NCERT Textbook - Climate Humanities/Arts Notes | EduRev

 Page 1


CLIMATE, VEGETATION AND SOIL
This unit deals with
• Weather and climate – spatial and temporal distribution of temperature,
pressure, winds and rainfall; Indian monsoons: mechanism, onset and
variability – spatial and temporal; climatic types
 Natural vegetation – forest types and distribution; wild life
conservation; biosphere reserves
 Soils – major types and their distribution, soil degradation and
conservation
UNIT
III
Page 2


CLIMATE, VEGETATION AND SOIL
This unit deals with
• Weather and climate – spatial and temporal distribution of temperature,
pressure, winds and rainfall; Indian monsoons: mechanism, onset and
variability – spatial and temporal; climatic types
 Natural vegetation – forest types and distribution; wild life
conservation; biosphere reserves
 Soils – major types and their distribution, soil degradation and
conservation
UNIT
III
W
e drink more water during summers.
Your uniform during the summer is
different from the winters.  Why do
you wear lighter clothes during summers and
heavy woollen clothes during winters in north
India?  In southern India, woollen clothes are
not required. In northeastern states, winters
are mild except in the hills. There are variations
in weather conditions during different seasons.
These changes occur due to the changes in the
elements of weather (temperature, pressure,
wind direction and velocity, humidity and
precipitation, etc.).
Weather is the momentary state of the
atmosphere while climate refers to the
average of the weather conditions over a
longer period of time. Weather changes
quickly, may be within a day or week but
climate changes imperceptively and may
be noted after 50 years or even more.
You have already studied about the
monsoon in your earlier classes.  You are also
aware of the meaning of the word, “monsoon”.
Monsoon connotes the climate associated with
seasonal reversal in the direction of winds.
India has hot monsoonal climate which is the
prevalent climate in south and southeast Asia.
UNITY AND DIVERSITY IN THE MONSOON CLIMATE
The monsoon regime emphasises the unity of
India with the rest of southeast Asian region.
This view of broad unity of the monsoon type
of climate should not, however, lead one to
ignore its regional variations which differentiate
the weather and climate of different regions of
India. For example, the climate of Kerala and
Tamil Nadu in the south are so different from
that of  Uttar Pradesh and Bihar in the north,
and yet all of these have a monsoon type of
climate. The climate of India has many regional
variations expressed in the pattern of winds,
temperature and rainfall, rhythm of seasons
and the degree of wetness or dryness. These
regional diversities may be described as
sub-types of monsoon climate. Let us take a
closer look at these regional variations in
temperature, winds and rainfall.
While in the summer the mercury
occasionally touches 55°C in the western
Rajasthan, it drops down to as low as minus
45°C in winter around Leh. Churu in Rajasthan
may record a temperature of 50°C or more on a
June day while the mercury hardly touches
19°C in Tawang (Arunachal Pradesh)
 
on the
same day. On a December night, temperature
in Drass (Jammu and Kashmir) may drop down
to minus 45°C while Thiruvananthapuram or
Chennai on the same night records 20°C or
22°C. These examples confirm that there are
seasonal variations in temperature from place
to place and from region to region in India. Not
only this, if we take only a single place and
record the temperature for just one day,
variations are no less striking. In Kerala and in
the Andaman Islands, the difference between
day and night temperatures may be hardly
seven or eight degree Celsius. But in the Thar
desert, if the day temperature is around 50°C,
at night, it may drop down considerably upto
15°-20°C.
CLIMATE
CHAPTER
Page 3


