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Atmospheric Circulation/Winds

Atmospheric Circulation - Pressure Belts Winds

Atmospheric Circulation Pressure Belts

• The movement of air in the atmosphere due to the uneven distribution of temperature over the surface of the earth is known as Atmospheric Circulation.
• Air expands when heated and gets compressed when cooled. 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 motion.
• Atmospheric pressure also determines when the air will rise or sink.

Wind
• Air in horizontal motion is wind.
• 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.

Atmospheric Pressure
• The weight of a column of air contained in a unit area from the mean sea level to the top of the atmosphere is called the atmospheric pressure.
• The atmospheric pressure is expressed in units of millibars.
• At sea level the average atmospheric pressure is 1,013.2 millibars.
• Due to gravity the air at the surface is denser and hence has higher pressure.
• Pressure is measured with the help of a mercury barometer or the aneroid barometer.
• The pressure decreases with height. At any elevation it varies from place to place and its variation is the primary cause of air motion, i.e. wind which moves from high pressure areas to low pressure areas

Vertical Variation of Pressure
• In the lower atmosphere the pressure decreases rapidly with height. The decrease amounts to about 1 mb for each 10 m increase in elevation.
• It does not always decrease at the same rate the vertical pressure gradient force is much larger than that of the horizontal pressure gradient.
• But, it is generally balanced by a nearly equal but opposite gravitational force. Hence, we do not experience strong upward winds.

Horizontal Distribution of Pressure
• Small differences in pressure are highly significant in terms of the wind direction and velocity. Horizontal distribution of pressure is studied by drawing isobars at constant levels. Isobars are lines connecting places having equal pressure. 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.

• 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.

Forces Affecting the Velocity and Direction of Wind
The wind blows from high pressure to low pressure. The wind at the surface experiences friction. In addition, rotation of the earth also affects the wind movement. The force exerted by the rotation of the earth is known as the Coriolis force. Thus, the horizontal winds near the earth surface respond to the combined effect of three forces – the pressure gradient force, the frictional force and the Coriolis force. In addition, the gravitational force acts downward.

Convergence and Divergence of winds

Pressure Gradient Force
The differences in atmospheric pressure produce a force. The rate of change of pressure with respect to distance is the pressure gradient. The pressure gradient is strong where the isobars are close to each other and is weak where the isobars are apart.

Frictional Force
It affects the speed of the wind. It is greatest at the surface and its influence generally extends up to an elevation of 1 – 3 km. Over the sea surface the friction is minimal.

Coriolis Force

• The rotation of the earth about its axis affects the direction of the wind.
• This force is called the Coriolis force after the French physicist who described it in 1844. It deflects the wind to the right direction in the northern hemisphere and to the left in the southern hemisphere.
• The deflection is more when the wind velocity is high.
• The Coriolis force is directly proportional to the angle of latitude.
• It is maximum at the poles and is absent at the equator.
• The Coriolis force acts perpendicular to the pressure gradient force. The pressure gradient force is perpendicular to an isobar.
• The higher the pressure gradient force, the more is the velocity of the wind and the larger is the deflection in the direction of wind.
• As a result of these two forces operating perpendicular to each other, in the low-pressure areas the wind blows around it.
• At the equator, the Coriolis force is zero and the wind blows perpendicular to the isobars. The low pressure gets filled instead of getting intensified. That is the reason why tropical cyclones are not formed near the equator.
• The winds in the upper atmosphere, 2 – 3 km above the surface, are free from frictional effect of the surface and are controlled mainly by the pressure gradient and the Coriolis force.

Geotropic Wind
• When isobars are straight and when there is no friction, the pressure gradient force is balanced by the Coriolis force and the resultant wind blows parallel to the isobar. This wind is known as the Geotropic wind

Cyclonic Circulation
• The wind circulation around a low is called cyclonic circulation. Around a high it is called anti cyclonic circulation. The direction of winds around such systems changes according to their location in different hemispheres

Other important patterns of atmospheric circulation over the globe are
• The wind is strong where the isobars are crowed and weak where they are wide apart. The normal sea level pressure is expressed as 1013.2 millibars or 29.92 inches.
• The relation between isobar spacing and wind speed is rather firm in high and mid-latitudes but weakens as we approach the equator. Between 10°N and 10°S, it is difficult to relate the winds to pressure distribution.
• In the large wind systems, the air is slow starter, but when it has worked up some speed, it will carry on for a longer time.
• Along and near the earth’s surface, wind does not move freely in a horizontal plain. The irregularities of the earth surface (e.g. mountains, hills etc,) influence the direction of winds
• Other factors being equal, the difference in wind speed and direction between the surface and upper levels is greatest over rough land surface. Over water the surface wind nearly equals the gradient wind.
• The maximum speed of wind usually occurs in the early afternoon and the minimum in the early morning hours just before the sunrise.
• Winds are named for the direction from which they come. A wind blowing from north to south is a north wind, a wind blowing from west to east is a west wind and a wind blowing from east to west is an east wind.

