NCERT Textbook - Climate Class 9 Notes | EduRev

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UPSC : NCERT Textbook - Climate Class 9 Notes | EduRev

 Page 1


I
n the last two chapters you have read about
the landforms and the drainage of
our country.  These are the two of the three
basic elements that one learns about the natural
environment of any area.  In this chapter you
will learn about the third, that is, the
atmospheric conditions that prevail over our
country.  Why do we wear woollens in December
or why it is hot and uncomfortable in the month
of May, and why it rains in June - July?  The
answers to all these questions can be found out
by studying about the climate of India.
Climate refers to the sum total of  weather
conditions and variations over a large  area for
a long period of time (more than thirty years).
Weather refers to the state of the atmosphere
over an area at any point of time. The elements
of weather and climate are the same, i.e.
temperature, atmospheric pressure, wind,
humidity and precipitation. You may have
observed that the weather conditions fluctuate
very often even within a day. But there is some
common pattern over a few weeks or months,
i.e. days are cool or hot, windy or calm, cloudy
or bright, and wet or dry. On the basis of the
generalised monthly atmospheric conditions,
the year is divided into seasons such as winter,
summer or rainy seasons.
The world is divided into a number of
climatic regions. Do you know what type of
climate India has and why it is so? We will
learn about it in this chapter.
•  The word monsoon is
derived from the Arabic word ‘mausim’ which
literally means season.
• ‘Monsoon’ refers to the seasonal reversal in the
wind direction during a year.
CLIMATE
The climate of India is described as the
‘monsoon’ type. In Asia, this type of climate
is found mainly in the south and the southeast.
Despite an overall unity in the general pattern,
there are perceptible regional variations in
climatic conditions within the country. Let us
take two important elements – temperature and
precipitation, and examine how they vary from
place to place and season to season.
In summer, the mercury occasionally
touches 50°C in some parts of the Rajasthan
desert, whereas it may be around 20°C in
Pahalgam in Jammu and Kashmir. On a winter
night, temperature at Drass in Jammu and
Kashmir may be as low as minus 45°C.
Thiruvananthapuram, on the other hand, may
have a temperature of 22°C.
In certain places there is a
wide difference between day and night
temperatures. In the Thar Desert the day
temperature may rise to 50°C, and drop down
to near 15°C the same night. On the other hand,
there is hardly any difference in day and night
temperatures in the Andaman and Nicobar
islands or in Kerala.
Let us now look at precipitation. There are
variations not only in the form and types of
precipitation but also in its amount and the
seasonal distribution. While precipitation is
mostly in the form of snowfall in the upper parts
of Himalayas, it rains over the rest of the
country. The annual precipitation varies from
over 400 cm in Meghalaya to less than 10 cm
in Ladakh and western Rajasthan. Most parts
of the country receive rainfall from June to
September. But some parts like the Tamil Nadu
4
2015-16
Page 2


I
n the last two chapters you have read about
the landforms and the drainage of
our country.  These are the two of the three
basic elements that one learns about the natural
environment of any area.  In this chapter you
will learn about the third, that is, the
atmospheric conditions that prevail over our
country.  Why do we wear woollens in December
or why it is hot and uncomfortable in the month
of May, and why it rains in June - July?  The
answers to all these questions can be found out
by studying about the climate of India.
Climate refers to the sum total of  weather
conditions and variations over a large  area for
a long period of time (more than thirty years).
Weather refers to the state of the atmosphere
over an area at any point of time. The elements
of weather and climate are the same, i.e.
temperature, atmospheric pressure, wind,
humidity and precipitation. You may have
observed that the weather conditions fluctuate
very often even within a day. But there is some
common pattern over a few weeks or months,
i.e. days are cool or hot, windy or calm, cloudy
or bright, and wet or dry. On the basis of the
generalised monthly atmospheric conditions,
the year is divided into seasons such as winter,
summer or rainy seasons.
The world is divided into a number of
climatic regions. Do you know what type of
climate India has and why it is so? We will
learn about it in this chapter.
•  The word monsoon is
derived from the Arabic word ‘mausim’ which
literally means season.
• ‘Monsoon’ refers to the seasonal reversal in the
wind direction during a year.
CLIMATE
The climate of India is described as the
‘monsoon’ type. In Asia, this type of climate
is found mainly in the south and the southeast.
