Trade winds

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

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.

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 the rule 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.

Wind Flow of gases or air on a large scale

Wind is the flow of gases on a large scale. On the surface of the Earth, wind consists of the bulk movement of air. In outer space, solar wind is the movement of gases or charged particles from the Sun through space, while planetary wind is the outgassing of light chemical elements from a planet's atmosphere into space. Winds are commonly classified by their spatial scale, their speed, the types of forces that cause them, the regions in which they occur, and their effect. The strongest observed winds on a planet in the Solar System occur on Neptune and Saturn. Winds have various aspects, an important one being its velocity ; another the density of the gas involved; another its energy content or wind energy. Wind is also a great source of transportation for seeds and small birds; with time things can travel thousands of miles in the wind.

Tropics region of the Earth surrounding the Equator

The tropics are the region of the Earth surrounding the Equator. They are delimited in latitude by The Tropic of Cancer in the Northern Hemisphere at 23°26′12.5″ (or 23.4368°) N and the Tropic of Capricorn in the Southern Hemisphere at 23°26′12.5″ (or 23.4368°) S; these latitudes correspond to the axial tilt of the Earth. The tropics are also referred to as the tropical zone and the torrid zone. The tropics include all the areas on the Earth where the Sun contacts a point directly overhead at least once during the solar year - thus the latitude of the tropics is roughly equal to the angle of the Earth's axial tilt.

Contents

In meteorology, the trade winds act as the steering flow for tropical storms that form over the Atlantic, Pacific, and southern Indian Oceans and make landfall in North America, Southeast Asia, and Madagascar and eastern Africa, respectively. Trade winds also transport African dust westward across the Atlantic Ocean into the Caribbean Sea, as well as portions of southeastern North America. Shallow cumulus clouds are seen within trade wind regimes, and are capped from becoming taller by a trade wind inversion, which is caused by descending air aloft from within the subtropical ridge. The weaker the trade winds become, the more rainfall can be expected in the neighboring landmasses.

Meteorology Interdisciplinary scientific study of the atmosphere focusing on weather forecasting

Meteorology is a branch of the atmospheric sciences which includes atmospheric chemistry and atmospheric physics, with a major focus on weather forecasting. The study of meteorology dates back millennia, though significant progress in meteorology did not occur until the 18th century. The 19th century saw modest progress in the field after weather observation networks were formed across broad regions. Prior attempts at prediction of weather depended on historical data. It was not until after the elucidation of the laws of physics and more particularly, the development of the computer, allowing for the automated solution of a great many equations that model the weather, in the latter half of the 20th century that significant breakthroughs in weather forecasting were achieved. An important domain of weather forecasting is marine weather forecasting as it relates to maritime and coastal safety, in which weather effects also include atmospheric interactions with large bodies of water.

Indian Ocean The ocean between Africa, Asia, Australia and Antarctica (or the Southern Ocean)

The Indian Ocean is the third largest of the world's oceanic divisions, covering 70,560,000 km2 (27,240,000 sq mi). It is bounded by Asia on the north, on the west by Africa, on the east by Australia, and on the south by the Southern Ocean or, depending on definition, by Antarctica.

Atlantic Ocean Ocean between Europe, Africa and the Americas

The Atlantic Ocean is the second largest of the world's oceans, with an area of about 106,460,000 square kilometers. It covers approximately 20 percent of the Earth's surface and about 29 percent of its water surface area. It separates the "Old World" from the "New World".

