Where Does The Wind Come From?
The source of the wind is the sun. The winds come from the suns energy falling on the earth's surface. This will last as long as the sun continues, and so is renewable for practical purposes into the distant future. This leads to convection currents in the atmosphere, i.e. the movement of air due to changes in its density and pressure. We can gain an understanding of how global circulation works by developing two simplified models of processes that produce this system. Drivers of the winds are examined; gravitational force, pressure gradient force, Coriolis force, centrifugal force, friction force. The major zones, Arctic, Mid-latitudes, and Tropical are are also compared.
The source of the wind is the sun. The winds come from the energy of the sun falling on the earth's surface. This will last as long as the sun continues, and so is renewable for practical purposes into the distant future. Due to the orientation of the earth's surface to the sun's rays near the equator the rays strike the surface at more optimum angles. The effect is that the air near the surface in tropical regions is heated more than the air near the surface of the polar regions. This leads to convection currents in the atmosphere, i.e. the movement of air due to changes in its density and pressure. This air movement is the principal cause of the winds.
For descriptive and analytic purposes the atmosphere is commonly divided up into the following scales:
|Synoptic||300 to 3,000km|
|Mesoscale||2 to 300km|
|Microscale||2mm to 2km|
Drivers of the Wind at a Global (or Planetary) Scale
1. Gravitational Force
This is related to the mass of the air and is directed downwards perpendicular to the earths surface. This is usually counter balanced largely by the upwards pressure gradient forces.
2. Pressure Gradient Force
The pressure gradient force is the change in pressure over a distance. This is manifested mostly in a horizontal direction, but there is also a vertical component, which was referred to in 1 above as counterbalancing the gravitational force on air. The differences in pressure can usually be traced back to uneven heating of the earth surface by the sun. The force can be derived from the following equation:
3. Coriolis Force
This is due to the fact that the surface of the earth is traveling at different speeds as one move from the equator to the poles. This is an apparent force. It only comes into play if the air is moving.
Coriolis Force = +/- f V
where v is the velocity of the wind,
and f = 2X Earth's rate of rotation X the sine of the latitude.
4. Centrifugal Force
When wind moves in a circular path, e.g. in an anticyclone or cyclone it experiences a force directed outwards from the centre of rotation. This effect does not play a role at the small scale level.
Centrifugal Force = V² / R
where V is the wind velocity
R is the radius of the curved path.
5. Friction Force
This is caused by wind flowing over the rough surface of the earth.
Friction Force = - kV
where V is the wind velocity
K is a constant.
Drivers of the Wind at a Small Scale
Small scale refers to distances of some tens of kilometers and a little more. In both cases, for large scale and for small scale, the geographic features of the landscape, the shape and the surface, combine with the forces in forming the winds. Some of the same forces that were encountered in understanding the origins of the large scale global winds are found again i.e.
1. Gravitational force
2. Pressure gradient force
3. Friction force
At this scale the coriolis or centrifugal forces do not play a significant role.
Air Masses and their role in relation to winds
An air mass is a large body of air with generally uniform temperature and humidity. The area from where an air mass originates is called a "source region." Air mass source regions range from extensive snow covered polar areas to deserts to tropical oceans. The longer the air mass stays over its source region, the more likely it will acquire the properties of the surface below.
The five principal air mass classifications according to their source region are:
- Polar (P) - formed pole ward of 60° north and south.
- Tropical (T) - formed within about 30° of the equator.
- Arctic (A) - formed over the Arctic
- Continental (c) - formed over large land masses.
- Maritime (m) - formed over the oceans. * (3)
As these air masses move around the earth they can begin to acquire additional attributes. The six air mass classifications that influence Ireland and U.K. are:
- Tropical continental (Tc)
- Polar continental (Pc)
- Tropical maritime (Tm)
- Polar maritime (Pm)
- Returning polar maritime (rPm)
- Arctic maritime (Am) * (4)
Global Scale Circulation of the Atmosphere - A Simple Model of Global Circulation
The general circulation of the atmosphere refers to the mean global flow over a long enough period to remove variations caused by weather systems but short enough to capture seasonal and monthly variability. The major influences on the general circulation of the atmosphere are:
- Differential heating
- Rotation of the planet
We can gain an understanding of how global (or planetary) circulation works by developing two simplified graphical models of processes that produce this system.* (5) The first model will be founded on the following simplifying assumptions:
- The Earth is not rotating in space.
