![Map_prevailing_winds_on_earth](https://upload.wikimedia.org/wikipedia/commons/1/18/Map_prevailing_winds_on_earth.png)
In
meteorology
Meteorology is a branch of the atmospheric sciences (which include atmospheric chemistry and physics) with a major focus on weather forecasting. The study of meteorology dates back millennia, though significant progress in meteorology did no ...
, prevailing wind in a region of the
Earth
Earth is the third planet from the Sun and the only astronomical object known to harbor life. While large volumes of water can be found throughout the Solar System, only Earth sustains liquid surface water. About 71% of Earth's surfa ...
's surface is a surface
wind
Wind is the natural movement of air or other gases relative to a planet's surface. Winds occur 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 ho ...
that blows predominantly from a particular direction. The dominant winds are the trends in direction of wind with the highest speed over a particular point on the Earth's surface at any given time. A region's prevailing and dominant winds are the result of global patterns of movement in the
Earth's atmosphere
The atmosphere of Earth is the layer of gases, known collectively as air, retained by Earth's gravity that surrounds the planet and forms its planetary atmosphere. The atmosphere of Earth protects life on Earth by creating pressure allowing fo ...
. In general, winds are predominantly easterly at low
latitude
In geography, latitude is a coordinate that specifies the north– south position of a point on the surface of the Earth or another celestial body. Latitude is given as an angle that ranges from –90° at the south pole to 90° at the north pol ...
s globally. In the mid-latitudes, westerly winds are dominant, and their strength is largely determined by the
polar cyclone
A circumpolar vortex, or simply polar vortex, is a large region of cold, rotating air that encircles both of Earth's polar regions. Polar vortices also exist on other rotating, low-obliquity planetary bodies. The term polar vortex can be used to ...
. In areas where winds tend to be light, the
sea breeze
A sea breeze or onshore breeze is any wind that blows from a large body of water toward or onto a landmass; it develops due to differences in air pressure created by the differing heat capacities of water and dry land. As such, sea breezes ar ...
/land breeze cycle is the most important cause of 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
Thermal lows, or heat lows, are non-frontal low-pressure areas that occur over the continents in the subtropics during the warm season, as the result of intense heating when compared to their surrounding environments.Glossary of Meteorology (200 ...
, which then augments the environmental wind flow.
Wind rose
A wind rose is a graphic tool used by meteorologists to give a succinct view of how wind speed and direction are typically distributed at a particular location. Historically, wind roses were predecessors of the compass rose (found on charts), as ...
s are tools used to display the direction of the prevailing wind. Knowledge of the prevailing wind allows the development of prevention strategies for
wind erosion
Aeolian processes, also spelled eolian, pertain to wind activity in the study of geology and weather and specifically to the wind's ability to shape the surface of the Earth (or other planets). Winds may erode, transport, and deposit materials ...
of agricultural land, such as across the
Great Plains.
Sand dunes can orient themselves perpendicular to the prevailing wind direction in coastal and
desert locations. Insects drift along with the prevailing wind, but the flight of birds is less dependent on it. Prevailing winds in mountain locations can lead to significant
rainfall
Rain is water droplets that have condensed from atmospheric water vapor and then fall under gravity. Rain is a major component of the water cycle and is responsible for depositing most of the fresh water on the Earth. It provides water f ...
gradients, ranging from wet across windward-facing slopes to desert-like conditions along their lee slopes. Prevailing winds can vary due to the uneven heating of the Earth.
Wind rose
A
wind rose
A wind rose is a graphic tool used by meteorologists to give a succinct view of how wind speed and direction are typically distributed at a particular location. Historically, wind roses were predecessors of the compass rose (found on charts), as ...
is a graphic tool used by
meteorologist
A meteorologist is a scientist who studies and works in the field of meteorology aiming to understand or predict Earth's atmospheric phenomena including the weather. Those who study meteorological phenomena are meteorologists in research, while t ...
s to give a succinct view of how
wind speed
In meteorology, wind speed, or wind flow speed, is a fundamental atmospheric quantity caused by air moving from high to low pressure, usually due to changes in temperature. Wind speed is now commonly measured with an anemometer.