GENERAL CIRCULATION OF THE ATMOSPHERE
• The pattern of the movement of the planetary winds is called the general circulation of the atmosphere.
• The general circulation of the atmosphere also sets in motion the ocean water circulation which influences the earth’s climate. 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 pattern of planetary winds largely depends on:
(i) Latitudinal variation of atmospheric heating;
(ii) Emergence of pressure belts
(iii) The migration of belts following apparent path of the sun
(iv) The distribution of continents and oceans
(v) The rotation of earth
• The converged air rises along with the convective cell. 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 30o 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 itreaches 30 N and S latitudes.
• Down below near the land surface the air flows towards the equator as the easterlies. The easterlies from either side of the equator converge in the Inter Tropical Convergence Zone (ITCZ). Such circulations from the surface upwards and vice-versa are called cells. Such a cell in the tropics is called Hadley Cell.
• In the middle latitudes the circulation is that of sinking cold air that comes from the poles and the rising warm air that blows from the subtropical high. At the surface these winds are called westerlies and the cell is known as the Ferrel cell.
• At polar latitudes the cold dense air subsides near the poles and blows towards middle latitudes as the polar easterlies. This cell is called the polar cell. These three cells set the pattern for the general circulation of the atmosphere. The transfer of heat energy from lower latitudes to higher latitudes maintains the general circulation.

The Tricellular Circulation
The Tricellular Circulation is the atmospheric circulation in the upper atmosphere.

Mechanism of Circulation:
The heat in the atmosphere is transferred:
1. Horizontally – The horizontal distribution of heat is mainly because of the unequal heating at different latitudes, while the vertical circulation is because of the ascent and descent of heated and cold air, respectively.
2. Vertically – The meridional circulation of heat transfer and vertical circulation of atmosphere result into the formation of certain cells which are as under:
• Tropical Cell (Hadley Cell)
• Polar Front Cell (Ferrell Cell)
• Polar or Sub-Polar Cell

a) Tropical-Cell (Hadley Cell): The Idea of tropical cell was given by Hadley in 1735. In his opinion there is a vertical cell in each hemisphere.
• In the equatorial zone, the Sun’s rays fall vertical. Consequently, the air becomes light resulting into the formation of a low pressure area along the equator, known as Doldrums.
• The warm ascending air current releases latent heat. This process results into the formation of cumulous clouds. The formation of cumulous clouds provides the required energy to drive the tropical cell. The cumulous clouds give torrential rains in the equatorial regions.
• The rising air from thermally driven tropical cell moves pole-ward in the upper troposphere. The air of the Hadley Cell descends at 30° North and 30° South. The Hadley Cells are more pronounced in the Southern Hemisphere than that of the Northern Hemisphere. It is mainly because of the less proportion of land in the Southern Hemisphere.

b) The Polar Front Cells (Ferrell Cell):
• The polar front cell, also known as Ferrell Cell develops between the 30° and 60° in both the Hemispheres. In these latitudes the wind blows from southwest to northeast in the Northern Hemisphere and from northwest to southeast in the Southern Hemisphere and because of the Coriolis force the winds blow almost from west to east.
• In the upper part of the atmosphere in these latitudes (30°C and 60° N and S) the movement of winds is parallel to the trade winds in both the Hemisphere.
• The prevailing westerly in this zone is frequently influenced by the migratory temperate cyclones. The direction of winds in the temperate cyclones is variable, coming from different directions and thus helps in the mixing of temperature.
• The middle latitude circulation cell plays a very vital role in maintaining the terrestrial heat balance. There is plenty of rainfall in these latitudes from the temperate cyclones throughout the year.

c) Polar or Sub-Polar Cells:
• This cell is located between 60° to 90° in both the hemispheres. These are the areas of high pressures or anticyclones. In these latitudes, the air descends downward from the upper atmosphere.
• From these high pressures the air moves towards the sub-polar low pressure. Though the direction of winds is from northeast to southwest in the Northern hemisphere and from southeast to northwest in the Southern Hemisphere, but under the impact of coriolis force the direction of winds is generally from east to west.
• The cold polar easterlies in their equator ward movement clash with the warmer westerlies (anti-trades) of the temperate regions. The zone of convergence of these two airflows of contrasting nature is known’s “Polar Front”. In this cell the mixing of heat transfer is accomplished by waves in the westerlies.
• In the upper atmosphere of this cell the wind blows from the 60° towards the poles.
• In brief, in the tropical cell (Hadley Cell), the exchange of heat and movement of air are accomplished by direct circulation, while in the Ferrell Cells and Polar Cells have a tendency to move north and southward with the shifts in pressure belts and change in seasons. In these areas the transfer or energy is influenced by the temperate cyclones.

