NCERT Textbook - Climate | Geography for UPSC CSE PDF Download

 Page 1


CLIMATE AND VEGETATION
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
UNIT
III
2024-25
Page 2


CLIMATE AND VEGETATION
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
UNIT
III
2024-25
28 INDIA : PHYSICAL ENVIRONMENT
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 (Ladakh) 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
2024-25
Page 3


CLIMATE AND VEGETATION
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
UNIT
III
2024-25
28 INDIA : PHYSICAL ENVIRONMENT
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 (Ladakh) 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
2024-25
29 CLIMATE
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 Odisha 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.
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
2024-25
Page 4


CLIMATE AND VEGETATION
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
UNIT
III
2024-25
28 INDIA : PHYSICAL ENVIRONMENT
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 (Ladakh) 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
2024-25
29 CLIMATE
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 Odisha 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.
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
2024-25
30 INDIA : PHYSICAL ENVIRONMENT
of Western Ghats and Assam receive high
rainfall during June-September whereas the
southern plateau remains dry due to its
leeward situation along the Western Ghats.
THE NATURE OF INDIAN MONSOON
Monsoon is a familiar though a little known
climatic phenomenon. Despite the observations
spread over centuries, the monsoon continues
to puzzle the scientists. Many attempts have
been made to discover the exact nature and
causation of monsoon, but so far, no single
theory has been able to explain the monsoon
fully.  A real breakthrough has come recently
when it was studied at the global rather than
at regional level.
Systematic studies of the causes of rainfall
in the South Asian region help to understand
the causes and salient features of the monsoon,
particularly some of its important aspects,
such as:
(i) The onset of the monsoon.
(ii) Break in the monsoon.
Onset of the Monsoon
Towards the end of the nineteenth
century, it
was believed that the differential heating of
land and sea during the summer months is
the mechanism which sets the stage for
the monsoon winds to drift towards the
subcontinent.  During April and May when the
sun shines vertically over the Tropic of Cancer,
the large landmass in the north  of Indian ocean
gets intensely heated. This causes the
formation of an intense low pressure in the
northwestern part of the subcontinent. Since
the pressure in the Indian Ocean in the south
of the landmass is high as water gets heated
slowly, the low pressure cell attracts the
southeast trades across the Equator. These
conditions help in the northward shift in the
position of the ITCZ.  The southwest monsoon
may thus, be seen as a continuation of the
southeast trades deflected towards the Indian
subcontinent after crossing the Equator. These
winds cross the Equator between 40°°E and
60°°E longitudes.
Inter Tropical Convergence Zone (ITCZ)
The Inter Tropical Convergence Zone (ITCZ) is a low pressure zone located at the equator
where trade winds converge, and so, it is a zone where air tends to ascend.  In July, the
ITCZ is located around 20°N-25°N latitudes (over the Gangetic plain), sometimes called
the monsoon trough.  This monsoon trough encourages the development of thermal low
over north and northwest India.  Due to the shift of ITCZ, the trade winds of the southern
hemisphere cross the equator between 40° and 60°E longitudes and start blowing from
southwest to northeast due to the Coriolis force.  It becomes southwest monsoon. In winter,
the ITCZ moves southward, and so the reversal of winds from northeast to south and
southwest, takes place. They are called northeast monsoons.
Figure 4.1 : Onset of Monsoon
Inter Tropical Convergence Zone (ITCZ)
The Inter Tropical Convergence Zone (ITCZ) is a low pressure zone located at the equator
where trade winds converge, and so, it is a zone where air tends to ascend.  In July, the
ITCZ is located around 20°°N-25°°N latitudes (over the Gangetic plain), sometimes called
the monsoon trough.  This monsoon trough encourages the development of thermal low
over north and northwest India.  Due to the shift of ITCZ, the trade winds of the southern
hemisphere cross the equator between 40°° and 60°°E longitudes and start blowing from
southwest to northeast due to the Coriolis force.  It becomes southwest monsoon. In winter,
the ITCZ moves southward, and so the reversal of winds from northeast to south and
southwest, takes place. They are called northeast monsoons.
2024-25
Page 5