History

A Spanish galleon Spanish Galleon.jpg
A Spanish galleon

The term trade winds originally derives from the early fourteenth century late Middle English word "trade," meaning "path" or "track." [1] The Portuguese recognized the importance of the trade winds (then the Volta do mar , meaning in Portuguese "turn of the sea" but also "return from the sea") in navigation in both the north and south Atlantic ocean as early as the 15th century. [2] From West Africa, the Portuguese had to sail away from continental Africa, that is, to west and northwest. They could then turn northeast, to the area around the Azores islands, and finally east to mainland Europe. They also learned that to reach South Africa, they needed to go far out in the ocean, head for Brazil, and around 30°S go east again. Following the African coast southbound means upwind in the Southern hemisphere. In the Pacific ocean, the full wind circulation, which included both the trade wind easterlies and higher-latitude Westerlies, was unknown to Europeans until Andres de Urdaneta's voyage in 1565. [3]

Middle English Stage of the English language from about the 12th through 15th centuries

Middle English was a form of the English language, spoken after the Norman conquest (1066) until the late 15th century. English underwent distinct variations and developments following the Old English period. Scholarly opinion varies, but the Oxford English Dictionary specifies the period when Middle English was spoken as being from 1150 to 1500. This stage of the development of the English language roughly followed the High to the Late Middle Ages.

<i>Volta do mar</i> navigational technique

Volta do mar, volta do mar largo, or volta do largo is a navigational technique perfected by Portuguese navigators during the Age of Discovery in the late fifteenth century, using the dependable phenomenon of the great permanent wind circle, the North Atlantic Gyre. This was a major step in the history of navigation, when an understanding of winds in the age of sail was crucial to success: the European sea empires would never have been established had the Europeans not figured out how the trade winds worked.

Azores Portuguese archipelago in the North Atlantic Ocean

The Azores, officially the Autonomous Region of the Azores, is one of the two autonomous regions of Portugal. It is an archipelago composed of nine volcanic islands in the North Atlantic Ocean about 1,360 km (850 mi) west of continental Portugal, about 1,643 km (1,021 mi) west of Lisbon, in continental Portugal, about 1,507 km (936 mi) northwest of Morocco, and about 1,925 km (1,196 mi) southeast of Newfoundland, Canada.

The captain of a sailing ship seeks a course along which the winds can be expected to blow in the direction of travel. [4] During the Age of Sail, the pattern of prevailing winds made various points of the globe easy or difficult to access, and therefore had a direct effect on European empire-building and thus on modern political geography. For example, Manila galleons could not sail into the wind at all. [3]

Age of Sail era dominated by sailing vesels out at sea

The Age of Sail was a period roughly corresponding to the early modern period in which international trade and naval warfare were dominated by sailing ships, lasting from the mid-16th to the mid-19th century.

Manila galleon Spanish trading ships

The Manila Galleons were Spanish trading ships which for two and a half centuries linked the Philippines with Mexico across the Pacific Ocean, making one or two round-trip voyages per year between the ports of Acapulco and Manila, which were both part of New Spain. The name of the galleon changed to reflect the city that the ship sailed from. The term Manila Galleons is also used to refer to the trade route itself between Acapulco and Manila, which lasted from 1565 to 1815.

Edmond Halley's map of the trade winds, 1686 Edmond Halley's map of the trade winds, 1686.jpg
Edmond Halley's map of the trade winds, 1686

By the 18th century, the importance of the trade winds to England's merchant fleet for crossing the Atlantic Ocean had led both the general public and etymologists to identify the name with a later meaning of "trade": "(foreign) commerce". [5] Between 1847 and 1849, Matthew Fontaine Maury collected enough information to create wind and current charts for the world's oceans. [6]

Matthew Fontaine Maury United States Navy officer

Matthew Fontaine Maury was an American astronomer, United States Navy officer, historian, oceanographer, meteorologist, cartographer, author, geologist, and educator.

Cause

3D map showing Hadley cells in relationship to trade winds on the surface. Earth Global Circulation.svg
3D map showing Hadley cells in relationship to trade winds on the surface.