CLIMATE, VEGETATION AND SOIL
This unit deals with
• Weather and climate – spatial and temporal distribution of temperature,
pressure, winds and rainfall; Indian monsoons: mechanism, onset and
variability – spatial and temporal; climatic types
 Natural vegetation – forest types and distribution; wild life
conservation; biosphere reserves
 Soils – major types and their distribution, soil degradation and
conservation
UNIT
III
W
e drink more water during summers.
Your uniform during the summer is
different from the winters.  Why do
you wear lighter clothes during summers and
heavy woollen clothes during winters in north
India?  In southern India, woollen clothes are
not required. In northeastern states, winters
are mild except in the hills. There are variations
in weather conditions during different seasons.
These changes occur due to the changes in the
elements of weather (temperature, pressure,
wind direction and velocity, humidity and
precipitation, etc.).
Weather is the momentary state of the
atmosphere while climate refers to the
average of the weather conditions over a
longer period of time. Weather changes
quickly, may be within a day or week but
climate changes imperceptively and may
be noted after 50 years or even more.
You have already studied about the
monsoon in your earlier classes.  You are also
aware of the meaning of the word, “monsoon”.
Monsoon connotes the climate associated with
seasonal reversal in the direction of winds.
India has hot monsoonal climate which is the
prevalent climate in south and southeast Asia.
UNITY AND DIVERSITY IN THE MONSOON CLIMATE
The monsoon regime emphasises the unity of
India with the rest of southeast Asian region.
This view of broad unity of the monsoon type
of climate should not, however, lead one to
ignore its regional variations which differentiate
the weather and climate of different regions of
India. For example, the climate of Kerala and
Tamil Nadu in the south are so different from
that of  Uttar Pradesh and Bihar in the north,
and yet all of these have a monsoon type of
climate. The climate of India has many regional
variations expressed in the pattern of winds,
temperature and rainfall, rhythm of seasons
and the degree of wetness or dryness. These
regional diversities may be described as
sub-types of monsoon climate. Let us take a
closer look at these regional variations in
temperature, winds and rainfall.
While in the summer the mercury
occasionally touches 55°C in the western
Rajasthan, it drops down to as low as minus
45°C in winter around Leh. Churu in Rajasthan
may record a temperature of 50°C or more on a
June day while the mercury hardly touches
19°C in Tawang (Arunachal Pradesh)
 
on the
same day. On a December night, temperature
in Drass (Jammu and Kashmir) may drop down
to minus 45°C while Thiruvananthapuram or
Chennai on the same night records 20°C or
22°C. These examples confirm that there are
seasonal variations in temperature from place
to place and from region to region in India. Not
only this, if we take only a single place and
record the temperature for just one day,
variations are no less striking. In Kerala and in
the Andaman Islands, the difference between
day and night temperatures may be hardly
seven or eight degree Celsius. But in the Thar
desert, if the day temperature is around 50°C,
at night, it may drop down considerably upto
15°-20°C.
CLIMATE
CHAPTER
34 INDIA : PHYSICAL ENVIRONMENT
Now, let us see the regional variations in
precipitation. While snowfall occurs in the
Himalayas, it only rains over the rest of the
country. Similarly, variations are noticeable not
only in the type of precipitation but also in its
amount. While Cherrapunji and Mawsynram
in the Khasi Hills of Meghalaya receive rainfall
over 1,080 cm in a year, Jaisalmer in Rajasthan
rarely gets more than 9 cm of rainfall during
the same period.
Tura situated in the Garo Hills of
Meghalaya may receive an amount of rainfall
in a single day which is equal to 10 years of
rainfall at Jaisalmer.  While the annual
precipitation is less than 10 cm in the north-
west Himalayas and the western deserts, it
exceeds 400 cm in Meghalaya.
The Ganga delta and the coastal plains of
Orissa are hit by strong rain-bearing storms
almost every third or fifth day in July and
August while the Coromandal coast, a
thousand km to the south, goes generally dry
during these months. Most parts of the country
get rainfall during June-September, but on the
coastal areas of Tamil Nadu, it rains in the
beginning of the winter season.
In spite of these differences and variations,
the climate of India is monsoonal in rhythm
and character.
FACTORS DETERMINING THE CLIMATE OF INDIA
India’s climate is controlled by a number of
factors which can be broadly divided into two
groups — factors related to location and relief,
and factors related to air pressure and winds.
Factors related to Location and Relief
Latitude : You already know the latitudinal and
longitudinal extent of the land of India.  You
also know that the Tropic of  Cancer passes
through the central part of India in east-west
direction. Thus, northern part of the India lies
in sub-tropical and temperate zone and the
part lying south of the Tropic of Cancer falls in
the tropical zone.  The tropical zone being
nearer to the equator, experiences high
temperatures throughout the year with small
daily and annual range.  Area north of the
Tropic of Cancer being away from the equator,
experiences extreme climate with high daily
and annual range of temperature.
The Himalayan Mountains : The lofty Himalayas
in the north along with its extensions act as an
effective climatic divide. The towering mountain
chain provides an invincible shield to protect
the subcontinent from the cold northern winds.
These cold and chilly winds originate near the
Arctic circle and blow across central and eastern
Asia. The Himalayas also trap the monsoon
winds, forcing them to shed their moisture
within the subcontinent.
Distribution of Land and Water : India is
flanked by the Indian Ocean on three sides in
the south and girdled by a high and
continuous mountain-wall in the north. As
compared to the landmass, water heats up or
cools down slowly. This differential heating of
land and sea creates different air pressure
zones in different seasons in and around the
Indian subcontinent. Difference in air pressure
causes reversal in the direction of monsoon
winds.
Distance from the Sea : With a long coastline,
large coastal areas have an equable climate.
Areas in the interior of India are far away from
the moderating influence of  the sea. Such
areas have extremes of climate. That is why,
the people of Mumbai and the Konkan coast
have hardly any idea of extremes of
temperature and the seasonal rhythm of
weather. On the other hand, the seasonal
contrasts in weather at places in the interior of
the country such as Delhi, Kanpur and
Amritsar affect the entire sphere of life.
Altitude :
 