- The Earth's surface is composed of similar materials.
- The global reception of solar insolation and outgoing longwave radiation cause a temperature gradient of hotter air at the equator and colder air at the poles.
Based on these assumptions, air circulation on the Earth should approximate the patterns shown on Figure 1. In this illustration, each hemisphere contains one three-dimensional circulation cell.
As described in the diagram above, surface air flow is from the poles to the equator. When the air reaches the equator, it is lifted vertically by the processes of convection and convergence. When it reaches the top of the troposphere, it begins to flow once again horizontally. However, the direction of flow is now from the equator to the poles. At the poles, the air in the upper atmosphere then descends to the Earth's surface to complete the cycle of flow.
Three Cell Model of Global Circulation
If we eliminate the first assumption, i.e. the Earth is not rotating in space, the pattern of flow described in the model above would be altered, and the mesoscale flow of the atmosphere would more closely approximate the actual global circulation on the Earth. Planetary rotation would cause the development of three circulation cells in each hemisphere rather than one (see Figure 2). These three circulation cells are known as the: Hadley cell; Ferrel cell; and Polar cell.
In the new model, the equator still remains the warmest location on the Earth. This area of greater heat acts as zone of thermal lows known as the intertropical convergence zone (ITCZ). The Intertropical Convergence Zone draws in surface air from the subtropics. When this subtropical air reaches the equator, it rises into the upper atmosphere because of convergence and convection. It attains a maximum vertical altitude of about 14 kilometers (top of the troposphere), and then begins flowing horizontally to the North and South Poles. Coriolis force causes the deflection of this moving air in the upper atmosphere, and by about 30° of latitude the air begins to flow zonally from west to east. This zonal flow is known as the subtropical jet stream. The zonal flow also causes the accumulation of air in the upper atmosphere as it is no longer flowing meridionally. To compensate for this accumulation, some of the air in the upper atmosphere sinks back to the surface creating the subtropical high pressure zone. From this zone, the surface air travels in two directions. A portion of the air moves back toward the equator completing the circulation system known as the Hadley cell. This moving air is also deflected by the Coriolis effect to create the Northeast Trades (right deflection) and Southeast Trades (left deflection). The surface air moving towards the poles from the subtropical high zone from 30 latitude to 60 is also deflected by Coriolis acceleration producing the Westerlies. Coriolis force deflects this wind to cause it to flow west to east forming the polar jet stream at roughly 60° North and South. On the Earth's surface at 60° North and South latitude, the subtropical Westerlies collide with cold air traveling from the poles. This collision results in frontal uplift and the creation of the sub polar lows or mid- latitude cyclones. Most of this lifted air is directed to the polar vortex where it moves downward to create the polar high.
Actual Global Surface Circulation
Figure 3 and 4 describes the actual surface pressures for the Earth as determined from 39 years of records. The circulation patterns produced by these pressures seem different somewhat from the three cell model in Figure 2. These differences are caused primarily by two factors. First, the earth's surface is not composed of uniform materials. The two surface materials that dominate are water and land. These two materials behave differently in terms of heating and cooling, causing latitudinal pressure zones to be less uniform. The second factor influencing actual circulation patterns is elevation. Elevation tends to cause pressure centers to become intensified when altitude is increased. This is especially true of high pressure systems.
In figures 3 and 4 monthly average sea-level pressure and prevailing winds for the Earth's surface are for the years 1959-1997. Atmosphere pressure values are adjusted for elevation and are described relative to sea-level. Pressure values are indicated by color shading (see the legend in the graphic). Blue shades indicate pressure lower than the global average, while yellow to orange shades are higher than average measurements. Wind direction is shown with arrows. Wind speed is indicated by the length of these arrows (see the legend on the graphic. * (5)
On these modified graphics, we can better visualize the intertropical convergence zone (ITCZ), subtropical high pressure zone, and the sub polar lows. The intertropical convergence zone is identified on the figures by a red line. The formation of this band of low pressure is the result of solar heating and the convergence of the trade winds. In January, the intertropical convergence zone is found south of the equator (Figure 3). During this time period, the southern hemisphere is tilted towards the sun and receives higher inputs of shortwave radiation. Note that the line representing the intertropical convergence zone is not straight and parallel to the lines of latitude. Bends in the line occur because of the different heating characteristics of land and water. Over the continents of Africa, South America, and Australia, these bends are toward the South Pole. This phenomenon occurs because land heats up faster then ocean.