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, [7] near-equatorial trough, [8] intertropical front, or the Intertropical Convergence Zone. [9] When located within a monsoon region, this zone of low pressure and wind convergence is also known as the monsoon trough. [10] 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 absolute humidity remains constant, which lowers the relative humidity of the air mass. This warm, dry air is known as a superior air mass and normally resides above a maritime tropical (warm and moist) air mass. An increase of temperature with height is known as a temperature inversion. When it occurs within a trade wind regime, it is known as a trade wind inversion. [11]

Hadley cell A global scale tropical atmospheric circulation feature

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.

Low-pressure area region where the atmospheric pressure is lower than that of surrounding locations

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.

Doldrums

The doldrums is a colloquial expression derived from historical maritime usage, which refers to those parts of the Atlantic Ocean and the Pacific Ocean affected by a low-pressure area around the equator where the prevailing winds are calm. The doldrums are also noted for calm periods when the winds disappear altogether, trapping sailing ships for periods of days or weeks. The term appears to have arisen in the eighteenth century, when trans-equator sailing voyages became more common. Since this zone is where two trade winds meet, it is also called the Intertropical Convergence Zone. They roughly lie between latitudes 5° north and south.

The surface air that flows from these subtropical high-pressure belts toward the Equator is deflected toward the west in both hemispheres by the Coriolis effect. [12] These winds blow predominantly from the northeast in the Northern Hemisphere and from the southeast in the Southern Hemisphere. [13] Because winds are named for the direction from which the wind is blowing, [14] these winds are called the northeasterly trade winds in the Northern Hemisphere and the southeasterly trade winds in the Southern Hemisphere. The trade winds of both hemispheres meet at the doldrums. [7]

As they blow across tropical regions, air masses heat up over lower latitudes due to more direct sunlight. Those that develop over land (continental) are drier and hotter than those that develop over oceans (maritime), and travel northward on the western periphery of the subtropical ridge. [15] Maritime tropical air masses are sometimes referred to as trade air masses. [16] The one region of the Earth which has an absence of trade winds is the north Indian ocean. [17]

Weather effects

Clouds which form above regions within trade wind regimes are typically composed of cumulus which extend no more than 4 kilometres (13,000 ft) in height, and are capped from being taller by the trade wind inversion. [18] Trade winds originate more from the direction of the poles (northeast in the Northern Hemisphere, southeast in the Southern Hemisphere) during the cold season, and are stronger in the winter than the summer. [19] As an example, the windy season in the Guianas, which lie at low latitudes in South America, occurs between January and April. [20] When the phase of the Arctic oscillation (AO) is warm, trade winds are stronger within the tropics. The cold phase of the AO leads to weaker trade winds. [21] When the trade winds are weaker, more extensive areas of rain fall upon landmasses within the tropics, such as Central America. [22]

During mid-summer in the Northern Hemisphere (July), the westward-moving trade winds south of the northward-moving subtropical ridge expand northwestward from the Caribbean sea into southeastern North America (Florida and Gulf Coast). When dust from the Sahara moving around the southern periphery of the ridge travels over land, rainfall is suppressed and the sky changes from a blue to a white appearance which leads to an increase in red sunsets. Its presence negatively impacts air quality by adding to the count of airborne particulates. [23] Although the Southeast USA has some of the cleanest air in North America, much of the African dust that reaches the United States affects Florida. [24] Since 1970, dust outbreaks have worsened due to periods of drought in Africa. There is a large variability in the dust transport to the Caribbean and Florida from year to year. [25] Dust events have been linked to a decline in the health of coral reefs across the Caribbean and Florida, primarily since the 1970s. [26]

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

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.

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.

Air mass a volume of air defined by its temperature and water vapor content

In meteorology, an air mass is a volume of air defined by its temperature and water vapor content. Air masses cover many hundreds or thousands of miles, and adapt to the characteristics of the surface below them. They are classified according to latitude and their continental or maritime source regions. Colder air masses are termed polar or arctic, while warmer air masses are deemed tropical. Continental and superior air masses are dry while maritime and monsoon air masses are moist. Weather fronts separate air masses with different density characteristics. Once an air mass moves away from its source region, underlying vegetation and water bodies can quickly modify its character. Classification schemes tackle an air mass' characteristics, as well as modification.