Wind speed ...
and direction are typically distributed at a particular location. Presented in a
polar coordinate
In mathematics, the polar coordinate system is a two-dimensional coordinate system in which each point on a plane is determined by a distance from a reference point and an angle from a reference direction. The reference point (analogous to the ...
grid, the wind rose shows the frequency of winds blowing from particular directions. The length of each spoke around the circle is related to the proportion of the time that the wind blows from each direction. Each concentric circle represents a different proportion, increasing outwards from zero at the center. A wind rose plot may contain additional information, in that each spoke is broken down into color-coded bands that show wind speed ranges. Wind roses typically show 8 or 16
cardinal directions, such as north (N), NNE, NE, etc., although they may be subdivided into as many as
32 directions.
Climatology
Trades and their impact
The trade winds (also called trades) are the prevailing pattern of
easterly surface winds found in the
tropics
The tropics are the regions of Earth surrounding the Equator. They are defined in latitude by the Tropic of Cancer in the Northern Hemisphere at N and the Tropic of Capricorn in
the Southern Hemisphere at S. The tropics are also referr ...
near the Earth's
equator, equatorward of the
subtropical ridge. These winds blow predominantly from the northeast in the
Northern Hemisphere and from the southeast in the
Southern Hemisphere.
The trade winds act as the
steering flow for
tropical cyclones
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 and squalls. Dependi ...
that form over world's oceans, guiding their path westward. Trade winds also steer African dust westward across the Atlantic Ocean into the
Caribbean sea
The Caribbean Sea ( es, Mar Caribe; french: Mer des Caraïbes; ht, Lanmè Karayib; jam, Kiaribiyan Sii; nl, Caraïbische Zee; pap, Laman Karibe) is a sea of the Atlantic Ocean in the tropics of the Western Hemisphere. It is bounded by Mexico ...
, as well as portions of southeast North America.
[Science Daily (1999-07-14)]
African Dust Called A Major Factor Affecting Southeast U.S. Air Quality.
Retrieved on 2007-06-10.
Westerlies and their impact
The
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 t ...
or the prevailing westerlies are the prevailing winds in the
middle latitudes
The middle latitudes (also called the mid-latitudes, sometimes midlatitudes, or moderate latitudes) are a spatial region on Earth located between the Tropic of Cancer ( latitudes 23°26'22") to the Arctic Circle (66°33'39"), and Tropic of Cap ...
(i.e. between 35 and 65 degrees
latitude
In geography, latitude is a coordinate that specifies the north– south position of a point on the surface of the Earth or another celestial body. Latitude is given as an angle that ranges from –90° at the south pole to 90° at the north pol ...
), which blow in areas poleward of the
high pressure area
A high-pressure area, high, or anticyclone, is an area near the surface of a planet where the atmospheric pressure is greater than the pressure in the surrounding regions. Highs are middle-scale meteorological features that result from interpl ...
known as the subtropical ridge in the
horse latitudes
The horse latitudes are the latitudes about 30 degrees north and south of the Equator. They are characterized by sunny skies, calm winds, and very little precipitation. They are also known as subtropical ridges, or highs. It is a high-pressure ...
.
These prevailing winds blow from the
west
West or Occident is one of the four cardinal directions or points of the compass. It is the opposite direction from east and is the direction in which the Sun sets on the Earth.
Etymology
The word "west" is a Germanic word passed into some ...
to the
east
East or Orient is one of the four cardinal directions or points of the compass. It is the opposite direction from west and is the direction from which the Sun rises on the Earth.
Etymology
As in other languages, the word is formed from the fac ...
, and steer extra-tropical cyclones in this general direction. The winds are predominantly from the southwest in the Northern Hemisphere and from the northwest in the Southern Hemisphere.
They are strongest in the winter when the pressure is lower over the poles, such as when the polar cyclone is strongest, and weakest during the summer when the polar cyclone is weakest and when pressures are higher over the poles.
Together with the
trade wind
The trade winds or easterlies are the permanent east-to-west prevailing winds that flow in the Earth's equatorial region. The trade winds blow mainly from the northeast in the Northern Hemisphere and from the southeast in the Southern Hemisph ...
s, the westerlies enabled a round-trip trade route for sailing ships crossing the Atlantic and Pacific oceans, as the westerlies lead to the development of strong ocean currents in both hemispheres. The westerlies can be particularly strong, especially in the southern hemisphere, where there is less land in the middle latitudes to cause the flow pattern to amplify, which slows the winds down. The strongest westerly winds in the middle latitudes are called the
Roaring Forties
The Roaring Forties are strong westerly winds found in the Southern Hemisphere, generally between the latitudes of 40°S and 50°S. The strong west-to-east air currents are caused by the combination of air being displaced from the Equator ...