Despite an overall unity in the general pattern,
there are perceptible regional variations in
climatic conditions within the country. Let us
take two important elements – temperature and
precipitation, and examine how they vary from
place to place and season to season.
In summer, the mercury occasionally
touches 50°C in some parts of the Rajasthan
desert, whereas it may be around 20°C in
Pahalgam in Jammu and Kashmir. On a winter
night, temperature at Drass in Jammu and
Kashmir may be as low as minus 45°C.
Thiruvananthapuram, on the other hand, may
have a temperature of 22°C.
In certain places there is a
wide difference between day and night
temperatures. In the Thar Desert the day
temperature may rise to 50°C, and drop down
to near 15°C the same night. On the other hand,
there is hardly any difference in day and night
temperatures in the Andaman and Nicobar
islands or in Kerala.
Let us now look at precipitation. There are
variations not only in the form and types of
precipitation but also in its amount and the
seasonal distribution. While precipitation is
mostly in the form of snowfall in the upper parts
of Himalayas, it rains over the rest of the
country. The annual precipitation varies from
over 400 cm in Meghalaya to less than 10 cm
in Ladakh and western Rajasthan. Most parts
of the country receive rainfall from June to
September. But some parts like the Tamil Nadu
4
2015-16
CLIMATE 27
•Why most of the world’s deserts are
located in the western margins of continents in the
subtropics?
Finally, relief too plays a major role in
determining the climate of a place. High mountains
act as barriers for cold or hot winds; they may
also cause precipitation if they are high enough
and lie in the path of rain-bearing winds. The
leeward side of mountains remains relatively dry.
FACTORS AFFECTING INDIA’S CLIMATE
Latitude
The Tropic of Cancer passes through the middle
of the country from the Rann of Kuchchh in the
west to Mizoram in the east. Almost half of the
country, lying south of the Tropic of Cancer,
belongs to the tropical area. All the remaining
area, north of the Tropic, lies in the sub-tropics.
Therefore, India’s climate has characteristics of
tropical as well as subtropical climates.
Altitude
India has mountains to the north, which have
an average height of about 6,000 metres. India
also has a vast coastal area where the
maximum elevation is about 30 metres. The
Himalayas prevent the cold winds from Central
Asia from entering the subcontinent. It is
because of these mountains that this
subcontinent experiences comparatively
milder winters as compared to central Asia.
Pressure and Winds
The climate and associated weather conditions
in India are governed by the following
atmospheric conditions:
• Pressure and surface winds;
• Upper air circulation; and
• Western cyclonic disturbances and
tropical cyclones.
India lies in the region of north easterly
winds. These winds originate from the
subtropical high-pressure belt of the northern
coast gets a large portion of its rain during
October and November.
 In general, coastal areas experience less
contrasts in temperature conditions. Seasonal
contrasts are more in the interior of the
country. There is decrease in rainfall generally
from east to west in the Northern Plains. These
variations have given rise to variety in lives of
people – in terms of the food they eat, the
clothes they wear and also the kind of houses
they live in.
• Why the houses in Rajasthan have thick
walls and flat roofs?
•  Why is it that the houses in the Tarai region and
in Goa and Mangalore have sloping roofs?
• Why houses in Assam are built on stilts?
CLIMATIC CONTROLS
There are six major controls of  the climate of
any place. They are: latitude, altitude,
pressure and wind system, distance from
the sea (continentality), ocean currents and
relief features.
Due to the curvature of the earth, the
amount of solar energy received varies
according to latitude. As a result, air
temperature generally decreases from the
equator towards the poles. As one goes from
the surface of the earth to higher altitudes,
the atmosphere becomes less dense and
temperature decreases. The hills are therefore
cooler during summers. The pressure and
wind system of any area depend on the
latitude and altitude of the place. Thus it
influences the temperature and rainfall
pattern. The sea exerts a moderating influence
on climate: As the distance from the sea
increases, its moderating influence decreases
and the people experience extreme weather
conditions. This condition is known as
continentality (i.e. very hot during summers
and very cold during winters). Ocean currents
along with onshore winds affect the climate of
the coastal areas, For example, any coastal
area with warm or cold currents flowing past
it, will be warmed or cooled if the winds are
onshore.
