Chapter-10- ATMOSPHERIC CIRCULATION AND WEATHER SYSTEMS
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. 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.
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 milibar. At sea level the average
atmospheric pressure is 1,013.2 milibar. Due to gravity the air at the surface is denser and
hence has higher pressure.
Air 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.
World Distribution of Sea Level Pressure
Near the equator the sea level pressure is low and the area is known as equatorial low.
Along 30° N and 30o S are found the high-pressure areas known as the subtropical
highs.
Further pole wards along 60o N and 60o S, the low-pressure belts are termed as the sub
polar lows.
Near the poles the pressure is high and it is known as the polar high.
These pressure belts are not permanent in nature. They oscillate with the apparent
movement of the sun.
In the northern hemisphere in winter they move southwards and in the summer northwards.
Forces Affecting the Velocity and Direction of Wind
The air in motion is called 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.
Pressure Gradient Force
The differences in atmospheric pressure produces 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 upto 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.(important need more point )
Pressure and Wind
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.
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 geostrophic wind (Figure 10.4).
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
The wind circulation at the earth’s surface around low and high on many
occasions is closely related to the wind circulation at higher level. Generally, over low
pressure area the air will converge and rise.
Over high pressure area the air will subside from above and diverge at the surface
(Figure10.5).
Apart from convergence, some eddies, convection currents, orographic uplift and
uplift along fronts cause the rising of air, which is essential for the formation of clouds and
precipitation
General circulation of the atmosphere
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 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.
Pressure System Pressure Condition
at the Centre
Pattern of Wind Direction
Northern
Hemisphere
Southern
Hemisphere
Cyclone Anticyclone Low
High
Anticlockwise
Clockwise
Anticlock
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
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.
Another reason for sinking is the cooling of air when it reaches 30o 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 general circulation of the atmosphere also affects the oceans. The large-scale winds of the
atmosphere initiate large and slow moving currents of the ocean. Oceans in turn provide
input of energy and water vapour into the air. These interactions take place rather slowly
over a large part of the ocean.
General Atmospheric Circulation and its Effects on Oceans
Warming and cooling of the Pacific Ocean is most important in terms of general
atmospheric circulation. The warm water of the central Pacific Ocean slowly drifts
towards South American coast and replaces the cool Peruvian current.
Such appearance of warm water off the coast of Peru is known as the El Nino.
The El Nino event is closely associated with the pressure changes in the Central Pacific
and Australia. This change in pressure condition over Pacific is known as the southern
oscillation.
The combined phenomenon of southern oscillation and El Nino is known as ENSO.
In the years when the ENSO is strong, large-scale variations in weather occur over
the world.
The arid west coast of South America receives heavy rainfall, drought occurs in
Australia and sometimes in India and floods in China. This phenomenon is closely
monitored and is used for long range forecasting in major parts of the world.
Seasonal Wind
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. The other local deviations from the general circulation system are as follows.
Local Winds
Differences in the heating and cooling of earth surfaces and the cycles those develop daily or
annually can create several common, local or regional winds.
Land and Sea Breezes
As explained earlier, the land and sea absorb and transfer heat differently. During the day the
land heats up faster and becomes warmer than the sea. Therefore, over the land the air rises
giving rise to a low pressure area, whereas the sea is relatively cool and the pressure over sea is
relatively high. Thus, pressure gradient from sea to land is created and the wind blows from the
sea to the land as the sea breeze.
In the night the reversal of condition takes place. The land loses heat faster and is cooler than the
sea. The pressure gradient is from the land to the sea and hence land breeze results
Mountain and Valley Winds
In mountainous regions, during the day the slopes get heated up and air moves
upslope and to fill the resulting gap the air from the valley blows up the valley. This
wind is known as the valley breeze.
During the night the slopes get cooled 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 called
katabatic wind.
Another type of warm wind occurs on the leeward side of the mountain ranges. The
moisture in these winds, while crossing the mountain ranges condense and
precipitate. When it descends down the leeward side of the slope the dry air gets
warmed up by adiabatic process. This dry air may melt the snow in a short time.
Air Masses
When the air remains over a homogenous area for a sufficiently longer time, it
acquires the characteristics of the area. The homogenous regions can be the vast
ocean surface or vast plains.
The air with distinctive characteristics in terms of temperature and humidity is called
an airmass.
It is defined as a large body of air having little horizontal variation in temperature and
moisture. The homogenous surfaces, over which air masses form, are called the
source regions.
The air masses are classified according to the source regions. There are five major
source regions. These are: (i) Warm tropical and subtropical oceans; (ii) The
subtropical hot deserts; (iii) The relatively cold high latitude oceans; (iv) The very
cold snow covered continents in high latitudes; (v) Permanently ice covered
continents in the Arctic and Antarctica.
following types of air- masses are recognised: (i) Maritime tropical (mT); (ii)
Continental tropical (cT); (iii) Maritime polar (mP); (iv) Continental polar (cP); (v)
Continental arctic (cA).
Tropical air masses are warm and polar air masses are cold.
Fronts
When two different air masses meet, the boundary zone between them is called
a front. The process of formation of the fronts is known as frontogenesis.
