Winds and Their Types | Atmospheric Circulation

Winds, What is Atmospheric Circulation, Atmospheric Pressure, Pressure Variations, Forces Governing Air Movement, Geostrophic Wind, Distribution of Pressure Belts, Shifting Belts, General Circulation of the Atmosphere, Upper Air Circulation

What is Atmospheric Circulation

Varying amount of insolation received by the earth causes differential heating of the earth and its atmosphere. Temperature difference thus produced account for the density differences in the air. Air expands when heated and gets compress when cool. This results in variations in the atmospheric pressure. The result is that it causes the movement of air from high pressure to low pressure, setting the air in three-dimensional motion on global scale. Air in horizontal motion is wind.

Atmospheric pressure also determines when the air will rise or sink. The wind redistributes the heat and moisture across the planet, thereby, maintaining a constant temperature for the planet as a whole. The vertical rising of moist air cools it down to form the clouds and bring precipitation. There is, in fact, an intimate relationship between winds and pressure, and knowledge of pressure variations is a prerequisite to understanding air motion.

Atmospheric Pressure

The atmosphere is held on the earth by the gravitational pull of the earth. A column of air exerts weight in terms of pressure on the surface of the earth. The weight of a column of air contain in a unit area from the mean sea level to the top of the atmosphere is call the atmospheric pressure. Pressure is normally measure in millibars or pascals and spatial variations of pressure are depict on maps by means of isobars, which are lines connecting places having the same barometric pressure. The actual pressure at a given place and at a given time fluctuates and it generally ranges between 950 and 1050 millibars.

Air pressure is measure with the help of a mercury barometer or the aneroid barometer. The gradual change of pressure between different areas is known as the barometric slope or pressure gradient. The closer the isobars are together, the greater the pressure gradient; for example, widely spaced isobars indicate a weak pressure gradient.

Pressure Variations

In the lower atmosphere the pressure decreases rapidly with height with decrease in density of air. It does not always decrease at the same rate. But to make calculations simple, a decrease of about 1 mb for each 10 m increase in elevation is taken into consideration. In spite of high vertical pressure gradient, we do not experience strong vertical air currents. This is possible because of equal and opposite gravitation force acting upon air.

The effects of low pressure are more clearly experienced by the people living in the hilly areas as compared to those who live in plains. In high mountainous areas rice takes more time to cook because low pressure reduces the boiling point of water. Breathing problem such as faintness and nose bleedings are also faced by many trekkers from outside in such areas because of low pressure conditions in which the air is thin and it has low amount of oxygen content.

Unlike vertical high pressure gradient, small horizontal pressure gradients are highly significant in terms of the wind direction and velocity. In order to eliminate the effect of altitude on pressure, it is measured at any station after being reduced to sea level for purposes of comparison. Figure 12 shows the patterns of isobars corresponding to pressure systems. Low pressure system is enclosed by one or more isobars with the lowest pressure in the centre. High-pressure system is also enclosed by one or more isobars with the highest pressure in the centre. The terms ‘high pressure’ and ‘low pressure’ do not usually signify any particular absolute values, but are used relatively.

Forces Governing Air Movement

We know that the air pressure is unevenly distributed in the atmosphere and air attempts to balance this unevenness. Hence, it moves from high pressure areas to low pressure areas. Horizontal movement of air in response to difference in pressure is termed as wind while vertical or nearly vertical moving air is called air current. Both winds and air currents form the system of circulation in the atmosphere.

1. Pressure Gradient

The existence of pressure differentials in the atmosphere is the immediate primary force causing air movement. The rate of change of pressure with respect to distance is the pressure gradient. The pressure gradient force always acts down the pressure gradient, attempting to cause the general movement of air away from high-pressure towards low pressure areas. The force exerted is proportional to the steepness of the gradient. The gentler the pressure gradient slower is the speed of the wind and vice-versa. If alone this force is exert to the air, wind would have direction perpendicular to the isobars. However, there are other forces also which, in fact, make wind to flow more nearly parallel to the isobars.