2015-16
Page 3


I
n the last two chapters you have read about
the landforms and the drainage of
our country.  These are the two of the three
basic elements that one learns about the natural
environment of any area.  In this chapter you
will learn about the third, that is, the
atmospheric conditions that prevail over our
country.  Why do we wear woollens in December
or why it is hot and uncomfortable in the month
of May, and why it rains in June - July?  The
answers to all these questions can be found out
by studying about the climate of India.
Climate refers to the sum total of  weather
conditions and variations over a large  area for
a long period of time (more than thirty years).
Weather refers to the state of the atmosphere
over an area at any point of time. The elements
of weather and climate are the same, i.e.
temperature, atmospheric pressure, wind,
humidity and precipitation. You may have
observed that the weather conditions fluctuate
very often even within a day. But there is some
common pattern over a few weeks or months,
i.e. days are cool or hot, windy or calm, cloudy
or bright, and wet or dry. On the basis of the
generalised monthly atmospheric conditions,
the year is divided into seasons such as winter,
summer or rainy seasons.
The world is divided into a number of
climatic regions. Do you know what type of
climate India has and why it is so? We will
learn about it in this chapter.
•  The word monsoon is
derived from the Arabic word ‘mausim’ which
literally means season.
• ‘Monsoon’ refers to the seasonal reversal in the
wind direction during a year.
CLIMATE
The climate of India is described as the
‘monsoon’ type. In Asia, this type of climate
is found mainly in the south and the southeast.
Despite an overall unity in the general pattern,
there are perceptible regional variations in
climatic conditions within the country. Let us
take two important elements – temperature and
precipitation, and examine how they vary from
place to place and season to season.
In summer, the mercury occasionally
touches 50°C in some parts of the Rajasthan
desert, whereas it may be around 20°C in
Pahalgam in Jammu and Kashmir. On a winter
night, temperature at Drass in Jammu and
Kashmir may be as low as minus 45°C.
Thiruvananthapuram, on the other hand, may
have a temperature of 22°C.
In certain places there is a
wide difference between day and night
temperatures. In the Thar Desert the day
temperature may rise to 50°C, and drop down
to near 15°C the same night. On the other hand,
there is hardly any difference in day and night
temperatures in the Andaman and Nicobar
islands or in Kerala.
Let us now look at precipitation. There are
variations not only in the form and types of
precipitation but also in its amount and the
seasonal distribution. While precipitation is
mostly in the form of snowfall in the upper parts
of Himalayas, it rains over the rest of the
country. The annual precipitation varies from
over 400 cm in Meghalaya to less than 10 cm
in Ladakh and western Rajasthan. Most parts
of the country receive rainfall from June to
September. But some parts like the Tamil Nadu
4
2015-16
CLIMATE 27
•Why most of the world’s deserts are
located in the western margins of continents in the
subtropics?
Finally, relief too plays a major role in
determining the climate of a place. High mountains
act as barriers for cold or hot winds; they may
also cause precipitation if they are high enough
and lie in the path of rain-bearing winds. The
leeward side of mountains remains relatively dry.
FACTORS AFFECTING INDIA’S CLIMATE
Latitude
The Tropic of Cancer passes through the middle
of the country from the Rann of Kuchchh in the
west to Mizoram in the east. Almost half of the
country, lying south of the Tropic of Cancer,
belongs to the tropical area. All the remaining
area, north of the Tropic, lies in the sub-tropics.
Therefore, India’s climate has characteristics of
tropical as well as subtropical climates.
Altitude
India has mountains to the north, which have
an average height of about 6,000 metres. India
also has a vast coastal area where the
maximum elevation is about 30 metres. The
Himalayas prevent the cold winds from Central
Asia from entering the subcontinent. It is
because of these mountains that this
subcontinent experiences comparatively
milder winters as compared to central Asia.
Pressure and Winds
The climate and associated weather conditions
in India are governed by the following
atmospheric conditions:
• Pressure and surface winds;
• Upper air circulation; and
• Western cyclonic disturbances and
tropical cyclones.
India lies in the region of north easterly
winds. These winds originate from the
subtropical high-pressure belt of the northern
coast gets a large portion of its rain during
October and November.
 In general, coastal areas experience less
contrasts in temperature conditions. Seasonal
contrasts are more in the interior of the
country. There is decrease in rainfall generally
from east to west in the Northern Plains. These
variations have given rise to variety in lives of
people – in terms of the food they eat, the
clothes they wear and also the kind of houses
they live in.
• Why the houses in Rajasthan have thick
walls and flat roofs?