Temperature decreases with height.
Due to thin air, places in the mountains are
cooler than places on the plains. For example,
Agra and Darjiling are located on the same
latitude, but temperature of January in Agra
is 16°C whereas it is only 4°C in Darjiling.
Relief :
  
The physiography or relief of India also
affects the temperature, air pressure, direction
and speed of wind and the amount and
distribution of rainfall. The windward sides of
Western Ghats and Assam receive high rainfall
Page 4


CLIMATE, VEGETATION AND SOIL
This unit deals with
• Weather and climate – spatial and temporal distribution of temperature,
pressure, winds and rainfall; Indian monsoons: mechanism, onset and
variability – spatial and temporal; climatic types
 Natural vegetation – forest types and distribution; wild life
conservation; biosphere reserves
 Soils – major types and their distribution, soil degradation and
conservation
UNIT
III
W
e drink more water during summers.
Your uniform during the summer is
different from the winters.  Why do
you wear lighter clothes during summers and
heavy woollen clothes during winters in north
India?  In southern India, woollen clothes are
not required. In northeastern states, winters
are mild except in the hills. There are variations
in weather conditions during different seasons.
These changes occur due to the changes in the
elements of weather (temperature, pressure,
wind direction and velocity, humidity and
precipitation, etc.).
Weather is the momentary state of the
atmosphere while climate refers to the
average of the weather conditions over a
longer period of time. Weather changes
quickly, may be within a day or week but
climate changes imperceptively and may
be noted after 50 years or even more.
You have already studied about the
monsoon in your earlier classes.  You are also
aware of the meaning of the word, “monsoon”.
Monsoon connotes the climate associated with
seasonal reversal in the direction of winds.
India has hot monsoonal climate which is the
prevalent climate in south and southeast Asia.
UNITY AND DIVERSITY IN THE MONSOON CLIMATE
The monsoon regime emphasises the unity of
India with the rest of southeast Asian region.
This view of broad unity of the monsoon type
of climate should not, however, lead one to
ignore its regional variations which differentiate
the weather and climate of different regions of
India. For example, the climate of Kerala and
Tamil Nadu in the south are so different from
that of  Uttar Pradesh and Bihar in the north,
and yet all of these have a monsoon type of
climate. The climate of India has many regional
variations expressed in the pattern of winds,
temperature and rainfall, rhythm of seasons
and the degree of wetness or dryness. These
regional diversities may be described as
sub-types of monsoon climate. Let us take a
closer look at these regional variations in
temperature, winds and rainfall.
While in the summer the mercury
occasionally touches 55°C in the western
Rajasthan, it drops down to as low as minus
45°C in winter around Leh. Churu in Rajasthan
may record a temperature of 50°C or more on a
June day while the mercury hardly touches
19°C in Tawang (Arunachal Pradesh)
 