2. Coriolis Force

Winds do not cross the isobars at right angles as the pressure gradient directs them. They get deflected from their original paths. One of the most potent influences on wind direction is the deflection caused by the earth’s rotation on its axis. This deflection is always to the right of the direction of motion in the northern hemisphere and to the left in the southern hemisphere. This influence is known as Coriolis force.

The degree of the deflecting force varies with the speed of the moving air and with latitude. The faster the wind, the greater the effect of rotation can be. Similarly, the rate of deflection increases with the increasing distance from the Equator because the Coriolis force is zero at the Equator and maximum at Poles. It must be note that it is an apparent or relative deflection. If view from outer space, objects moving across the face of the earth would not in fact appear to be deflect. In relation to star positions, they would travel in a straight line, while the earth rotates beneath them. The phenomenon affects all freely moving objects – air, ocean currents, rockets and projectiles etc. Thus, it is not actually any force. But it is simplest to accept that deflection is caused by a force.

3. Centripetal Force

This force applies when the isobars are curved, as within cyclones. The fact that air is following a curved path means that in addition to the pressure gradient and the Coriolis force, a third force is acting centripetally, pulling air inwards. Wind which is in balance with these three forces is known as the gradient wind.

4. Frictional Force

It lessens the speed of the wind. It is greatest at the surface and its influence generally extends upto an elevation of 1 – 3 km. Over the sea surface the friction is minimal. By reducing speed of wind, it weakens the Coriolis force. This allows the pressure gradient to assert its greater strength by causing the air to flow more towards low pressure. Thus, the usual situation is that surface winds flow at a slight angle to the isobars.

Geostrophic Wind

The velocity and direction of the wind are the net result of the wind generating forces. The winds in the upper atmosphere, 2 – 3 km above the surface, are free from frictional effect of the surface and are controlled by the pressure gradient and the Coriolis force. At such height in the free atmosphere, winds generally blow at right angles to the pressure gradient: this indicate that the pressure gradient force is exactly balanced by the Coriolis force acting in a diametrically opposite direction. This sort of air motion is known as the geostrophic wind.

Not all winds are exactly geostrophic. As pressure pattern change, the balance is upset, but the wind always strives to readjust itself until it obtains the new geostrophic speed.

Distribution of Pressure Belts

The horizontal distribution of air pressure across the latitudes is characterized by high or low pressure belts. These pressure belts are:

1. Equatorial low pressure belt

This belt extends from equator to 100 N and 100 S latitudes. This belt is thermally produce due to heating by Sun. Due to excessive heating horizontal movement of air is absent here and only vertical currents are experience in this belt. Therefore, this belt is call as doldrums (the zone of calm).  This belt is also known as-Inter Tropical Convergence Zone (ITCZ) because the trade winds flowing from sub tropical high pressure belts converge here.

2. Sub-tropical high pressure belt

These extend roughly between 250 and 350 latitudes in both the Hemispheres. The existence of these pressure belts is due to the fact that the up rising air of the equatorial region is deflect towards poles due to the earth’s rotation. After becoming cold and heavy, it start descending in these regions and get piled up. This results in high pressure. Calm conditions with feeble and variable winds are present here. In southern  hemisphere, this belt is broken by small low-pressure areas in summer over Australia and South Africa. In northern hemisphere, the belt is more discontinuous by the presence of land masses, and high pressure occurs only over the ocean areas as discrete cells; these are termed the Azores and Hawaiian cells in the Atlantic and Pacific areas respectively.

These belts are also call as Horse latitudes. In older days, vessels with cargo of horses passing through these belts found difficult in sailing under these calm conditions. They used to throw the horses in the sea in order to make the vessels lighter. In the upper atmosphere over this belt the upper level westerlies and anti-trade winds converge and set up descending currents in the atmosphere.

3. Sub-polar low pressure belt

It extends along 600 latitudes (550 -650 ) in both the hemisphere. These belts are not thermally induced instead the winds coming from the sub tropics and the polar regions converge in this belt and rise upward. The great temperature contrast between the subtropical and the polar regions, gives rise to cyclonic storms in this belt. In Southern hemisphere, this low pressure belt is more pronounced due to vast presence of ocean and also referred as the sub-antarctic low. But in the northern hemisphere, there are large land masses along 600 latitudes which are very cold. Therefore, the pressures over these landmasses get increase. Thus, the continuity of the belt is broken.