•  Why is it that the houses in the Tarai region and
in Goa and Mangalore have sloping roofs?
• Why houses in Assam are built on stilts?
CLIMATIC CONTROLS
There are six major controls of  the climate of
any place. They are: latitude, altitude,
pressure and wind system, distance from
the sea (continentality), ocean currents and
relief features.
Due to the curvature of the earth, the
amount of solar energy received varies
according to latitude. As a result, air
temperature generally decreases from the
equator towards the poles. As one goes from
the surface of the earth to higher altitudes,
the atmosphere becomes less dense and
temperature decreases. The hills are therefore
cooler during summers. The pressure and
wind system of any area depend on the
latitude and altitude of the place. Thus it
influences the temperature and rainfall
pattern. The sea exerts a moderating influence
on climate: As the distance from the sea
increases, its moderating influence decreases
and the people experience extreme weather
conditions. This condition is known as
continentality (i.e. very hot during summers
and very cold during winters). Ocean currents
along with onshore winds affect the climate of
the coastal areas, For example, any coastal
area with warm or cold currents flowing past
it, will be warmed or cooled if the winds are
onshore.
2015-16
CONTEMPORARY INDIA 28
hemisphere. They blow south, get deflected to
the right due to the Coriolis force, and move
on towards the equatorial low-pressure area.
Generally, these winds carry very little
moisture as they originate and blow over land.
Therefore, they bring little or no rain. Hence,
India should have been an arid land, but, it is
not so. Let us see why?
Coriolis force: An apparent force caused by the earth’s
rotation. The Coriolis force is responsible for deflecting
winds towards the right in the northern hemisphere
and towards the left in the southern hemisphere.  This
is also known as ‘Ferrel’s Law’.
The pressure and wind conditions over
India are unique. During winter, there is a
high-pressure area north of the Himalayas.
Cold dry winds blow from this region to the
low-pressure areas over the oceans to the
south. In summer, a low-pressure area
develops over interior Asia as well as over
northwestern India. This causes a complete
reversal of the direction of winds during
summer. Air moves from the high-pressure
area over the southern Indian Ocean, in a
south-easterly direction, crosses the equator,
and turns right towards the low-pressure areas
over the Indian subcontinent. These are known
as the Southwest Monsoon winds. These winds
blow over the warm oceans, gather moisture
and bring widespread rainfall over the
mainland of India.
The upper air circulation in this region is
dominated by a westerly flow. An important
component of this flow is the jet stream.
These jet streams are located approximately
over 27°-30° north latitude, therefore, they are
known as subtropical westerly jet streams. Over
India, these jet streams blow south of the
Himalayas, all through the year except in
summer. The western cyclonic disturbances
experienced in the north and north-western parts
of the country are brought in by this westerly
flow. In summer, the subtropical westerly jet
stream moves north of the Himalayas with the
apparent movement of the sun. An easterly jet
stream, called the sub-tropical easterly jet stream
blows over peninsular India, approximately over
14°N during the summer months.
Western Cyclonic Disturbances
The western cyclonic disturbances are weather
phenomena of the winter months brought in by the
westerly flow from the Mediterranean region. They
usually influence the weather of the north and
north-western regions of India. Tropical cyclones
occur during the monsoon as well as in October -
November, and are part of the easterly flow. These
distrurbances affect the coastal regions of the
country. Have you read or heard about the
disasters caused by them on Orissa and Andhra
Pradesh coast?
THE INDIAN M ONSOON
The climate of India is strongly influenced by
monsoon winds. The sailors who came to India
in historic times were one of the first to have
noticed the phenomenon of the monsoon. They
benefited from the reversal of the wind system
as they came by sailing ships at the mercy of
winds. The Arabs, who had also come to India
as traders named this seasonal reversal of the
wind system ‘monsoon’.
Figure 4.1 : Arrival of Monsoon
Jet stream: These are a narrow belt of high
altitude (above 12,000 m) westerly winds in the
troposphere. Their speed varies from about 110
km/h in summer to about 184 km/h in winter.
A number of separate jet streams have been
identified. The most constant are the mid-latitude
and the sub tropical jet stream.
2015-16
Page 4


I
n the last two chapters you have read about
the landforms and the drainage of
our country.  These are the two of the three
basic elements that one learns about the natural
environment of any area.  In this chapter you
will learn about the third, that is, the
atmospheric conditions that prevail over our
country.  Why do we wear woollens in December
or why it is hot and uncomfortable in the month
of May, and why it rains in June - July?  The
answers to all these questions can be found out
by studying about the climate of India.