on the
same day. On a December night, temperature
in Drass (Jammu and Kashmir) may drop down
to minus 45°C while Thiruvananthapuram or
Chennai on the same night records 20°C or
22°C. These examples confirm that there are
seasonal variations in temperature from place
to place and from region to region in India. Not
only this, if we take only a single place and
record the temperature for just one day,
variations are no less striking. In Kerala and in
the Andaman Islands, the difference between
day and night temperatures may be hardly
seven or eight degree Celsius. But in the Thar
desert, if the day temperature is around 50°C,
at night, it may drop down considerably upto
15°-20°C.
CLIMATE
CHAPTER
34 INDIA : PHYSICAL ENVIRONMENT
Now, let us see the regional variations in
precipitation. While snowfall occurs in the
Himalayas, it only rains over the rest of the
country. Similarly, variations are noticeable not
only in the type of precipitation but also in its
amount. While Cherrapunji and Mawsynram
in the Khasi Hills of Meghalaya receive rainfall
over 1,080 cm in a year, Jaisalmer in Rajasthan
rarely gets more than 9 cm of rainfall during
the same period.
Tura situated in the Garo Hills of
Meghalaya may receive an amount of rainfall
in a single day which is equal to 10 years of
rainfall at Jaisalmer.  While the annual
precipitation is less than 10 cm in the north-
west Himalayas and the western deserts, it
exceeds 400 cm in Meghalaya.
The Ganga delta and the coastal plains of
Orissa are hit by strong rain-bearing storms
almost every third or fifth day in July and
August while the Coromandal coast, a
thousand km to the south, goes generally dry
during these months. Most parts of the country
get rainfall during June-September, but on the
coastal areas of Tamil Nadu, it rains in the
beginning of the winter season.
In spite of these differences and variations,
the climate of India is monsoonal in rhythm
and character.
FACTORS DETERMINING THE CLIMATE OF INDIA
India’s climate is controlled by a number of
factors which can be broadly divided into two
groups — factors related to location and relief,
and factors related to air pressure and winds.
Factors related to Location and Relief
Latitude : You already know the latitudinal and
longitudinal extent of the land of India.  You
also know that the Tropic of  Cancer passes
through the central part of India in east-west
direction. Thus, northern part of the India lies
in sub-tropical and temperate zone and the
part lying south of the Tropic of Cancer falls in
the tropical zone.  The tropical zone being
nearer to the equator, experiences high
temperatures throughout the year with small
daily and annual range.  Area north of the
Tropic of Cancer being away from the equator,
experiences extreme climate with high daily
and annual range of temperature.
The Himalayan Mountains : The lofty Himalayas
in the north along with its extensions act as an
effective climatic divide. The towering mountain
chain provides an invincible shield to protect
the subcontinent from the cold northern winds.
These cold and chilly winds originate near the
Arctic circle and blow across central and eastern
Asia. The Himalayas also trap the monsoon
winds, forcing them to shed their moisture
within the subcontinent.
Distribution of Land and Water : India is
flanked by the Indian Ocean on three sides in
the south and girdled by a high and
continuous mountain-wall in the north. As
compared to the landmass, water heats up or
cools down slowly. This differential heating of
land and sea creates different air pressure
zones in different seasons in and around the
Indian subcontinent. Difference in air pressure
causes reversal in the direction of monsoon
winds.
Distance from the Sea : With a long coastline,
large coastal areas have an equable climate.
Areas in the interior of India are far away from
the moderating influence of  the sea. Such
areas have extremes of climate. That is why,
the people of Mumbai and the Konkan coast
have hardly any idea of extremes of
temperature and the seasonal rhythm of
weather. On the other hand, the seasonal
contrasts in weather at places in the interior of
the country such as Delhi, Kanpur and
Amritsar affect the entire sphere of life.
Altitude :
 
Temperature decreases with height.
Due to thin air, places in the mountains are
cooler than places on the plains. For example,
Agra and Darjiling are located on the same
latitude, but temperature of January in Agra
is 16°C whereas it is only 4°C in Darjiling.
Relief :
  