During July, the intertropical convergence zone (ITCZ) is generally found north of the equator (Figure 4). This shift in position occurs because the altitude of the sun is now higher in the Northern Hemisphere. The greatest spatial shift in the ITCZ, from January to July, occurs in the eastern half of the image. This shift is about 40° of latitude in some places. The more intense July sun causes land areas of Northern Africa and Asia rapidly warm creating the Asiatic Low which becomes part of the ITCZ. In the winter months, the intertropical convergence zone is pushed south by the development of an intense high pressure system over central Asia (compare Figures 3 and 4). The extreme movement of the ITCZ in this part of the world also helps to intensify the development of a regional winds system called the Asian monsoon.
The subtropical high pressure zone does not form a uniform area of high pressure stretching around the world in reality. Instead, the system consists of several localized anticyclonic cells of high pressure. These systems are located roughly at about 20 to 30 degrees of latitude and are labeled with the letter H on Figures 3 and 4. The subtropical high pressure systems develop because of the presence of descending air currents from the Hadley cell. These systems intensify over the ocean during the summer or high sun season. During this season, the air over the ocean bodies remains relatively cool because of the slower heating of water relative to land surfaces. Over land, intensification takes place in the winter months. At this time, land cools off quickly, relative to ocean, forming large cold continental air masses.
The sub polar lows form a continuous zone of low pressure in the southern hemisphere at a latitude of between 50 and 70° (Figures 3 and 4). The intensity of the sub polar lows varies with season. This zone is most intense during southern hemisphere summer (Figure 4). At this time, greater differences in temperature exist between air masses found either side of this zone. North of the sub polar low belt, summer heating warms subtropical air masses. South of the zone, the ice covered surface of Antarctica reflects much of the incoming solar radiation back to space. As a consequence, air masses above Antarctica remain cold because very little heating of the ground surface takes place. The meeting of the warm subtropical and cold polar air masses at the sub polar low zone enhances frontal uplift and the formation of intense low pressure systems.
In the northern hemisphere, the sub polar lows do not form a continuous belt circling the globe (Figures 3 and 4). Instead, they exist as localized cyclonic centers of low pressure. In the northern hemisphere winter, these pressure centers are intense and located over the oceans just to the south of Greenland and the Aleutian Islands. These areas of low pressure are responsible for spawning many mid-latitude cyclones. The development of the sub polar lows in summer only occurs weakly (Figure 4 - over Greenland and Baffin Island, Canada), unlike the southern hemisphere. The reason for this phenomenon is that considerable heating of the Earth's surface occurs from 60 to 90° north. As a result, cold polar air masses generally do not form. * (5)
Most European windstorms are caused by extratropical cyclones (ETCs). These mid-latitude weather systems derive their energy from horizontal temperature contrasts between cold, polar air masses and warm, subtropical air masses. Because the temperature contrasts between these air masses are greatest during winter, the frequency and intensity of European windstorms peak during this season as well. With respect to the origins of the cyclones, including the temperate difference between the different air masses referred to above, may be added the effect of the jet stream, and the presence of mountains or other surface boundary, e.g. a coastline.* (3)
European windstorms differ dynamically from Atlantic hurricanes or IndoPacific typhoons, which are strong tropical cyclones. Tropical cyclones derive their energy from the vertical temperature contrast between the warm lower layer and cold upper reaches of the tropical atmosphere. Because the hurricane ultimately derives its energy from warm seawater, its winds quickly diminish when its eye moves over cooler water or land.
The horizontal temperature gradient that powers an extratropical cyclone can persist as the storm center moves over land. Thus wind speeds in these storms can occasionally remain high, or even increase, after landfall. Also, while a hurricane can sustain the same minimum central pressure for days, the energy that drives an extratropical cyclone rapidly decays as the air masses within it intermix—a single cyclone typically exists independently for three to five days.