Climate refers to the sum total of  weather
conditions and variations over a large  area for
a long period of time (more than thirty years).
Weather refers to the state of the atmosphere
over an area at any point of time. The elements
of weather and climate are the same, i.e.
temperature, atmospheric pressure, wind,
humidity and precipitation. You may have
observed that the weather conditions fluctuate
very often even within a day. But there is some
common pattern over a few weeks or months,
i.e. days are cool or hot, windy or calm, cloudy
or bright, and wet or dry. On the basis of the
generalised monthly atmospheric conditions,
the year is divided into seasons such as winter,
summer or rainy seasons.
The world is divided into a number of
climatic regions. Do you know what type of
climate India has and why it is so? We will
learn about it in this chapter.
•  The word monsoon is
derived from the Arabic word ‘mausim’ which
literally means season.
• ‘Monsoon’ refers to the seasonal reversal in the
wind direction during a year.
CLIMATE
The climate of India is described as the
‘monsoon’ type. In Asia, this type of climate
is found mainly in the south and the southeast.
Despite an overall unity in the general pattern,
there are perceptible regional variations in
climatic conditions within the country. Let us
take two important elements – temperature and
precipitation, and examine how they vary from
place to place and season to season.
In summer, the mercury occasionally
touches 50°C in some parts of the Rajasthan
desert, whereas it may be around 20°C in
Pahalgam in Jammu and Kashmir. On a winter
night, temperature at Drass in Jammu and
Kashmir may be as low as minus 45°C.
Thiruvananthapuram, on the other hand, may
have a temperature of 22°C.
In certain places there is a
wide difference between day and night
temperatures. In the Thar Desert the day
temperature may rise to 50°C, and drop down
to near 15°C the same night. On the other hand,
there is hardly any difference in day and night
temperatures in the Andaman and Nicobar
islands or in Kerala.
Let us now look at precipitation. There are
variations not only in the form and types of
precipitation but also in its amount and the
seasonal distribution. While precipitation is
mostly in the form of snowfall in the upper parts
of Himalayas, it rains over the rest of the
country. The annual precipitation varies from
over 400 cm in Meghalaya to less than 10 cm
in Ladakh and western Rajasthan. Most parts
of the country receive rainfall from June to
September. But some parts like the Tamil Nadu
4
2015-16
CLIMATE 27
•Why most of the world’s deserts are
located in the western margins of continents in the
subtropics?
Finally, relief too plays a major role in
determining the climate of a place. High mountains
act as barriers for cold or hot winds; they may
also cause precipitation if they are high enough
and lie in the path of rain-bearing winds. The
leeward side of mountains remains relatively dry.
FACTORS AFFECTING INDIA’S CLIMATE
Latitude
The Tropic of Cancer passes through the middle
of the country from the Rann of Kuchchh in the
west to Mizoram in the east. Almost half of the
country, lying south of the Tropic of Cancer,
belongs to the tropical area. All the remaining
area, north of the Tropic, lies in the sub-tropics.
Therefore, India’s climate has characteristics of
tropical as well as subtropical climates.
Altitude
India has mountains to the north, which have
an average height of about 6,000 metres. India
also has a vast coastal area where the
maximum elevation is about 30 metres. The
Himalayas prevent the cold winds from Central
Asia from entering the subcontinent. It is
because of these mountains that this
subcontinent experiences comparatively
milder winters as compared to central Asia.
Pressure and Winds
The climate and associated weather conditions
in India are governed by the following
atmospheric conditions:
• Pressure and surface winds;
• Upper air circulation; and
• Western cyclonic disturbances and
tropical cyclones.
India lies in the region of north easterly
winds. These winds originate from the
subtropical high-pressure belt of the northern
coast gets a large portion of its rain during
October and November.
 In general, coastal areas experience less
contrasts in temperature conditions. Seasonal
contrasts are more in the interior of the
country. There is decrease in rainfall generally
from east to west in the Northern Plains. These
variations have given rise to variety in lives of
people – in terms of the food they eat, the
clothes they wear and also the kind of houses
they live in.
• Why the houses in Rajasthan have thick
walls and flat roofs?
•  Why is it that the houses in the Tarai region and
in Goa and Mangalore have sloping roofs?