The physiography or relief of India also
affects the temperature, air pressure, direction
and speed of wind and the amount and
distribution of rainfall. The windward sides of
Western Ghats and Assam receive high rainfall
35 CLIMATE
during June-September whereas the southern
plateau remains dry due to its leeward
situation along the Western Ghats.
Factors Related to Air Pressure and Wind
To understand the differences in local climates
of India, we need to understand the
mechanism of the following three factors:
(i) Distribution of air pressure and winds
on the surface of the earth.
(ii) Upper air circulation caused by factors
controlling global weather and the inflow
of different air masses and jet streams.
(iii) Inflow of western cyclones generally
known as disturbances during the winter
season and tropical depressions during
the south-west monsoon period into
India, creating weather conditions
favourable to rainfall.
The mechanism of these three factors can
be understood with reference to winter and
summer seasons of the year separately.
Mechanism of Weather in the Winter Season
Surface Pressure and Winds : In winter
months, the weather conditions over India are
generally influenced by the distribution of
pressure in Central and Western Asia. A high
pressure centre in the region lying to the north
of the Himalayas develops during winter. This
centre of high pressure gives rise to the flow of
air at the low level from the north towards the
Indian subcontinent, south of the mountain
range. The surface winds blowing out of the high
pressure centre over Central Asia reach India
in the form of a dry continental air mass. These
continental winds come in contact with trade
winds over northwestern India. The position of
this contact zone is not, however, stable.
Occasionally, it may shift its position as far east
as the middle  Ganga valley with the result that
the  whole of the northwestern and northern
India up to the middle Ganga valley comes
under the influence of dry northwestern winds.
Jet Stream and Upper Air Circulation : The
pattern of air circulation discussed above is
witnessed only at the lower level of the
atmosphere near the surface of the earth. Higher
up in the lower troposphere, about three km
above the surface of the earth, a different pattern
of air circulation is observed. The variations in
the atmospheric pressure closer to the surface
of the earth have no role to play in the making of
upper air circulation. All of  Western and Central
Asia remains under the influence of westerly
winds along the altitude of 9-13 km from west
to east. These winds blow across the Asian
continent at latitudes north of the Himalayas
roughly parallel  to the Tibetan highlands
(Figure 4.1). These are known as jet streams.
Tibetan highlands act as a barrier in the path of
these jet streams.  As a result, jet streams get
bifurcated. One of its branches blows to the north
of the Tibetan highlands, while the  southern
branch blows in an eastward direction, south of
the Himalayas. It has its mean position at 25°N
in February at 200-300 mb level. It is believed
that this southern branch of the jet stream
exercises an important influence on the winter
weather in India.
Western Cyclonic Disturbance and Tropical
Cyclones : The western cyclonic disturbances
which enter the Indian subcontinent from the west
and the northwest during the winter months,
originate over the Mediterranean Sea and are
Figure 4.1 : Direction of Winds in India  in
Winter at the Height of 9-13 km
Page 5


CLIMATE, VEGETATION AND SOIL
This unit deals with
• Weather and climate – spatial and temporal distribution of temperature,
pressure, winds and rainfall; Indian monsoons: mechanism, onset and
variability – spatial and temporal; climatic types
 Natural vegetation – forest types and distribution; wild life
conservation; biosphere reserves
 Soils – major types and their distribution, soil degradation and
conservation
UNIT
III
W
e drink more water during summers.
Your uniform during the summer is
different from the winters.  Why do
you wear lighter clothes during summers and
heavy woollen clothes during winters in north
India?  In southern India, woollen clothes are
not required. In northeastern states, winters
are mild except in the hills. There are variations
in weather conditions during different seasons.
These changes occur due to the changes in the
elements of weather (temperature, pressure,
wind direction and velocity, humidity and
precipitation, etc.).
Weather is the momentary state of the
atmosphere while climate refers to the
average of the weather conditions over a
longer period of time. Weather changes
quickly, may be within a day or week but
climate changes imperceptively and may
be noted after 50 years or even more.
You have already studied about the
monsoon in your earlier classes.  You are also
aware of the meaning of the word, “monsoon”.
Monsoon connotes the climate associated with
seasonal reversal in the direction of winds.
India has hot monsoonal climate which is the
prevalent climate in south and southeast Asia.
UNITY AND DIVERSITY IN THE MONSOON CLIMATE
The monsoon regime emphasises the unity of
India with the rest of southeast Asian region.
This view of broad unity of the monsoon type
of climate should not, however, lead one to
ignore its regional variations which differentiate
the weather and climate of different regions of
India. For example, the climate of Kerala and
Tamil Nadu in the south are so different from
that of  Uttar Pradesh and Bihar in the north,
and yet all of these have a monsoon type of
climate. The climate of India has many regional
variations expressed in the pattern of winds,
temperature and rainfall, rhythm of seasons
and the degree of wetness or dryness. These
regional diversities may be described as
sub-types of monsoon climate. Let us take a
closer look at these regional variations in
temperature, winds and rainfall.
While in the summer the mercury
occasionally touches 55°C in the western
Rajasthan, it drops down to as low as minus
45°C in winter around Leh. Churu in Rajasthan
may record a temperature of 50°C or more on a
June day while the mercury hardly touches
19°C in Tawang (Arunachal Pradesh)
 