Climatic Significance of Tricellular Circulation
Climatic Significance of Meridional Circulation
Some of the significant climatic influences are as under:
• The Tricellular circulation is very significant in the transfer of heat from the lower to the upper atmosphere.
• The convergence of trade winds in the Inter Tropical Convergence Zone and in the Subtropical High Pressure (Divergent) Zone makes a substantial contribution in the transfer of energy.
• The Tricellular Cells help in the development of the temperate and tropical cyclones.
• The mechanism of origin of Indian Monsoon is closely influenced by these cells.
• The Origin of tornados and vertical disturbances are the results or heat transfer in the Hadley Cells.
• The formation of hot deserts, horse latitudes, roaring forties are because of the meridional circulation of the atmosphere.
• In brief, the seasons, climates, climatic belts, vegetation belts, and the life style of people in the different regions of the world are directly or indirectly influenced by the Tricellular atmospheric circulation.

Winds

When the movement of the air in the atmosphere is in a horizontal direction over the surface of the earth, it is known as the wind. Movement of the wind is directly controlled by pressure.
Horizontally, at the Earth’s surface wind always blows from areas of high pressure to areas of low pressure usually at speeds determined by the rate of air pressure change between pressure centers. Wind speed is a function of the steepness or gradient of atmospheric air pressure found between high and low pressure systems. When expressed scientifically, pressure change over a unit distance is called pressure gradient force and the greater this force the faster the winds will blow.


PLANETARY WINDS

Equatorial Low Pressure Belt (inter-Tropical Convergence Zone):
– This is known as Doldrums (Gloomy and sultry air).
– NE and SE Trade wind converge on Doldrums.
– In this zone strong heating causes surface air to expand and rise.
– The humid, rising and expanding air loses moisture as Convectional rainfall (tropical rain forests).
– Doldrums migrates about 5°N and 5°S.

Subtropical High Pressure Belts
– The subtropical high pressure belts lie adjacent to Tropical zone but just outside the Tropic of Cancer and Tropic of Capricorn (40°N and 40°S)
– These are the regions of anticyclones.
– The Subtropical High Pressure Belts are, however, not contiguous. They are best developed on oceans.
– In these belts the air descends. The descending are is generally arid.
– The great hot deserts are found in both the hemispheres in the sub-tropical High pressure Belts.
– Also known as Horse Latitudes.

Low Pressure Belts
– The Sub-polar low Pressure Belts lie between 60° and 65° latitude is both of the Hemispheres.
– These are dynamically produced by the rotation of earth on its axis.
– The Sub-polar Low Pressure Belt is more developed in the Southern Hemisphere.
– In the Northern Hemisphere it is more developed on the Aleutian Islands and Iceland.

Polar High Pressure Belts
– Polar regions are cold throughout the year.
– These are the areas of high pressure.
– The polar winds move outward.
– More developed in Canada and Siberia.

Trade Winds (Easterlies)
– Origin from the Latin word ‘trado’ meaning constant direction.
– The trade winds are the surface winds of the Hadley cells as they move from the horse latitudes to the doldrums.
– In the Northern Hemisphere they blow from NE to SW and in the Southern Hemisphere from SE to NW. (Winds are named by the direction from which they blow).
– Trade winds blow with great regularity on oceans.
– Trade winds help in maintaining the global heat balance.
– With the change in seasons, the Trade winds move 5 degree latitudes.
– Between the two trade winds is the Doldrum.

Anti-Trade Winds (Westerlies)
– Blow from the Sub-tropical High Pressure towards the Poles.
– In the N. Hemisphere they blow from SW to NE and in the S. Hemisphere from NW to SE.
– In winter they move southward and in summer northward affecting the Mediterranean region.
– They blow throughout the year.
– Cyclonic weather.
– In the S. Hemisphere they blow with greater strength.
– Roaring Forties (40°S to 50°S)

Polar Easterlies
– They blow from polar areas towards the mid latitudes.
– They are more pronounced is the Southern Hemisphere.
– In Northern Hemisphere they blow from NE to SW and the Southern Hemisphere form NW to SE.
– Extremely cold.


Shifting of Pressure Belts
– Summer Solstice (21st June) : The pressure belts move northward (Equator upto 10°N)
– Winter Solstice (23rd Dec.): The Pressure belts move southward.
– Spring Equinox (21st March) and Autumn Equinox (23rd Sept): The pressure belts occupy normal positions.
– The occurrence of the Mediterranean and Monsoon climates, are closely influenced by the shift in pressure belts.

