler, drier air. Fog can also form overnight within a region of higher pressure. Mid-tropospheric systems, such as the subtropical ridge, deflect tropical cyclones around their periphery and cause a temperature inversion inhibiting free convection near their center, building up surface-based haze under their base. Anticyclones aloft can form within warm core lows such as tropical cyclones, due to descending cool air from the backside of upper troughs such as polar highs, or from large scale sinking such as the subtropical ridge. The evolution of an anticyclone depends on a few variables such as its size, intensity, moist-convection, Coriolis force etc.

Atmospheric circulation The large-scale movement of air, a process which distributes thermal energy about the Earths surface

Atmospheric circulation is the large-scale movement of air, and together with ocean circulation is the means by which thermal energy is redistributed on the surface of the Earth.

High-pressure area region where the atmospheric pressure at the surface of the planet is greater than its surrounding environment

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.

A low-pressure area, low, depression or cyclone is a region on the topographic map where the atmospheric pressure is lower than that of surrounding locations. Low-pressure systems form under areas of wind divergence that occur in the upper levels of the troposphere. The formation process of a low-pressure area is known as cyclogenesis. Within the field of meteorology, atmospheric divergence aloft occurs in two areas. The first area is on the east side of upper troughs, which form half of a Rossby wave within the Westerlies. A second area of wind divergence aloft occurs ahead of embedded shortwave troughs, which are of smaller wavelength. 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.

The synoptic scale in meteorology is a horizontal length scale of the order of 1000 kilometers or more. This corresponds to a horizontal scale typical of mid-latitude depressions. Most high and low-pressure areas seen on weather maps such as surface weather analyses are synoptic-scale systems, driven by the location of Rossby waves in their respective hemisphere. Low-pressure areas and their related frontal zones occur on the leading edge of a trough within the Rossby wave pattern, while high-pressure areas form on the back edge of the trough. Most precipitation areas occur near frontal zones. The word synoptic is derived from the Greek word συνοπτικός, meaning seen together.

Trade winds

The trade winds are the prevailing pattern of surface winds from the east toward the west (easterly) found in the tropics, within the lower portion of the Earth's atmosphere, in the lower part of the troposphere near the Earth's equator. The trade winds blow predominantly from the northeast in the Northern Hemisphere and from the southeast in the Southern Hemisphere, strengthening during the winter and when the Arctic oscillation is in its warm phase. Trade winds have been used by captains of sailing ships to cross the world's oceans for centuries, and enabled colonial expansion into the Americas and trade routes to become established across the Atlantic and Pacific oceans.

1986 Pacific typhoon season typhoon season in the Pacific Ocean

The 1986 Pacific typhoon season has no official bounds; it ran year-round in 1986, but most tropical cyclones tend to form in the northwestern Pacific Ocean between May and December. These dates conventionally delimit the period of each year when most tropical cyclones form in the northwestern Pacific Ocean. Tropical Storms formed in the entire west pacific basin were assigned a name by the Joint Typhoon Warning Center. Tropical depressions that enter or form in the Philippine area of responsibility are assigned a name by the Philippine Atmospheric, Geophysical and Astronomical Services Administration or PAGASA. This can often result in the same storm having two names.

1981 Pacific typhoon season typhoon season in the Pacific Ocean

The 1981 Pacific typhoon season was a slightly above average season that produced 29 tropical storms, 13 typhoons and two intense typhoons. The season ran throughout 1981, though most tropical cyclones typically develop between May and October. The season's first named storm, Freda, developed on March 12 while the final storm, Lee, dissipated on December 29. Tropical cyclones only accounted for 12 percent of the rainfall in Hong Kong this season, the lowest percentage for the protectorate since 1972.

Atlantic hurricane tropical cyclone that forms in the North Atlantic Ocean

An Atlantic hurricane or tropical storm is a tropical cyclone that forms in the Atlantic Ocean, usually between the months of June and November. A hurricane differs from a cyclone or typhoon only on the basis of location. A hurricane is a storm that occurs in the Atlantic Ocean and northeastern Pacific Ocean, a typhoon occurs in the northwestern Pacific Ocean, and a cyclone occurs in the south Pacific or Indian Ocean.