(a) There are four types of fronts: (a) Cold; (b) Warm; (C)Stationary; (d) Occluded.
When the front remains stationary, it is called a stationary front.
When the cold air moves towards the warm air mass, its contact zone is called the
cold front,
whereas if the warm air mass moves towards the cold air mass, the contact zone is a
warm front.
If an air mass fully lifted above the land surface, it is called the
occluded front.
The fronts occur in middle latitudes and are characterised by steep gradient in
temperature and pressure. They bring abrupt changes in temperature and cause
the air to rise to form clouds and cause precipitation.
Extra Tropical Cyclones
The systems developing in the mid and high latitude, beyond the tropics are called the middle
latitude or extra tropical cyclones.
Extra tropical cyclones form along the polar front. Initially, the front is stationary.
In the northern hemisphere, warm air blows from the south and cold air from the north of the
front. When the pressure drops along the front, the warm air moves northwards and the cold
air move towards, south setting in motion an anticlockwise cyclonic circulation.
The cyclonic circulation leads to a well-developed extra tropical cyclone, with a warm front
and a cold front.
There are pockets of warm air or warm sector wedged between the forward and the rear cold
air or cold sector.
The warm air glides over the cold air and a sequence of clouds appear over the sky ahead of
the warm front and cause precipitation.
The cold front approaches the warm air from behind and pushes the warm air up. As a
result, cumulus clouds develop along the cold front. The cold front moves faster than the warm
front ultimately overtaking the warm front. The warm air is completely lifted up and the front
is occluded and the cyclone dissipates.
which is not present in the tropical cyclones. They cover a larger area and can originate over
the land and sea. Whereas the tropical cyclones originate only over the seas and on reaching
the land they dissipate. The extra tropical cyclone affects a much larger area as
compared to the tropical cyclone. The wind velocity in a tropical cyclone is much higher
and it is more destructive. The extra tropical cyclones move from west to east but tropical
cyclones, move from east to west.
Tropical Cyclones
are violent storms that originate over oceans in tropical areas and move over to the coastal
areas bringing about large scale destruction caused by violent winds, very heavy rainfall and
storm surges.
one of the most devastating natural calamities. They are known as Cyclones in the Indian
Ocean, Hurricanes in the Atlantic, Typhoons in the Western Pacific and South China Sea, and
Willy-willies in the Western Australia.
Tropical cyclones originate and intensify over warm tropical oceans. The conditions
favourable for the formation and intensification of tropical storms are: (i) Large sea surface
with temperature higher than 27° C; (ii) Presence of the Coriolis force; (iii) Small variations in
the vertical wind speed; (iv) A pre-existing weak- low-pressure area or low-level-cyclonic
circulation; (v) Upper divergence above the sea level system.
The energy that intensifies the storm, comes from the condensation process in the towering
cumulonimbus clouds, surrounding the centre of the storm. With continuous supply of
moisture from the sea, the storm is further strengthened. On reaching the land the
moisture supply is cut off and the storm dissipates. The place where a tropical cyclone
crosses the coast is called the landfall of the cyclone. The cyclones, which cross 20o N
latitude generally, recurve and they are more destructive.
They cover a larger area and can originate over the land and sea. Whereas the tropical cyclones
originate only over the seas and on reaching the land they dissipate. The extra tropical
cyclone affects a much larger area as compared to the tropical cyclone. The wind
velocity in a tropical cyclone is much higher and it is more destructive. The extra tropical
cyclones move from west to east but tropical cyclones, move from east to west.
The energy that intensifies the storm, comes from the condensation process in the towering
cumulonimbus clouds, surrounding the centre of the storm. With continuous supply of
moisture from the sea, the storm is further strengthened. On reaching the land the
moisture supply is cut off and the storm dissipates.
The place where a tropical cyclone crosses the coast is called the landfall of the cyclone.
The cyclones, which cross 20o N latitude generally, recurve and they are more destructive.
A mature tropical cyclone is characterised by the strong spirally circulating wind
around the centre, called the eye. The diameter of the circulating system can vary
between 150 and 250 km.
The eye is a region of calm with subsiding air. Around the eye is the eye wall, where
there is a strong spiralling ascent of air to greater height reaching the tropopause.
The wind reaches maximum velocity in this region, reaching as high as 250 km
per hour. Torrential rain occurs here. From the eye wall rain bands may radiate and
trains of cumulus and cumulonimbus clouds may drift into the outer region.
Thunderstorms and Tornadoes
Thunderstorms are caused by intense convection on moist hot days. A thunderstorm is a
well-grown cumulonimbus cloud producing thunder and lightening. When the clouds
extend to heights where sub-zero temperature prevails, hails are formed and they come down as
hailstorm. If there is insufficient moisture, a thunderstorm can generate dust- storms.
A thunderstorm is characterised by intense updraft of rising warm air, which causes the clouds
to grow bigger and rise to greater height. This causes precipitation. Later, downdraft brings
down to earth the cool air and the rain.
From severe thunderstorms sometimes spiralling wind descends like a trunk of an elephant
with great force, with very low pressure at the centre, causing massive destruction on its way.
Such a phenomenon is called a tornado. Tornadoes generally occur in middle latitudes. The
tornado over the sea is called water sprouts.
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