on the
same day. On a December night, temperature
in Drass (Jammu and Kashmir) may drop down
to minus 45°C while Thiruvananthapuram or
Chennai on the same night records 20°C or
22°C. These examples confirm that there are
seasonal variations in temperature from place
to place and from region to region in India. Not
only this, if we take only a single place and
record the temperature for just one day,
variations are no less striking. In Kerala and in
the Andaman Islands, the difference between
day and night temperatures may be hardly
seven or eight degree Celsius. But in the Thar
desert, if the day temperature is around 50°C,
at night, it may drop down considerably upto
15°-20°C.
CLIMATE
CHAPTER
34 INDIA : PHYSICAL ENVIRONMENT
Now, let us see the regional variations in
precipitation. While snowfall occurs in the
Himalayas, it only rains over the rest of the
country. Similarly, variations are noticeable not
only in the type of precipitation but also in its
amount. While Cherrapunji and Mawsynram
in the Khasi Hills of Meghalaya receive rainfall
over 1,080 cm in a year, Jaisalmer in Rajasthan
rarely gets more than 9 cm of rainfall during
the same period.
Tura situated in the Garo Hills of
Meghalaya may receive an amount of rainfall
in a single day which is equal to 10 years of
rainfall at Jaisalmer.  While the annual
precipitation is less than 10 cm in the north-
west Himalayas and the western deserts, it
exceeds 400 cm in Meghalaya.
The Ganga delta and the coastal plains of
Orissa are hit by strong rain-bearing storms
almost every third or fifth day in July and
August while the Coromandal coast, a
thousand km to the south, goes generally dry
during these months. Most parts of the country
get rainfall during June-September, but on the
coastal areas of Tamil Nadu, it rains in the
beginning of the winter season.
In spite of these differences and variations,
the climate of India is monsoonal in rhythm
and character.
FACTORS DETERMINING THE CLIMATE OF INDIA
India’s climate is controlled by a number of
factors which can be broadly divided into two
groups — factors related to location and relief,
and factors related to air pressure and winds.
Factors related to Location and Relief
Latitude : You already know the latitudinal and
longitudinal extent of the land of India.  You
also know that the Tropic of  Cancer passes
through the central part of India in east-west
direction. Thus, northern part of the India lies
in sub-tropical and temperate zone and the
part lying south of the Tropic of Cancer falls in
the tropical zone.  The tropical zone being
nearer to the equator, experiences high
temperatures throughout the year with small
daily and annual range.  Area north of the
Tropic of Cancer being away from the equator,
experiences extreme climate with high daily
and annual range of temperature.
The Himalayan Mountains : The lofty Himalayas
in the north along with its extensions act as an
effective climatic divide. The towering mountain
chain provides an invincible shield to protect
the subcontinent from the cold northern winds.
These cold and chilly winds originate near the
Arctic circle and blow across central and eastern
Asia. The Himalayas also trap the monsoon
winds, forcing them to shed their moisture
within the subcontinent.
Distribution of Land and Water : India is
flanked by the Indian Ocean on three sides in
the south and girdled by a high and
continuous mountain-wall in the north. As
compared to the landmass, water heats up or
cools down slowly. This differential heating of
land and sea creates different air pressure
zones in different seasons in and around the
Indian subcontinent. Difference in air pressure
causes reversal in the direction of monsoon
winds.
Distance from the Sea : With a long coastline,
large coastal areas have an equable climate.
Areas in the interior of India are far away from
the moderating influence of  the sea. Such
areas have extremes of climate. That is why,
the people of Mumbai and the Konkan coast
have hardly any idea of extremes of
temperature and the seasonal rhythm of
weather. On the other hand, the seasonal
contrasts in weather at places in the interior of
the country such as Delhi, Kanpur and
Amritsar affect the entire sphere of life.
Altitude :
 
Temperature decreases with height.
Due to thin air, places in the mountains are
cooler than places on the plains. For example,
Agra and Darjiling are located on the same
latitude, but temperature of January in Agra
is 16°C whereas it is only 4°C in Darjiling.
Relief :
  