Storms are steered by the polar jet stream, so the position of the jet stream defines the storm track. The polar jet stream and storm track move from month to month and from year to year, but storms moving along the storm track tend to reinforce the jet, and keep it from shifting. This is called eddy feedback. By reinforcing the jet, eddy feedback helps the jet and storm track "remember" their position over weeks to months, and thus enhances the persistence and predictability of mid-latitude variability. It is also responsible for the tendency of European windstorms to occur in series. Eddy feedback is not, however, sufficiently robust to provide persistence from year to year.
The storms that affect Europe are the eastern outliers of the approximately 200 disturbances that form in the north Atlantic each year and track eastward along with the jet stream. The track of the jet is affected by the position of the Azores high pressure cell and the Icelandic low. Shifts in the relative strength of these pressure cells control where a storm might make landfall in Europe. * (7)
The jet stream is an area of strong winds that are concentrated in a relatively narrow band in the upper troposphere of the middle latitudes and subtropical regions of the northern and southern hemispheres. Flowing in a semi- continuous band around the globe from west to east, it is caused by the changes in air temperature which is greatest where the cold polar air moving towards the equator meets the warmer equatorial air moving polar ward. The jet stream is found at heights between 10 to 15km above the ground. It is hundreds of kilometers wide and has speeds of 50 to 100m/ s. The pathway followed by jet streams is quite variable. They may break apart into two separate streams and then rejoin, or not. They also tend to meander north and south from a central west-east axis. The movement of the jet streams is an important factor in determining weather conditions in mid-latitude regions. In the northern hemisphere it is found between the latitudes of 40ºN to 70ºN. It is strongest in the winter due to higher thermal contrast. Its edges are very turbulent. The Polar Jet stream is stronger than the Subtropical Jet stream. * (8)
Weather patterns in the UK and Ireland are dominated by the polar jet stream. The position of the polar jet stream varies with the seasons. During the autumn and winter, the polar jet stream generally lies directly over the UK and Ireland, feeding in west to south-westerly winds off the Atlantic along with several low pressure systems, travelling from west to east. The polar jet stream is at its strongest during the autumn when the thermal contrast between the cold air to the north and warm air to the south is at its greatest. A stronger polar jet stream has a greater potential to produce deeper, more intense areas of low pressure and stronger winds. Therefore, the autumn months tend to be the windiest months across the UK and Ireland. During the summer months, the warm air to the south of Britain generally shifts further northwards and the jet stream pushes further north across Iceland. This takes the areas of low pressure further north and also allows the Azores High to move further north. The UK and Ireland’s most settled weather with light winds therefore tends to occur in the summer when either the Azores High shifts further northwards or a ridge of high pressure extends north giving a period of lighter winds.
Further Notes on Extratropical Cyclones
The forward motion of an extratropical cyclone generally ranges from 9- 20m/s, but can reach as high as 40m/s. Both the average and extreme values greatly exceed the forward speeds of low-latitude hurricanes. The storm’s wind field thus becomes highly asymmetrical, with damaging winds generally restricted to the south or right-hand side of the track. As an extratropical cyclone travels eastward toward the European coastline, it may interact with a trough of low pressure high in the atmosphere. This interaction can cause the storm to intensify rapidly, as happened during the October 1987 windstorm. European windstorms may produce wind gusts of more than 67m/s in exposed coastal areas and the mountains. Inland, wind gusts may reach about 45m/s. Sustained wind speeds in the most intense Atlantic hurricanes can exceed 70m/s, with wind gusts exceeding 90m/s. * (7)
Extratropical cyclones differ structurally from tropical cyclones in that they have fronts, discrete boundaries between air masses of different temperature. The highest surface winds and heaviest precipitation often occur along these frontal boundaries. There are generally two fronts: one where warm air overrides cold air to the northeast of the center, another where cold air wedges beneath warm air to the southwest (this is where the highest surface winds generally occur).
Because the lower tropical atmosphere is typically of a uniform temperature, tropical cyclones have no fronts. Instead, their winds and precipitation are concentrated in the ring of intense thunderstorms surrounding the eye, and in the spiral rain bands that feed those thunderstorms. Another structural difference between extratropical and tropical cyclones is that the highest winds in the former occur high in the atmosphere, whereas the highest winds in a tropical cyclone occur near the surface.