, between
40 and
50 degrees south latitude, within the Southern Hemisphere. 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.
The westerlies explain why coastal Western North America tends to be wet, especially from Northern Washington to Alaska, during the winter. Differential heating from the Sun between the land which is quite cool and the ocean which is relatively warm causes areas of low pressure to develop over land. This results in moisture-rich air flowing east from the Pacific Ocean, causing frequent rainstorms and wind on the coast. This moisture continues to flow eastward until
orographic lift
Orographic lift occurs when an air mass is forced from a low elevation to a higher elevation as it moves over rising terrain. As the air mass gains altitude it quickly cools down adiabatically, which can raise the relative humidity to 100% and cr ...
caused by the Coast Ranges, and the Cascade, Sierra Nevada, Columbia, and Rocky Mountains causes a
rain shadow effect which limits further penetration of these systems and associated rainfall eastward. This trend reverses in the summer when strong heating of the land causes high pressure and tends to block moisture-rich air from the Pacific from reaching land. This explains why most of coastal Western North America in the highest latitude experiences dry summers, despite vast rainfall in the winter.
Polar easterlies
The
polar easterlies
In the study of Earth's atmosphere, polar easterlies are the dry, cold prevailing winds that blow around the high-pressure areas of the polar highs at the North and South Poles.Glossary of Meteorology (2009).They are called Easterlies because the ...
(also known as Polar Hadley cells) are the dry, cold prevailing winds that blow from the
high-pressure area
A high-pressure area, high, or anticyclone, is an area near the surface of a planet where the atmospheric pressure is greater than the pressure in the surrounding regions. Highs are middle-scale meteorological features that result from interpl ...
s of the
polar highs at the
North
North is one of the four compass points or cardinal directions. It is the opposite of south and is perpendicular to east and west. ''North'' is a noun, adjective, or adverb indicating direction or geography.
Etymology
The word ''north ...
and
South Pole
The South Pole, also known as the Geographic South Pole, Terrestrial South Pole or 90th Parallel South, is one of the two points where Earth's axis of rotation intersects its surface. It is the southernmost point on Earth and lies antipod ...
s towards the low-pressure areas within the westerlies at high latitudes. Like trade winds and unlike the westerlies, these prevailing winds blow from the
east
East or Orient is one of the four cardinal directions or points of the compass. It is the opposite direction from west and is the direction from which the Sun rises on the Earth.
Etymology
As in other languages, the word is formed from the fac ...
to the
west
West or Occident is one of the four cardinal directions or points of the compass. It is the opposite direction from east and is the direction in which the Sun sets on the Earth.
Etymology
The word "west" is a Germanic word passed into some ...
, and are often weak and irregular. Due to the low
sun
The Sun is the star at the center of the Solar System. It is a nearly perfect ball of hot plasma, heated to incandescence by nuclear fusion reactions in its core. The Sun radiates this energy mainly as light, ultraviolet, and infrared radi ...
angle,
cold
Cold is the presence of low temperature, especially in the atmosphere. In common usage, cold is often a subjective perception. A lower bound to temperature is absolute zero, defined as 0.00K on the Kelvin scale, an absolute thermodynamic ...
air builds up and
subsides at the pole creating surface high-pressure areas, forcing an outflow of air toward the
equator; that outflow is deflected westward by the
Coriolis effect
In physics, the Coriolis force is an inertial or fictitious force that acts on objects in motion within a frame of reference that rotates with respect to an inertial frame. In a reference frame with clockwise rotation, the force acts to the ...
.
Local considerations
Sea and land breezes
In areas where the wind flow is light, sea breezes and land breezes are important factors in a location's prevailing winds. The
sea is warmed by the sun to a greater depth than the land due to its greater
specific heat
In thermodynamics, the specific heat capacity (symbol ) of a substance is the heat capacity of a sample of the substance divided by the mass of the sample, also sometimes referred to as massic heat capacity. Informally, it is the amount of heat t ...
