NCERT Textbook: Atmospheric Circulation & Weather Systems - Notes | Study Geography for UPSC CSE - UPSC

 Page 1


ATMOSPHERIC CIRCULATION
AND WEATHER SYSTEMS
E
arlier Chapter 9 described the uneven
distribution of temperature over the
surface of the earth. 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. You already
know that 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. This chapter has been
devoted to explain the causes of pressure
differences, the forces that control the
atmospheric circulation, the turbulent pattern
of wind, the formation of air masses, the
disturbed weather when air masses interact
with each other and the phenomenon of violent
tropical storms.
ATMOSPHERIC PRESSURE
Do you realise that our body is subjected to a
lot of air pressure. As one moves up the air
gets varified and one feels breathless.
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.
Consult your book, Practical Work in
Geography — Part I (NCERT, 2006) and learn
about these instruments. 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. Table 10.1 gives
the average pressure and temperature at
selected levels of elevation for a standard
atmosphere.
Table 10.1 : Standard Pressure and Temperature at
Selected Levels
Level Pressure in mb Temperature °C
Sea Level 1,013.25 15.2
1 km 898.76 8.7
5 km 540.48 –17. 3
10 km 265.00 – 49.7
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.
CHAPTER
2015-16
Page 2


ATMOSPHERIC CIRCULATION
AND WEATHER SYSTEMS
E
arlier Chapter 9 described the uneven
distribution of temperature over the
surface of the earth. 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. You already
know that 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. This chapter has been
devoted to explain the causes of pressure
differences, the forces that control the
atmospheric circulation, the turbulent pattern
of wind, the formation of air masses, the
disturbed weather when air masses interact
with each other and the phenomenon of violent
tropical storms.
ATMOSPHERIC PRESSURE
Do you realise that our body is subjected to a
lot of air pressure. As one moves up the air
gets varified and one feels breathless.
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.
Consult your book, Practical Work in
Geography — Part I (NCERT, 2006) and learn
about these instruments. 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. Table 10.1 gives
the average pressure and temperature at
selected levels of elevation for a standard
atmosphere.
Table 10.1 : Standard Pressure and Temperature at
Selected Levels
Level Pressure in mb Temperature °C
Sea Level 1,013.25 15.2
1 km 898.76 8.7
5 km 540.48 –17. 3
10 km 265.00 – 49.7
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.
CHAPTER
2015-16
ATMOSPHERIC CIRCULATION AND WEATHER SYSTEMS 89
Horizontal Distribution of Pressure
Small differences in pressure are highly
significant in terms of the wind direction and
purposes of comparison. The sea level pressure
distribution is shown on weather maps.
Figure 10.1 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.
World Distribution of Sea Level Pressure
The world distribution of sea level pressure in
January and July has been shown in Figures
10.2 and 10.3. Near the equator the sea level
pressure is low and the area is known as
equatorial low. Along 30° N and 30
o
 S are
found the high-pressure areas known as the
subtropical highs. Further pole wards along
60
o
N and 60
o
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
Figure 10.2 : Distribution of pressure (in millibars) — January
Figure 10.1 : Isobars, pressure and wind systems in
Northern Hemisphere
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
2015-16
Page 3


ATMOSPHERIC CIRCULATION
AND WEATHER SYSTEMS
E
arlier Chapter 9 described the uneven
distribution of temperature over the
surface of the earth. 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. You already
know that 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. This chapter has been
devoted to explain the causes of pressure
differences, the forces that control the
atmospheric circulation, the turbulent pattern
of wind, the formation of air masses, the
disturbed weather when air masses interact
with each other and the phenomenon of violent
tropical storms.
ATMOSPHERIC PRESSURE
Do you realise that our body is subjected to a
lot of air pressure. As one moves up the air
gets varified and one feels breathless.
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.
Consult your book, Practical Work in
Geography — Part I (NCERT, 2006) and learn
about these instruments. 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. Table 10.1 gives
the average pressure and temperature at
selected levels of elevation for a standard
atmosphere.
Table 10.1 : Standard Pressure and Temperature at
Selected Levels
Level Pressure in mb Temperature °C
Sea Level 1,013.25 15.2
1 km 898.76 8.7
5 km 540.48 –17. 3
10 km 265.00 – 49.7
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.
CHAPTER
2015-16
ATMOSPHERIC CIRCULATION AND WEATHER SYSTEMS 89
Horizontal Distribution of Pressure
Small differences in pressure are highly
significant in terms of the wind direction and
purposes of comparison. The sea level pressure
distribution is shown on weather maps.
Figure 10.1 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.
World Distribution of Sea Level Pressure
The world distribution of sea level pressure in
January and July has been shown in Figures
10.2 and 10.3. Near the equator the sea level
pressure is low and the area is known as
equatorial low. Along 30° N and 30
o
 S are
found the high-pressure areas known as the
subtropical highs. Further pole wards along
60
o
N and 60
o
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
Figure 10.2 : Distribution of pressure (in millibars) — January
Figure 10.1 : Isobars, pressure and wind systems in
Northern Hemisphere
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
2015-16
FUNDAMENTALS OF PHYSICAL GEOGRAPHY 90
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
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 V elocity
and Direction of Wind
You already know that the air is set in motion
due to the differences in atmospheric pressure.
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.
Figure 10.3 : Distribution of pressure (in millibars) — July
2015-16
Page 4