Block (meteorology) term in meteorology for large-scale patterns in the atmospheric pressure field that are nearly stationary, effectively “blocking” or redirecting migratory cyclones

Blocks in meteorology are large-scale patterns in the atmospheric pressure field that are nearly stationary, effectively “blocking” or redirecting migratory cyclones. They are also known as blocking highs or blocking anticyclones. These blocks can remain in place for several days or even weeks, causing the areas affected by them to have the same kind of weather for an extended period of time. In the Northern Hemisphere, extended blocking occurs most frequently in the spring over the eastern Pacific and Atlantic Oceans.

Monsoon trough

The monsoon trough is a portion of the Intertropical Convergence Zone in the Western Pacific, as depicted by a line on a weather map showing the locations of minimum sea level pressure, and as such, is a convergence zone between the wind patterns of the southern and northern hemispheres.

Tropical cyclogenesis

Tropical cyclogenesis is the development and strengthening of a tropical cyclone in the atmosphere. The mechanisms through which tropical cyclogenesis occurs are distinctly different from those through which temperate cyclogenesis occurs. Tropical cyclogenesis involves the development of a warm-core cyclone, due to significant convection in a favorable atmospheric environment.

Extratropical cyclone type of cyclone

Extratropical cyclones, sometimes called mid-latitude cyclones or wave cyclones, are low-pressure areas which, along with the anticyclones of high-pressure areas, drive the weather over much of the Earth. Extratropical cyclones are capable of producing anything from cloudiness and mild showers to heavy gales, thunderstorms, blizzards, and tornadoes. These types of cyclones are defined as large scale (synoptic) low pressure weather systems that occur in the middle latitudes of the Earth. In contrast with tropical cyclones, extratropical cyclones produce rapid changes in temperature and dew point along broad lines, called weather fronts, about the center of the cyclone.

Tropical cyclone Is a rotating storm system

A tropical cyclone is a rapidly rotating storm system characterized by a low-pressure center, a closed low-level atmospheric circulation, strong winds, and a spiral arrangement of thunderstorms that produce heavy rain. Depending on its location and strength, a tropical cyclone is referred to by different names, including hurricane, typhoon, tropical storm, cyclonic storm, tropical depression, and simply cyclone. A hurricane is a tropical cyclone that occurs in the Atlantic Ocean and northeastern Pacific Ocean, and a typhoon occurs in the northwestern Pacific Ocean; in the south Pacific or Indian Ocean, comparable storms are referred to simply as "tropical cyclones" or "severe cyclonic storms".

Upper tropospheric cyclonic vortex

An upper tropospheric cyclonic vortex is a vortex, or a circulation with a definable center, that usually moves slowly from east-northeast to west-southwest and is prevalent across Northern Hemisphere's warm season. Its circulations generally do not extend below 6,080 metres (19,950 ft) in altitude, as it is an example of a cold-core low. A weak inverted wave in the easterlies is generally found beneath it, and it may also be associated with broad areas of high-level clouds. Downward development results in an increase of cumulus clouds and the appearance of circulation at ground level. In rare cases, a warm-core cyclone can develop in its associated convective activity, resulting in a tropical cyclone and a weakening and southwest movement of the nearby upper tropospheric cyclonic vortex. Symbiotic relationships can exist between tropical cyclones and the upper level lows in their wake, with the two systems occasionally leading to their mutual strengthening. When they move over land during the warm season, an increase in monsoon rains occurs.

Typhoon Lekima (2013)

Typhoon Lekima was the second most intense tropical cyclone worldwide in 2013, as well as the twenty-ninth named storm and the eleventh typhoon of the annual typhoon season. It developed into a tropical storm late on October 20. After Lekima intensified into a typhoon and underwent rapid deepening in a very favourable environment on October 22, the system reached peak intensity on the following day. Maintaining its strength for over one day, Lekima began to weaken on October 24, as stronger vertical wind shear and mid-latitude westerlies began to make the typhoon significantly decay. On October 26, Lekima transitioned into an extratropical cyclone east of Japan.


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