PERIODIC WINDS
The pattern of wind circulation is modified in different seasons due to the shifting of regions of maximum heating, pressure and wind belts. The most pronounced effect of such a shift is noticed in the monsoons, especially over southeast Asia.

(a) Monsoon Winds
• Monsoons are regional scale wind systems that predictably change direction with the passing of the seasons.

Summer Monsoon
During the summer, monsoon winds blow from the cooler ocean surfaces onto the warmer continents. In the summer, the continents become much warmer than the oceans because of a number of factors. These factors include:
(i) Specific heat differences between land and water.
(ii) Greater evaporation over water surfaces.
(iii) Subsurface mixing in ocean basins, which redistributes heat energy through a deeper layer.
Precipitation is normally associated with the summer monsoons. Onshore winds blowing inland from the warm ocean are very high in humidity, and slight cooling of these air masses causes condensation and rain. In some cases, this precipitation can be greatly intensified by orographic uplift. Some highland areas in Asia receive more than 10 meters of rain during the summer months.

Winter Monsoon
In the winter, the wind patterns reverses, as the ocean surfaces are now warmer. With little solar energy available, the continents begin cooling rapidly as longwave radiation is emitted to space. The ocean surface retains its heat energy longer because of water’s high specific heat and subsurface mixing. The winter monsoons bring clear dry weather and winds that flow from land to sea.
Besides, Asian continent, monsoon wind systems also exist in Australia, Africa, South America, and North America.


LOCAL WINDS
• Local Winds is a general term for winds generated as a direct effect of local terrain. Local winds generally develop as a result of variations in local temperature, pressure and humidity. The origin of local winds is also attributed to the formation of air-currents Crossing Mountain ranges, valley and other physical barriers.

Main Causes of Local Winds
– Unequal heating of land and sea resulting into the land and sea breeze.
– Unequal heating and cooling of the mountain slopes.
– Local winds originate because of the formation of air-currents, crossing the mountain ranges, and physical barriers.
– Convectional local winds are caused by steep pressure gradients and steep variations in local temperatures.

Some of the local winds are discussed as follows
Land Breeze and Sea Breeze
– Land and sea breezes are local periodic winds on a diurnal basis which change their direction after every twelve hours.
– They result when a differential heating takes place within a short distance near the sea coast.
– Their air movement is caused either by heating or cooling of a particular area.

These winds can be observed in Greenland, Arctic Island, Siberia, and Scandinavian countries, Alaska, Himalayas, Andes and Rockies. In the Andes these winds are known as Nevados. Also, Inversion of temperature due to valley breeze affects crops and orchards in the valley adversely.

OTHER LOCAL WINDS
Drainage (Gravitational) Winds
These local winds blow in the temperate latitudes during the winter season. In winter a high pressure area develops over plateaus.
Some of the important drainage winds are as under:
– Brick fielder : Blows from the desert of Australia in Summers (Dec., Jan.)
– Chili: A hot dry wind which blows southerly from the Sahara desert to the Mediterranean sea through Tunisia.
– Karaburn : (Tarim Basin of China) – Blows from March to July. Hazy weather, helps in loess deposits of China.
– Khamsin (Egypt) : Hot wind Blows for 50 days (April to June)
– Loo (N. W. India) : Blow in the months of May and June.
– Sirocco: (Algeria) Blows from the Sahara desert towards Malta and Sicily – April to July. It becomes hot and humid.
– Zonda: (Argentina and Uruguay) – A warm and dry wind.
– Blizzard: (Greenland, Canada, Antarctica) – Intensely cold, high wind, accompanied by falling snow, visibility reduced.
– Bora (Adriatic Sea): cold, Dec. to March, speed 120 to 140 km. per hour and occasionally upto 225 km per hour, may blow for several days.
– Buran (Siberia and Central Asia): Strong, cold north-easterly wind, temperature around -30°C.
– Mistral (Rhone Valley of France): Winter season, 130 km. per hour. Orchards are protected from it.
– Pampero (Pampas of Argentina): It is cold wind.
Descending Winds
– Berg (Germany, descends from the Alps) : Hot and dry, leads to irritation and headache.
– Fohn (Foehn, Fon): Blows northwards from the Alps in the upper Rhine Valley.
– Chinook (Snow and Ice-eater): Lows in U.S.A. (Colorado, Wyoming, Montana, N. Dakota, Oregon and Washington) and in Canada (Alberta, Manitoba, Mackenzie). Period Dec. to April. Warm and dry. It melts the winter snow and ice.
– Samoon (Iran and Kurdistan): Hot and Dry.
– Andhi (Dust Storm): May and June in N.W. India.
– Haboob (Sudan): Hot wind.
– Simoon (S. Arabia): Hot wind.

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