• Why houses in Assam are built on stilts?
CLIMATIC CONTROLS
There are six major controls of  the climate of
any place. They are: latitude, altitude,
pressure and wind system, distance from
the sea (continentality), ocean currents and
relief features.
Due to the curvature of the earth, the
amount of solar energy received varies
according to latitude. As a result, air
temperature generally decreases from the
equator towards the poles. As one goes from
the surface of the earth to higher altitudes,
the atmosphere becomes less dense and
temperature decreases. The hills are therefore
cooler during summers. The pressure and
wind system of any area depend on the
latitude and altitude of the place. Thus it
influences the temperature and rainfall
pattern. The sea exerts a moderating influence
on climate: As the distance from the sea
increases, its moderating influence decreases
and the people experience extreme weather
conditions. This condition is known as
continentality (i.e. very hot during summers
and very cold during winters). Ocean currents
along with onshore winds affect the climate of
the coastal areas, For example, any coastal
area with warm or cold currents flowing past
it, will be warmed or cooled if the winds are
onshore.
2015-16
CONTEMPORARY INDIA 28
hemisphere. They blow south, get deflected to
the right due to the Coriolis force, and move
on towards the equatorial low-pressure area.
Generally, these winds carry very little
moisture as they originate and blow over land.
Therefore, they bring little or no rain. Hence,
India should have been an arid land, but, it is
not so. Let us see why?
Coriolis force: An apparent force caused by the earth’s
rotation. The Coriolis force is responsible for deflecting
winds towards the right in the northern hemisphere
and towards the left in the southern hemisphere.  This
is also known as ‘Ferrel’s Law’.
The pressure and wind conditions over
India are unique. During winter, there is a
high-pressure area north of the Himalayas.
Cold dry winds blow from this region to the
low-pressure areas over the oceans to the
south. In summer, a low-pressure area
develops over interior Asia as well as over
northwestern India. This causes a complete
reversal of the direction of winds during
summer. Air moves from the high-pressure
area over the southern Indian Ocean, in a
south-easterly direction, crosses the equator,
and turns right towards the low-pressure areas
over the Indian subcontinent. These are known
as the Southwest Monsoon winds. These winds
blow over the warm oceans, gather moisture
and bring widespread rainfall over the
mainland of India.
The upper air circulation in this region is
dominated by a westerly flow. An important
component of this flow is the jet stream.
These jet streams are located approximately
over 27°-30° north latitude, therefore, they are
known as subtropical westerly jet streams. Over
India, these jet streams blow south of the
Himalayas, all through the year except in
summer. The western cyclonic disturbances
experienced in the north and north-western parts
of the country are brought in by this westerly
flow. In summer, the subtropical westerly jet
stream moves north of the Himalayas with the
apparent movement of the sun. An easterly jet
stream, called the sub-tropical easterly jet stream
blows over peninsular India, approximately over
14°N during the summer months.
Western Cyclonic Disturbances
The western cyclonic disturbances are weather
phenomena of the winter months brought in by the
westerly flow from the Mediterranean region. They
usually influence the weather of the north and
north-western regions of India. Tropical cyclones
occur during the monsoon as well as in October -
November, and are part of the easterly flow. These
distrurbances affect the coastal regions of the
country. Have you read or heard about the
disasters caused by them on Orissa and Andhra
Pradesh coast?
THE INDIAN M ONSOON
The climate of India is strongly influenced by
monsoon winds. The sailors who came to India
in historic times were one of the first to have
noticed the phenomenon of the monsoon. They
benefited from the reversal of the wind system
as they came by sailing ships at the mercy of
winds. The Arabs, who had also come to India
as traders named this seasonal reversal of the
wind system ‘monsoon’.
Figure 4.1 : Arrival of Monsoon
Jet stream: These are a narrow belt of high
altitude (above 12,000 m) westerly winds in the
troposphere. Their speed varies from about 110
km/h in summer to about 184 km/h in winter.
A number of separate jet streams have been
identified. The most constant are the mid-latitude
and the sub tropical jet stream.