CLIMATE AND VEGETATION
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
UNIT
III
2024-25
28 INDIA : PHYSICAL ENVIRONMENT
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 (Ladakh) 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
2024-25
29 CLIMATE
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 Odisha 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.
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
2024-25
30 INDIA : PHYSICAL ENVIRONMENT
of Western Ghats and Assam receive high
rainfall during June-September whereas the
southern plateau remains dry due to its
leeward situation along the Western Ghats.
THE NATURE OF INDIAN MONSOON
Monsoon is a familiar though a little known
climatic phenomenon. Despite the observations
spread over centuries, the monsoon continues
to puzzle the scientists. Many attempts have
been made to discover the exact nature and
causation of monsoon, but so far, no single
theory has been able to explain the monsoon
fully.  A real breakthrough has come recently
when it was studied at the global rather than
at regional level.
Systematic studies of the causes of rainfall
in the South Asian region help to understand
the causes and salient features of the monsoon,
particularly some of its important aspects,
such as:
(i) The onset of the monsoon.
(ii) Break in the monsoon.
Onset of the Monsoon
Towards the end of the nineteenth
century, it
was believed that the differential heating of
land and sea during the summer months is
the mechanism which sets the stage for
the monsoon winds to drift towards the
subcontinent.  During April and May when the
sun shines vertically over the Tropic of Cancer,
the large landmass in the north  of Indian ocean
gets intensely heated. This causes the
formation of an intense low pressure in the
northwestern part of the subcontinent. Since
the pressure in the Indian Ocean in the south
of the landmass is high as water gets heated
slowly, the low pressure cell attracts the
southeast trades across the Equator. These
conditions help in the northward shift in the
position of the ITCZ.  The southwest monsoon
may thus, be seen as a continuation of the
southeast trades deflected towards the Indian
subcontinent after crossing the Equator. These
winds cross the Equator between 40°°E and
60°°E longitudes.
Inter Tropical Convergence Zone (ITCZ)
The Inter Tropical Convergence Zone (ITCZ) is a low pressure zone located at the equator
where trade winds converge, and so, it is a zone where air tends to ascend.  In July, the
ITCZ is located around 20°N-25°N latitudes (over the Gangetic plain), sometimes called
the monsoon trough.  This monsoon trough encourages the development of thermal low
over north and northwest India.  Due to the shift of ITCZ, the trade winds of the southern
hemisphere cross the equator between 40° and 60°E longitudes and start blowing from
southwest to northeast due to the Coriolis force.  It becomes southwest monsoon. In winter,
the ITCZ moves southward, and so the reversal of winds from northeast to south and
southwest, takes place. They are called northeast monsoons.
Figure 4.1 : Onset of Monsoon
Inter Tropical Convergence Zone (ITCZ)
The Inter Tropical Convergence Zone (ITCZ) is a low pressure zone located at the equator
where trade winds converge, and so, it is a zone where air tends to ascend.  In July, the
ITCZ is located around 20°°N-25°°N latitudes (over the Gangetic plain), sometimes called
the monsoon trough.  This monsoon trough encourages the development of thermal low
over north and northwest India.  Due to the shift of ITCZ, the trade winds of the southern
hemisphere cross the equator between 40°° and 60°°E longitudes and start blowing from
southwest to northeast due to the Coriolis force.  It becomes southwest monsoon. In winter,
the ITCZ moves southward, and so the reversal of winds from northeast to south and
southwest, takes place. They are called northeast monsoons.
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31 CLIMATE
The shift in the position of the ITCZ is also
related to the phenomenon of the withdrawal
of the westerly jet stream from its position over
the north Indian plain, south of the Himalayas.
The easterly jet stream sets in along 15°°N
latitude only after the western jet stream has
withdrawn itself from the region.  This easterly
jet stream is held responsible for the burst of
the monsoon in India.
Entry of Monsoon into India : The southwest
monsoon sets in over the Kerala coast by 1st
June and moves swiftly to reach Mumbai
and Kolkata between 10th and 13th June.
By mid-July, southwest monsoon engulfs
the entire subcontinent (Figure 4.2)
Break in the Monsoon
During the south-west monsoon period after
having rains for a few days, if rain fails to occur
for one or more weeks, it is known as break in
the monsoon.  These dry spells are quite
common during the rainy season. These
breaks in the different regions are due to
different reasons:
(i) In northern India rains are likely to fail if
the rain-bearing storms are not very
frequent  along the monsoon trough or
the ITCZ over this region.
(ii) Over the west coast the dry spells are
associated with days when winds blow
parallel to the coast.
THE RHYTHM OF SEASONS
The climatic conditions of India can best be
described in terms of an annual cycle of
seasons. The meteorologists recognise the
following four seasons :
(i) the cold weather season
(ii) the hot weather season
(iii) the southwest monsoon season
(iv) the retreating monsoon season.
The Cold Weather Season
Temperature : Usually, the cold weather
season sets in by mid-November in northern
India.  December and January are the coldest
months in the northern plain. The mean
daily temperature remains below 21°°C over
most parts of northern India.  The night
temperature may be quite low, sometimes
going below freezing point in Punjab and
Rajasthan.
There are three main reasons for the excessive
cold in north India during this season :
(i) States like Punjab, Haryana and
Rajasthan being far away from the
moderating influence of sea
experience continental climate.
(ii) The snowfall in the nearby Himalayan
ranges creates cold wave situation;
and
(iii) Around February, the cold winds coming
from the Caspian Sea and Turkmenistan
EI-Nino and the Indian Monsoon
EI-Nino is a complex weather system that appears once every three to seven years, bringing
drought, floods and other weather extremes to different parts of the world.
The system involves oceanic and atmospheric phenomena with the appearance of warm
currents off the coast of Peru in the Eastern Pacific and affects weather in many places including
India. EI-Nino is merely an extension of the warm equatorial current which gets replaced temporarily
by cold Peruvian current or Humbolt current (locate these currents in your atlas).  This current
increases the temperature of water on the Peruvian coast by 10°°C.  This results in:
(i) the distortion of equatorial atmospheric circulation;
(ii) irregularities in the evaporation of sea water;
(iii) reduction in the amount of planktons which further reduces the number of fish in the sea.
The word EI-Nino means ‘Child Christ’ because this current appears around Christmas
in December.  December is a summer month in Peru (Southern Hemisphere).
EI-Nino is used in India for forecasting long range monsoon rainfall.  In 1990-91, there
was a wild EI-Nino event and the onset of southwest monsoon was delayed over most parts of
the country ranging from five to twelve days.
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