. The sea therefore has a greater capacity for absorbing heat than the land, so the surface of the sea warms up more slowly than the land's surface. As the temperature of the surface of the
land
Land, also known as dry land, ground, or earth, is the solid terrestrial surface of the planet Earth that is not submerged by the ocean or other bodies of water. It makes up 29% of Earth's surface and includes the continents and various isla ...
rises, the land heats the air above it. The warm air is less dense and so it rises. This rising air over the land lowers the
sea level pressure
Atmospheric pressure, also known as barometric pressure (after the barometer), is the pressure within the atmosphere of Earth. The standard atmosphere (symbol: atm) is a unit of pressure defined as , which is equivalent to 1013.25 millibars, 7 ...
by about 0.2%. The cooler air above the sea, now with higher sea level pressure, flows towards the land into the lower pressure, creating a cooler breeze near the coast.
The strength of the sea breeze is directly proportional to the temperature difference between the land mass and the sea. If an off-shore wind of exists, the sea breeze is not likely to develop. At night, the land cools off more quickly than the ocean due to differences in their
specific heat
In thermodynamics, the specific heat capacity (symbol ) of a substance is the heat capacity of a sample of the substance divided by the mass of the sample, also sometimes referred to as massic heat capacity. Informally, it is the amount of heat t ...
values, which forces the daytime sea breeze to dissipate. If the temperature onshore cools below the temperature offshore, the pressure over the water will be lower than that of the land, establishing a land breeze, as long as an onshore wind is not strong enough to oppose it.
[JetStream: An Online School For Weather (2008)]
The Sea Breeze.
National Weather Service
The National Weather Service (NWS) is an agency of the United States federal government that is tasked with providing weather forecasts, warnings of hazardous weather, and other weather-related products to organizations and the public for the ...
. Retrieved on 2006-10-24.
Circulation in elevated regions
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
Thermal lows, or heat lows, are non-frontal low-pressure areas that occur over the continents in the subtropics during the warm season, as the result of intense heating when compared to their surrounding environments.Glossary of Meteorology (200 ...
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
Topography is the study of the forms and features of land surfaces. The topography of an area may refer to the land forms and features themselves, or a description or depiction in maps.
Topography is a field of geoscience and planetary sc ...
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
rotor
Rotor may refer to:
Science and technology
Engineering
* Rotor (electric), the non-stationary part of an alternator or electric motor, operating with a stationary element so called the stator
*Helicopter rotor, the rotary wing(s) of a rotorcraft ...
s. Strong
updraft
In meteorology, an updraft is a small-scale current of rising air, often within a cloud.
Overview
Localized regions of warm or cool air will exhibit vertical movement. A mass of warm air will typically be less dense than the surrounding regi ...
s, downdrafts and
eddies
In fluid dynamics, an eddy is the swirling of a fluid and the reverse current created when the fluid is in a turbulent flow regime. The moving fluid creates a space devoid of downstream-flowing fluid on the downstream side of the object. Fluid b ...
develop as the air flows over hills and down valleys. Wind direction changes due to the contour of the land. If there is a
pass
Pass, PASS, The Pass or Passed may refer to:
Places
* Pass, County Meath, a townland in Ireland
* Pass, Poland, a village in Poland
* Pass, an alternate term for a number of straits: see List of straits
* Mountain pass, a lower place in a moun ...
in the mountain range, winds will rush through the pass with considerable speed due to the
Bernoulli principle
In fluid dynamics, Bernoulli's principle states that an increase in the speed of a fluid occurs simultaneously with a decrease in static pressure or a decrease in the fluid's potential energy. The principle is named after the Swiss mathematici ...
that describes an inverse relationship between speed and pressure. The airflow can remain turbulent and erratic for some distance downwind into the flatter countryside. These conditions are dangerous to ascending and descending airplanes.
Daytime heating and nighttime cooling of the hilly slopes lead to day to night variations in the airflow, similar to the relationship between sea breeze and land breeze. At night, the sides of the hills cool through radiation of the heat. The air along the hills becomes cooler and denser, blowing down into the valley, drawn by gravity. This is known a mountain breeze. If the slopes are covered with ice and snow, the mountain breeze will blow during the day, carrying the cold dense air into the warmer, barren valleys. The slopes of hills not covered by snow will be warmed during the day. The air that comes in contact with the warmed slopes becomes warmer and less dense and flows uphill. This is known as an
anabatic wind or valley breeze.