Shape and Size of Cyclones
Extratropical cyclones change shape during their development. When mature, their cloud patterns are often "comma" shaped, with a warm front and a cold front radiating from an area of low-pressure at the storm center. These storms grow in size during their life-cycle, with individual storms reaching a range of maximum sizes, 200-2000 km in greatest dimension. The size of an extratropical cyclone can best be defined by the swath of its damaging winds. This is usually found to the right of the storm track and is typically 150-500 km wide.
Tropical cyclones typically exhibit a more symmetric, concentric shape. The size of a tropical cyclone can best be defined as the radius of the area experiencing winds greater than or equal to hurricane force (≥ 33 m/s). This is typically ~140km, although values range widely, from 1100 km in Typhoon Tip (western Pacific, 1979) to 50 km in Tropical Cyclone Tracy (Darwin, Australia, 1974). * (7)
There are different definitions of the tropical zone. One that is appropriate to to our focus would be:
"The region in which winds blow primarily from the east (approximately ± 30° latitude), except for the regional monsoon. The easterly trade winds flow out of the subtropical high into the equatorial trough. They converge at the Intertropical Convergence Zone (ITCZ), which is usually identified as an intermittent band of clouds in the low pressure belt or equatorial trough." * (9)
The meteorology differs from that of the Mid-Latitudes, which is dominated by synoptic cyclones related to strong gradients in air temperature and density. The large-scale midlatitude atmosphere is noted for being baroclinic, while barotropic conditions are usually found in the tropics for these scales. The air masses are more homogeneous. The temperature variations are small. The Coriolis force is hardly present. Weather changes come from small differences in wind velocity gradients or small differences in heating. The tropical atmosphere is in near thermal inertia because of the warm ocean surface currents and moist planetary boundary layer. Tropical circulations, such as the semi-permanent pressure systems, the longitudinally dependent components of the general circulation, and the monsoons, together account for the low frequency variability of the tropics compared with the higher frequency variability of the mid-latitude general circulations. * (9)
The winds are dominated in winter by lows over the North Atlantic in the vicinity of Iceland, and North Pacific in the vicinity of the Aleutian Islands. An intense high is located in Siberia near Lake Baikal. In addition there are the weaker high pressure areas over the Beaufort Sea and North America. The winds are weaker in winter than in summer. This is related to the inversion that is present in winter, and less so in summer, which tends to decouple the stronger upper level winds from the surface winds. In winter also the coastal temperature gradients are less than in summer. * (10)
The land-sea breeze is one of the better known thermal winds. Differential heating between land and sea surface and different rates of cooling of sea and land during day and night are the causes. Of the two the day one is a stronger effect. They would be superimposed on the more regional pressure driven effects.
This is another of the thermal winds. During the day, the thin air above a high mountain warms quickly. The warm air rises and creates an upslope breeze that becomes strongest around noon. This is a valley breeze or anabatic wind. At night, the high mountain slopes cool very quickly. This cold dense air forms a local high-pressure area. The pressure gradient drives a gentle breeze down the slope into the valley. It is strongest just before sunrise. This is the mountain breeze. * (3)
A katabatic wind is created by air flowing downhill more violently than in the mountain breeze. When this air is warm, it may be called a foehn wind and regionally it may be known with a regional name, e.g. Chinook or Santa Ana. When this air is cold or cool, it is called a drainage wind. In mountainous areas with steep sided-sided snow covered large plateaus, sometimes associated with glaciers the wind can be quite strong. These are called katabatic winds. Well known ones are the Bora, in the Adriatic coast of Europe, and the Mistral, in the French Riviera, the Puelche on the west slopes of the Andes, the Zonda on the east slopes of the Andes, and the Fall wind in Greenland and the Antarctica. * (3)
Another down slope wind occurs in some parts of the world due to processes other than gravitational cold air drainage discussed above. These are due to stationary gravity waves. The gravity waves create a downward momentum flux which brings strong upper air flow down to the surface. This is found in the westerly airflow on the eastern slopes of the Rocky Mountains, but they can be found in non-mountainous areas as well. * (11)
An effect sometimes observed in mountainous regions are gap winds. Examples of this would be the Altamont Pass in California, in the Tehachapi Mountains south of Bakersfield in California, in the Colombia Gorge on the boundary between Oregon and Washinton States in the US, and in the Straits of Gibraltar between Southern Europe and North Africa. With a strong pressure difference present on either side of the feature, the wind seeks out a gap or passes and is funneled strongly through.