2015-16
CLIMATE 29
Figure 4.3 : Atmospheric Conditions over the Indian Subcontinent in the Month of June
Figure 4.2 : Atmospheric Conditions over the Indian Subcontinent in the Month of January
2015-16
Page 5


I
n the last two chapters you have read about
the landforms and the drainage of
our country.  These are the two of the three
basic elements that one learns about the natural
environment of any area.  In this chapter you
will learn about the third, that is, the
atmospheric conditions that prevail over our
country.  Why do we wear woollens in December
or why it is hot and uncomfortable in the month
of May, and why it rains in June - July?  The
answers to all these questions can be found out
by studying about the climate of India.
Climate refers to the sum total of  weather
conditions and variations over a large  area for
a long period of time (more than thirty years).
Weather refers to the state of the atmosphere
over an area at any point of time. The elements
of weather and climate are the same, i.e.
temperature, atmospheric pressure, wind,
humidity and precipitation. You may have
observed that the weather conditions fluctuate
very often even within a day. But there is some
common pattern over a few weeks or months,
i.e. days are cool or hot, windy or calm, cloudy
or bright, and wet or dry. On the basis of the
generalised monthly atmospheric conditions,
the year is divided into seasons such as winter,
summer or rainy seasons.
The world is divided into a number of
climatic regions. Do you know what type of
climate India has and why it is so? We will
learn about it in this chapter.
•  The word monsoon is
derived from the Arabic word ‘mausim’ which
literally means season.
• ‘Monsoon’ refers to the seasonal reversal in the
wind direction during a year.
CLIMATE
The climate of India is described as the
‘monsoon’ type. In Asia, this type of climate
is found mainly in the south and the southeast.
Despite an overall unity in the general pattern,
there are perceptible regional variations in
climatic conditions within the country. Let us
take two important elements – temperature and
precipitation, and examine how they vary from
place to place and season to season.
In summer, the mercury occasionally
touches 50°C in some parts of the Rajasthan
desert, whereas it may be around 20°C in
Pahalgam in Jammu and Kashmir. On a winter
night, temperature at Drass in Jammu and
Kashmir may be as low as minus 45°C.
Thiruvananthapuram, on the other hand, may
have a temperature of 22°C.
In certain places there is a
wide difference between day and night
temperatures. In the Thar Desert the day
temperature may rise to 50°C, and drop down
to near 15°C the same night. On the other hand,
there is hardly any difference in day and night
temperatures in the Andaman and Nicobar
islands or in Kerala.
Let us now look at precipitation. There are
variations not only in the form and types of
precipitation but also in its amount and the
seasonal distribution. While precipitation is
mostly in the form of snowfall in the upper parts
of Himalayas, it rains over the rest of the
country. The annual precipitation varies from
over 400 cm in Meghalaya to less than 10 cm
in Ladakh and western Rajasthan. Most parts
of the country receive rainfall from June to
September. But some parts like the Tamil Nadu
4
2015-16
CLIMATE 27
•Why most of the world’s deserts are
located in the western margins of continents in the
subtropics?
Finally, relief too plays a major role in
determining the climate of a place. High mountains
act as barriers for cold or hot winds; they may
also cause precipitation if they are high enough
and lie in the path of rain-bearing winds. The
leeward side of mountains remains relatively dry.
FACTORS AFFECTING INDIA’S CLIMATE
Latitude
The Tropic of Cancer passes through the middle
of the country from the Rann of Kuchchh in the
west to Mizoram in the east. Almost half of the
country, lying south of the Tropic of Cancer,
belongs to the tropical area. All the remaining
area, north of the Tropic, lies in the sub-tropics.
Therefore, India’s climate has characteristics of
tropical as well as subtropical climates.
Altitude
India has mountains to the north, which have
an average height of about 6,000 metres. India
also has a vast coastal area where the
maximum elevation is about 30 metres. The
Himalayas prevent the cold winds from Central
Asia from entering the subcontinent. It is
because of these mountains that this
subcontinent experiences comparatively
milder winters as compared to central Asia.
Pressure and Winds
The climate and associated weather conditions
in India are governed by the following
atmospheric conditions:
• Pressure and surface winds;
• Upper air circulation; and
• Western cyclonic disturbances and
tropical cyclones.
India lies in the region of north easterly
winds. These winds originate from the
subtropical high-pressure belt of the northern
coast gets a large portion of its rain during
October and November.
 In general, coastal areas experience less
contrasts in temperature conditions. Seasonal
contrasts are more in the interior of the
country. There is decrease in rainfall generally
from east to west in the Northern Plains. These
variations have given rise to variety in lives of
people – in terms of the food they eat, the
clothes they wear and also the kind of houses
they live in.