4. Polar high pressure belt

Because of low temperature, air compresses and its density increases. Hence, high pressure is present here throughout the year. This is more marked over the land area of the Antarctic continent than over the ocean of the North Pole. In northern hemisphere, high pressure is not centre at the pole, but it extends from Greenland to Islands situated in the northern part of Canada.

Shifting of Belts

Pressure belts are not fix. The main cause of their formation is the uneven distribution of temperature on the surface of earth. Consequently, the pressure belts swing either to the north (in July) or the south (in December) of the equator by following the apparent annual migration of the sun. Sun’s movement is record between tropic of Cancer and tropic of Capricorn. During the month of July, low pressure equatorial belt extends upto the tropic of Cancer in Asian region. While in January, it extends to latitudes 100 -150 S. Most profound effect of shifting of belts is seen in the temperate region. Winds blowing from the Horse latitudes in the form of westerlies create unique climatic conditions in the temperate parts of the world, especially in the Mediterranean region.

General Circulation of the Atmosphere

As discussed earlier that wind is the result of pressure gradient which is largely caused by differential heating of the earth. Winds in the atmosphere are neither unidirectional nor have a same pattern as we go up in the atmosphere. In fact, winds may change their direction and intensity multiple times within same day. Largely, wind movement in the atmosphere may be classified into three broad categories:

– Primary circulation: it includes planetary wind systems which are related to the general arrangement of pressure belts on the earth’s surface. The pattern of the movement of the planetary winds is called the general circulation of the atmosphere. In fact, it is the primary circulation patterns which prepare the broad framework for the other circulation patterns.
– Secondary circulation: it consists of cyclones and anti-cyclones, monsoon.
– Tertiary circulation: it include all the local winds which are produce by local causes such as topographical features, sea influences etc. Their impact is visible only in a particular area.

Planetary Winds

Primary or planetary winds blow from high pressure belts to low pressure belts in the same direction throughout the year. They blow over vast area of continents and oceans. Trade winds, Westerlies and polar easterlies together form the planetary wind circulation.
These are describe below:

– The air at the Inter Tropical Convergence Zone (ITCZ) rises because of convection caused by high insolation and a low pressure is created. The winds from the tropics converge at this low pressure zone. The converged air rises along up. It reaches the top of the troposphere up to an altitude of 14 km. and moves towards the poles. This causes accumulation of air at about 300 N and S. Part of the accumulated air sinks to the ground and forms a subtropical high. Another reason for sinking is the cooling of air when it reaches 300 N and S latitudes. Down below near the land surface the air flows towards the equator as the easterlies1 or tropical easterlies or trade winds. Because of Coriolis force, their direction becomes north-east and south-east in northern and southern hemisphere respectively.

What are Planetary Winds

– In the middle latitudes (300 -600 ) the circulation is that of sinking cold air that comes from the poles and the rising warm air that blows from the subtropical high pressure belt. These winds are deflect due to coriolis force and become westerly in both the hemisphere. Deflect wind are call as westerlies. These winds meet along the sub-polar low pressure belt to raise high in the troposphere. From here, air moves away in both directions – towards pole and equator.

– The prevailing westerlies are relatively more variable than the trade winds both in direction and intensity. There are more frequent invasions of polar air masses along with the travelling cyclones and anti-cyclones. These moving cells of low and high pressures largely affect the movement of westerlies. The westerlies are stronger in the cold. In the southern hemisphere, westerlies are so powerful and persistent due to absence of land between 400 – 600 S that these are called ‘roaring forties’, ‘furious fifties’ and ‘screaming sixties’ along 400 S, 500 S and 600 S latitudes.

– Winds move away from polar high pressure to sub-polar low pressure along the surface of the earth in Polar cell. Their direction becomes easterlies due to coriolis force. These are call as polar easterlies.

– Winds coming from the sub-tropical and the polar high belts converge to produce cyclonic storms or low pressure conditions. This zone of convergence is also call as polar front.