Night Time Jet
Another phenomenon of interest effecting winds in the US in the Great Plains and down into Colorado and Texas is the nocturnal low-level jet. * (12) This is most evident in the spring and summer. It has been observed that this has the effect of generating higher winds and steeper vertical wind profiles in the layer of the atmosphere in which wind turbines operate at night time. A jet phenomenon has also been observed in China. * (13)
A very useful addition to the literature is the book by Lars Landberg(14). It covers some of the same material. It adds insights, and grounds them in a scientific and historical perspective.
A very useful visualization of the winds in magnitude and direction for the planet is available at the following website. It also includes forecasts of the wind.
An excellent introduction to understanding Global Climate Models and how wind is treated is the paper of Prof. Peter Lynch. He has the distinction of having contributed to the methods of assimilation of measured data into these models.
1. Jacobson, Mark Z. Fundamentals of Atmospheric modelling. 2nd edition, Cambridge University Press, 2005.
2. personal communication, Prof Lynch, University College Dublin, 2007.
3. Ackerman, Steven A., Knox, John A., Meteorology Understanding the Atmosphere. 2nd edition, Thomson Brooks/Cole, 2007.
4. Fact sheet No. 10 – Air masses and weather fronts National Meteorological Library and Archive. Met Office, U.K.
5. Pidwirny, Michael. University of British Columbia Okanagan
6. Climate Lab Section of the Environmental Change Research Group, Department of Geography, University of Oregon - Global Climate Animations. http://geography.uoregon.edu/envchange/clim_animations/index.html
7. European Windstorms and the North Atlantic Oscillation: Impacts, Characteristics, and Predictability
Editor: David L. Malmquist, Risk Prediction Initiative, Bermuda Biological Station for Research, Inc.
David Cotton, Satellite Observing Systems, Godalming, UK
Tim Hewson, UK Meteorological Office/University of Reading, UK
David Malmquist, RPI/BBSR
Robert Muir-Wood, Risk Management Solutions, Inc., London, UK
Jean Palutikof, Climatic Research Unit, University of East Anglia, UK
Balaji Rajagopalan, Lamont-Doherty Earth Observatory of Columbia, USA
Andrew Robertson, University of California at Los Angeles, USA
Walt Robinson, University of Illinois, USA
Mark Saunders, Benfield Greig Hazard Research Centre, University College London, UK
Torben Schmith, Danish Climate Centre, Denmark
David Simmons, Benfield Greig ReMetrics, London, UK
8. Stull, Roland B., Meteorology for Scientists and Engineers 2nd Edition.
Brooks/Cole, Cengage Learning, 2000
9. Introduction to Tropical Meteorology 2nd EditionVersion 2.0, October 2011, produced by The COMET® Program
The source of this material is the COMET® Website at http://meted.ucar.edu/ of the University Corporation for Atmospheric Research (UCAR), sponsored in part through cooperative agreement(s) with the National Oceanic and Atmospheric Administration (NOAA), U.S. Department of Commerce (DOC). ©1997-2011 University Corporation for Atmospheric Research. All Rights Reserved.
10. Artic Meteorology and Oceanography. The COMET® Program. Feb. 2012. https://www.meted.ucar.edu/training_module.php?id=758
11. Meteorlogical aspects of the utilization of wind as an energy source. Technical note No.175. World Meteorological Organisation, Geneva, Switzerland.
12. Low Level Jet_files\NREL-Lamar Low Level Jet Project.htm
13. A diagnostic study of the low-level jet during TAMEX IOP 5, CHEN Y.-L. ; CHEN X. A. ; ZHANG Y.-X. ; Univ. Hawaii Manoa, school ocean earth sci. technology, dep. meteorology, Honolulu HI 96882, ETATS-UNIS
14. Meteorology for Wind Energy An Introduction. L. Landberg. John Wiley&Sons Ltd. 2016