Subtropics

The subtropics are geographic and climate zones located roughly between the tropics at latitude 23.5° and temperate zones north and south of the Equator.

Intertropical Convergence Zone

The Intertropical Convergence Zone (ITCZ), known by sailors as the doldrums or the calms, is the area encircling Earth near the Equator, where the northeast and southeast trade winds converge.

Tropical climate climate in the tropical region

A tropical climate in the Köppen climate classification is a non-arid climate in which all twelve months have mean temperatures of warmer than 18.4 °C (65.1 °F). In tropical climates there are often only two seasons: a wet season and a dry season. Tropical climates are frost-free, and changes in the solar angle are small. In tropical climates temperature remains relatively constant (hot) throughout the year. Sunlight is intense.

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.

Westerlies

The westerlies, anti-trades, 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. 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.

Tropical wave type of atmospheric trough

Tropical waves, easterly waves, or tropical easterly waves, also known as African easterly waves in the Atlantic region, are a type of atmospheric trough, an elongated area of relatively low air pressure, oriented north to south, which moves from east to west across the tropics, causing areas of cloudiness and thunderstorms. West-moving waves can also form from the tail end of frontal zones in the subtropics and tropics, and may be referred to as easterly waves, but these waves are not properly called tropical waves; they are a form of inverted trough sharing many characteristics with fully tropical waves. All tropical waves form in the easterly flow along the equatorward side of the subtropical ridge or belt of high pressure which lies north and south of the Intertropical Convergence Zone (ITCZ). Tropical waves are generally carried westward by the prevailing easterly winds along the tropics and subtropics near the equator. They can lead to the formation of tropical cyclones in the north Atlantic and northeastern Pacific basins. A tropical wave study is aided by Hovmöller diagrams, a graph of meteorological data.

This is a list of meteorology topics. The terms relate to meteorology, the interdisciplinary scientific study of the atmosphere that focuses on weather processes and forecasting.

Mesoscale convective system complex of thunderstorms organized on a larger scale

A mesoscale convective system (MCS) is a complex of thunderstorms that becomes organized on a scale larger than the individual thunderstorms but smaller than extratropical cyclones, and normally persists for several hours or more. A mesoscale convective system's overall cloud and precipitation pattern may be round or linear in shape, and include weather systems such as tropical cyclones, squall lines, lake-effect snow events, polar lows, and Mesoscale Convective Complexes (MCCs), and generally form near weather fronts. The type that forms during the warm season over land has been noted across North America, Europe, and Asia, with a maximum in activity noted during the late afternoon and evening hours.

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.

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.

There are a number of explanations of the asymmetry of the Intertropical Convergence Zone (ITCZ), known by sailors as The Doldrums.

Papagayo Jet

The Papagayo jet, also referred to as the Papagayo Wind or the Papagayo Wind Jet, are strong intermittent winds that blow approximately 70 km north of the Gulf of Papagayo, after which they are named. The jet winds travel southwest from the Caribbean and the Gulf of Mexico to the Pacific Ocean through a pass in the Cordillera mountains at Lake Nicaragua. The jet follows the same path as the northeast trade winds in this region; however, due to a unique combination of synoptic scale meteorology and orographic phenomena, the jet winds can reach much greater speeds than their trade wind counterparts. That is to say, the winds occur when cold high-pressure systems from the North American continent meet warm moist air over the Caribbean and Gulf of Mexico, generating winds that are then funneled through a mountain pass in the Cordillera. The Papagayo jet is also not unique to this region. There are two other breaks in the Cordillera where this same phenomenon occurs, one at the Chivela Pass in México and another at the Panama Canal, producing the Tehuano (Tehuantepecer) and the Panama jets respectively.

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