The physiography or relief of India also
affects the temperature, air pressure, direction
and speed of wind and the amount and
distribution of rainfall. The windward sides of
Western Ghats and Assam receive high rainfall
35 CLIMATE
during June-September whereas the southern
plateau remains dry due to its leeward
situation along the Western Ghats.
Factors Related to Air Pressure and Wind
To understand the differences in local climates
of India, we need to understand the
mechanism of the following three factors:
(i) Distribution of air pressure and winds
on the surface of the earth.
(ii) Upper air circulation caused by factors
controlling global weather and the inflow
of different air masses and jet streams.
(iii) Inflow of western cyclones generally
known as disturbances during the winter
season and tropical depressions during
the south-west monsoon period into
India, creating weather conditions
favourable to rainfall.
The mechanism of these three factors can
be understood with reference to winter and
summer seasons of the year separately.
Mechanism of Weather in the Winter Season
Surface Pressure and Winds : In winter
months, the weather conditions over India are
generally influenced by the distribution of
pressure in Central and Western Asia. A high
pressure centre in the region lying to the north
of the Himalayas develops during winter. This
centre of high pressure gives rise to the flow of
air at the low level from the north towards the
Indian subcontinent, south of the mountain
range. The surface winds blowing out of the high
pressure centre over Central Asia reach India
in the form of a dry continental air mass. These
continental winds come in contact with trade
winds over northwestern India. The position of
this contact zone is not, however, stable.
Occasionally, it may shift its position as far east
as the middle  Ganga valley with the result that
the  whole of the northwestern and northern
India up to the middle Ganga valley comes
under the influence of dry northwestern winds.
Jet Stream and Upper Air Circulation : The
pattern of air circulation discussed above is
witnessed only at the lower level of the
atmosphere near the surface of the earth. Higher
up in the lower troposphere, about three km
above the surface of the earth, a different pattern
of air circulation is observed. The variations in
the atmospheric pressure closer to the surface
of the earth have no role to play in the making of
upper air circulation. All of  Western and Central
Asia remains under the influence of westerly
winds along the altitude of 9-13 km from west
to east. These winds blow across the Asian
continent at latitudes north of the Himalayas
roughly parallel  to the Tibetan highlands
(Figure 4.1). These are known as jet streams.
Tibetan highlands act as a barrier in the path of
these jet streams.  As a result, jet streams get
bifurcated. One of its branches blows to the north
of the Tibetan highlands, while the  southern
branch blows in an eastward direction, south of
the Himalayas. It has its mean position at 25°N
in February at 200-300 mb level. It is believed
that this southern branch of the jet stream
exercises an important influence on the winter
weather in India.
Western Cyclonic Disturbance and Tropical
Cyclones : The western cyclonic disturbances
which enter the Indian subcontinent from the west
and the northwest during the winter months,
originate over the Mediterranean Sea and are
Figure 4.1 : Direction of Winds in India  in
Winter at the Height of 9-13 km
36 INDIA : PHYSICAL ENVIRONMENT
brought into India by the westerly jet stream. An
increase in the prevailing night temperature
generally indicates an advance in the arrival of these
cyclones disturbances.
Tropical cyclones originate over the Bay of
Bengal and the Indian ocean. These tropical
cyclones have very high wind velocity and heavy
rainfall and hit the Tamil Nadu, Andhra Pradesh
and Orissa coast.  Most of these cyclones are very
destructive due to high wind velocity and torrential
rain that accompanies it.  Have you seen their
movement in the weather report in the television?
Mechanism of W eather in the Summer Season
Surface Pressure and Winds : As the summer
sets in and the sun shifts northwards, the wind
circulation over the subcontinent undergoes
a complete reversal at  both, the lower as well
as the upper levels. By the middle of July, the
low pressure belt nearer the surface [termed
as Inter Tropical Convergence Zone (ITCZ)]
shifts northwards, roughly parallel to the
Himalayas between 20° N and 25° N. By this
time, the westerly jet stream withdraws from
the Indian region. In fact, meteorologists have
found an interrelationship between the
northward shift of the equatorial trough (ITCZ)
and the withdrawal of the westerly jet stream
from over the North Indian Plain. It is generally
believed that there is a cause and effect
relationship between the two. The ITCZ being
a zone of low pressure, attracts inflow of winds
from different directions. The maritime tropical
airmass (mT) from the southern hemisphere,
after crossing the equator, rushes to the low
pressure area in the general southwesterly
direction. It is this moist air current which is
popularly known as the southwest monsoon.
Jet Streams and Upper Air Circulation :  The
pattern of pressure and winds as mentioned
above is formed only at the level of the
troposphere. An easterly jet stream flows over
Figure 4.2 : Summer Monsoon Winds : Surface Circulation