ATMOSPHERIC CIRCULATION
AND WEATHER SYSTEMS
E
arlier Chapter 9 described the uneven
distribution of temperature over the
surface of the earth. 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. You already
know that 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. This chapter has been
devoted to explain the causes of pressure
differences, the forces that control the
atmospheric circulation, the turbulent pattern
of wind, the formation of air masses, the
disturbed weather when air masses interact
with each other and the phenomenon of violent
tropical storms.
ATMOSPHERIC PRESSURE
Do you realise that our body is subjected to a
lot of air pressure. As one moves up the air
gets varified and one feels breathless.
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.
Consult your book, Practical Work in
Geography — Part I (NCERT, 2006) and learn
about these instruments. 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. Table 10.1 gives
the average pressure and temperature at
selected levels of elevation for a standard
atmosphere.
Table 10.1 : Standard Pressure and Temperature at
Selected Levels
Level Pressure in mb Temperature °C
Sea Level 1,013.25 15.2
1 km 898.76 8.7
5 km 540.48 –17. 3
10 km 265.00 – 49.7
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.
CHAPTER
2015-16
ATMOSPHERIC CIRCULATION AND WEATHER SYSTEMS 89
Horizontal Distribution of Pressure
Small differences in pressure are highly
significant in terms of the wind direction and
purposes of comparison. The sea level pressure
distribution is shown on weather maps.
Figure 10.1 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.
World Distribution of Sea Level Pressure
The world distribution of sea level pressure in
January and July has been shown in Figures
10.2 and 10.3. Near the equator the sea level
pressure is low and the area is known as
equatorial low. Along 30° N and 30
o
 S are
found the high-pressure areas known as the
subtropical highs. Further pole wards along
60
o
N and 60
o
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
Figure 10.2 : Distribution of pressure (in millibars) — January
Figure 10.1 : Isobars, pressure and wind systems in
Northern Hemisphere
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
2015-16
FUNDAMENTALS OF PHYSICAL GEOGRAPHY 90
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
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 V elocity
and Direction of Wind
You already know that the air is set in motion
due to the differences in atmospheric pressure.
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.
Figure 10.3 : Distribution of pressure (in millibars) — July
2015-16
ATMOSPHERIC CIRCULATION AND WEATHER SYSTEMS 91
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.
Pressure and 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 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).
Table 10.2 : Pattern of Wind Direction in Cyclones and Anticyclones
Pressure System Pressure Condition Pattern of Wind Direction
at the Centre Northern Hemisphere Southern Hemisphere
Cyclone Low Anticlockwise Clockwise
Anticyclone High Clockwise Anticlockwise
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 (Table 10.2).
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.
Figure 10.4 : Geostropic Wind
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
Figure 10.5 : Convergence and divergence of winds
2015-16
Page 5


ATMOSPHERIC CIRCULATION
AND WEATHER SYSTEMS
E
arlier Chapter 9 described the uneven
distribution of temperature over the
surface of the earth. 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. You already
know that 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. This chapter has been
devoted to explain the causes of pressure
differences, the forces that control the
atmospheric circulation, the turbulent pattern
of wind, the formation of air masses, the
disturbed weather when air masses interact
with each other and the phenomenon of violent
tropical storms.
ATMOSPHERIC PRESSURE
Do you realise that our body is subjected to a
lot of air pressure. As one moves up the air
gets varified and one feels breathless.
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.
Consult your book, Practical Work in
Geography — Part I (NCERT, 2006) and learn
about these instruments. 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. Table 10.1 gives
the average pressure and temperature at
selected levels of elevation for a standard
atmosphere.
Table 10.1 : Standard Pressure and Temperature at
Selected Levels
Level Pressure in mb Temperature °C
Sea Level 1,013.25 15.2
1 km 898.76 8.7
5 km 540.48 –17. 3
10 km 265.00 – 49.7
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.
CHAPTER
2015-16
ATMOSPHERIC CIRCULATION AND WEATHER SYSTEMS 89
Horizontal Distribution of Pressure
Small differences in pressure are highly
significant in terms of the wind direction and
purposes of comparison. The sea level pressure
distribution is shown on weather maps.
Figure 10.1 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.
World Distribution of Sea Level Pressure
The world distribution of sea level pressure in
January and July has been shown in Figures
10.2 and 10.3. Near the equator the sea level
pressure is low and the area is known as
equatorial low. Along 30° N and 30
o
 S are
found the high-pressure areas known as the
subtropical highs. Further pole wards along
60
o
N and 60
o
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
Figure 10.2 : Distribution of pressure (in millibars) — January
Figure 10.1 : Isobars, pressure and wind systems in
Northern Hemisphere
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
2015-16
FUNDAMENTALS OF PHYSICAL GEOGRAPHY 90
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
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 V elocity
and Direction of Wind
You already know that the air is set in motion
due to the differences in atmospheric pressure.
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.
Figure 10.3 : Distribution of pressure (in millibars) — July
2015-16
ATMOSPHERIC CIRCULATION AND WEATHER SYSTEMS 91
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.
Pressure and 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 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).
Table 10.2 : Pattern of Wind Direction in Cyclones and Anticyclones
Pressure System Pressure Condition Pattern of Wind Direction
at the Centre Northern Hemisphere Southern Hemisphere
Cyclone Low Anticlockwise Clockwise
Anticyclone High Clockwise Anticlockwise
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 (Table 10.2).
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.
Figure 10.4 : Geostropic Wind
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
Figure 10.5 : Convergence and divergence of winds
2015-16
FUNDAMENTALS OF PHYSICAL GEOGRAPHY 92
climate. A schematic description of the general
circulation is shown in Figure 10.6.
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. You would be studying the
details of monsoon in the book India : Physical
Environment (NCERT, 2006). 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.
Figure 10. 6 : Simplified general circulation
    of the atmosphere
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 30
o
 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 30
o
 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.
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