Local Winds

Besides major wind systems of the earth’s surface, there are certain types of winds which are produce by purely local factors and therefore, are call as local winds. These local winds play a significant role in the weather and climate of a particular locality.
Following is a brief account of some of the well-known local winds which are there in different parts of the world.

1. The Land and Sea Breezes

These winds are define as the complete cycle of diurnal local winds occurring on sea coasts due to differences in the surface temperature of sea and adjacent land. There is complete reversal of wind direction of these coastal winds. The land and sea breeze system is very shallow with average depth of 1-2km. Over lakes, the height of circulation is much less. Warm tropical areas, where intense solar heating persists throughout the year, experience stronger and regular breezes compare to higher latitudes.

2. The Mountain and Valley Breezes

Another combination of local winds that undergoes a daily reversal consists of the mountain and valley breezes. During the day the slopes get heate up more than the valleys. Hence, the pressure is low over the slopes while it is comparatively high in the valleys below. Air moves up from slope and to fill the resulting gap the air from the valley blows up the valley. This wind is known as the valley breeze or anabatic wind. The valley breeze is sometimes accompanied by the formation of cumulus cloud near mountain peaks to cause orographic rainfall.

During the night the slopes get cool and the dense air descends into the valley as the mountain wind. The cool air, of the high plateaus and ice fields draining into the valley is call as mountain breeze or katabatic wind.

3. Hot Local Winds

Local winds that are hot are cause by the advection of hot air from a warm source region. They may also be produce by dynamic heating of air as it descends from an elevated area to lowland. Few famous hot winds are:

Loo

Is a hot and dry wind, which blows very strongly over the northern plains of India and Pakistan in the months of May and June. Their direction is from west to east and they are usually experience in the afternoons. Their temperature varies between 45°C to 50°C.

Foehn

Is strong, dusty, dry and warm local wind which develops on the leeward side of the Alps mountain ranges. Regional pressure gradient forces the air to ascend and cross the barrier. Ascending air sometimes causes precipitation on the windward side of the mountains. After crossing the mountain crest, the Foehn winds starts descending on the leeward side or northern slopes of the mountain as warm and dry wind. The temperature of the winds varies from 15°C to 20°C which help in melting snow. Thus making pasture land ready for animal grazing and help the grapes to ripe early.

Chinook

Is the name of hot and dry local wind, which moves down the eastern slopes of the Rockies in U.S.A. and Canada. The literal meaning of chinook is ‘snow eater’ as they help in melting the snow earlier. They keep the grasslands clear of snow. Hence, they are very helpful to ranchers.

Sirocco

Is a hot, dry dusty wind, which originates in the Sahara desert. It is most frequent in spring and normally lasts for only a few days. After crossing the Mediterranean sea, the Sirocco is slightly cool by the moisture from the sea. Still it is harmful for vegetation, crops in that region. Its other local names are Leveche in Spain, Khamsin in Egypt, Gharbi in Aegean Sea area.

Harmattan

Is a strong dry wind that blows over northwest Africa from the northeast. Blowing directly from the Sahara desert, it is a hot, dry and dusty wind. It provides a welcome relief from the moist heat and is beneficial to health of people hence also known as ‘the doctor’. It is full of fine desert dust which makes the atmosphere hazy and causes problems to the caravan traders. May also cause severe damage to the crops.

4. Cold Local Winds

There are certain local winds which originate in the snow-capped mountains during winter and move down the slopes towards the valleys. Few of important these are:

Mistral

It originates on the Alps and move over France towards the Mediterranean Sea through the Rhone valley. They are very cold, dry and high velocity winds. They bring down temperature below freezing point in areas of their influence. As a protective measure, many of the houses and orchards of the Rhone valley have thick rows of trees and hedges planted to shield them from the Minstral.

Bora

It is a cold, dry north-easterly wind blowing down from the mountains in the Adriatic Sea region. It is also cause by pressure difference between continental Europe and the Mediterranean Sea. This is usually occurs in winter. It sometimes attain speeds of over 150 kmph.

Blizzard

It is a violent and extremely cold wind laden with dry snow. Such blizzards are of common occurrence in the Antarctic. Wind velocity sometimes reaches 160 kmph and temperature is as low as -7° C.

Leave a Comment