Effect on precipitation
Orographic precipitation occurs on the
windward
Windward () and leeward () are terms used to describe the direction of the wind. Windward is ''upwind'' from the point of reference, i.e. towards the direction from which the wind is coming; leeward is ''downwind'' from the point of reference ...
side of mountains and is caused by the rising air motion of a large-scale flow of moist air across the mountain ridge, resulting in
adiabatic cooling and
condensation. In mountainous parts of the world subjected to consistent winds (for example, the
trade wind
The trade winds or easterlies are the permanent east-to-west prevailing winds that flow in the Earth's equatorial region. The trade winds blow mainly from the northeast in the Northern Hemisphere and from the southeast in the Southern Hemisph ...
s), a more moist
climate
Climate is the long-term weather pattern in an area, typically averaged over 30 years. More rigorously, it is the mean and variability of meteorological variables over a time spanning from months to millions of years. Some of the meteorologi ...
usually prevails on the windward side of a mountain than on the
leeward or downwind side. Moisture is removed by orographic lift, leaving drier air (see
foehn wind
A Foehn or Föhn (, , ), is a type of dry, relatively warm, downslope wind that occurs in the lee (downwind side) of a mountain range.
It is a rain shadow wind that results from the subsequent adiabatic warming of air that has dropped most of ...
) on the descending and generally warming, leeward side where a
rain shadow is observed.
[Dr. Michael Pidwirny (2008)]
CHAPTER 8: Introduction to the Hydrosphere (e). Cloud Formation Processes.
Physical Geography. Retrieved on 2009-01-01.
In South America, the
Andes
The Andes, Andes Mountains or Andean Mountains (; ) are the longest continental mountain range in the world, forming a continuous highland along the western edge of South America. The range is long, wide (widest between 18°S – 20°S ...
mountain range blocks
Pacific
The Pacific Ocean is the largest and deepest of Earth's five oceanic divisions. It extends from the Arctic Ocean in the north to the Southern Ocean (or, depending on definition, to Antarctica) in the south, and is bounded by the contine ...
moisture that arrives in that continent, resulting in a desertlike climate just downwind across western Argentina. The
Sierra Nevada range creates the same effect in North America forming the
Great Basin and
Mojave Desert
The Mojave Desert ( ; mov, Hayikwiir Mat'aar; es, Desierto de Mojave) is a desert in the rain shadow of the Sierra Nevada mountains in the Southwestern United States. It is named for the indigenous Mojave people. It is located primarily ...
s.
Effect on nature
Insects are swept along by the prevailing winds, while birds follow their own course. As such, fine line patterns within
weather radar
Weather radar, also called weather surveillance radar (WSR) and Doppler weather radar, is a type of radar used to locate precipitation, calculate its motion, and estimate its type (rain, snow, hail etc.). Modern weather radars are mostly pulse- ...
imagery, associated with converging winds, are dominated by insect returns. In the
Great Plains,
wind erosion
Aeolian processes, also spelled eolian, pertain to wind activity in the study of geology and weather and specifically to the wind's ability to shape the surface of the Earth (or other planets). Winds may erode, transport, and deposit materials ...
of agricultural land is a significant problem, and is mainly driven by the prevailing wind. Because of this,
wind barrier strips have been developed to minimize this type of erosion. The strips can be in the form of soil ridges, crop strips, crops rows, or trees which act as wind breaks. They are oriented perpendicular to the wind in order to be most effective. In regions with minimal vegetation, such as coastal and
desert areas, transverse
sand dunes orient themselves perpendicular to the prevailing wind direction, while longitudinal dunes orient themselves parallel to the prevailing winds.
[Ronald Greeley, James D. Iversen (1987)]
Wind as a geological process on Earth, Mars, Venus and Titan.
CUP Archive, pp. 158–162. . Retrieved on 2009-04-26.
See also
*
Trade winds
The trade winds or easterlies are the permanent east-to-west prevailing winds that flow in the Earth's equatorial region. The trade winds blow mainly from the northeast in the Northern Hemisphere and from the southeast in the Southern Hemisp ...
*
Wind speed
In meteorology, wind speed, or wind flow speed, is a fundamental atmospheric quantity caused by air moving from high to low pressure, usually due to changes in temperature. Wind speed is now commonly measured with an anemometer.
Wind speed ...
*
Atmospheric circulation
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. The Earth's atmospheric circulation varies from year to year, bu ...
*
Winds in the age of sail
The captain of a steam ship naturally chooses the shortest route to nearby destinations. Since a sailing ship is usually pushed by winds and currents, its captain must find a route where the wind will probably blow in the right direction. Tackin ...
References
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