• Why the houses in Rajasthan have thick
walls and flat roofs?
•  Why is it that the houses in the Tarai region and
in Goa and Mangalore have sloping roofs?
• Why houses in Assam are built on stilts?
CLIMATIC CONTROLS
There are six major controls of  the climate of
any place. They are: latitude, altitude,
pressure and wind system, distance from
the sea (continentality), ocean currents and
relief features.
Due to the curvature of the earth, the
amount of solar energy received varies
according to latitude. As a result, air
temperature generally decreases from the
equator towards the poles. As one goes from
the surface of the earth to higher altitudes,
the atmosphere becomes less dense and
temperature decreases. The hills are therefore
cooler during summers. The pressure and
wind system of any area depend on the
latitude and altitude of the place. Thus it
influences the temperature and rainfall
pattern. The sea exerts a moderating influence
on climate: As the distance from the sea
increases, its moderating influence decreases
and the people experience extreme weather
conditions. This condition is known as
continentality (i.e. very hot during summers
and very cold during winters). Ocean currents
along with onshore winds affect the climate of
the coastal areas, For example, any coastal
area with warm or cold currents flowing past
it, will be warmed or cooled if the winds are
onshore.
2015-16
CONTEMPORARY INDIA 28
hemisphere. They blow south, get deflected to
the right due to the Coriolis force, and move
on towards the equatorial low-pressure area.
Generally, these winds carry very little
moisture as they originate and blow over land.
Therefore, they bring little or no rain. Hence,
India should have been an arid land, but, it is
not so. Let us see why?
Coriolis force: An apparent force caused by the earth’s
rotation. The Coriolis force is responsible for deflecting
winds towards the right in the northern hemisphere
and towards the left in the southern hemisphere.  This
is also known as ‘Ferrel’s Law’.
The pressure and wind conditions over
India are unique. During winter, there is a
high-pressure area north of the Himalayas.
Cold dry winds blow from this region to the
low-pressure areas over the oceans to the
south. In summer, a low-pressure area
develops over interior Asia as well as over
northwestern India. This causes a complete
reversal of the direction of winds during
summer. Air moves from the high-pressure
area over the southern Indian Ocean, in a
south-easterly direction, crosses the equator,
and turns right towards the low-pressure areas
over the Indian subcontinent. These are known
as the Southwest Monsoon winds. These winds
blow over the warm oceans, gather moisture
and bring widespread rainfall over the
mainland of India.
The upper air circulation in this region is
dominated by a westerly flow. An important
component of this flow is the jet stream.
These jet streams are located approximately
over 27°-30° north latitude, therefore, they are
known as subtropical westerly jet streams. Over
India, these jet streams blow south of the
Himalayas, all through the year except in
summer. The western cyclonic disturbances
experienced in the north and north-western parts
of the country are brought in by this westerly
flow. In summer, the subtropical westerly jet
stream moves north of the Himalayas with the
apparent movement of the sun. An easterly jet
stream, called the sub-tropical easterly jet stream
blows over peninsular India, approximately over
14°N during the summer months.
Western Cyclonic Disturbances
The western cyclonic disturbances are weather
phenomena of the winter months brought in by the
westerly flow from the Mediterranean region. They
usually influence the weather of the north and
north-western regions of India. Tropical cyclones
occur during the monsoon as well as in October -
November, and are part of the easterly flow. These
distrurbances affect the coastal regions of the
country. Have you read or heard about the
disasters caused by them on Orissa and Andhra
Pradesh coast?
THE INDIAN M ONSOON
The climate of India is strongly influenced by
monsoon winds. The sailors who came to India
in historic times were one of the first to have
noticed the phenomenon of the monsoon. They
benefited from the reversal of the wind system
as they came by sailing ships at the mercy of
winds. The Arabs, who had also come to India
as traders named this seasonal reversal of the
wind system ‘monsoon’.
Figure 4.1 : Arrival of Monsoon
Jet stream: These are a narrow belt of high
altitude (above 12,000 m) westerly winds in the
troposphere. Their speed varies from about 110
km/h in summer to about 184 km/h in winter.
A number of separate jet streams have been
identified. The most constant are the mid-latitude
and the sub tropical jet stream.
2015-16
CLIMATE 29
Figure 4.3 : Atmospheric Conditions over the Indian Subcontinent in the Month of June
Figure 4.2 : Atmospheric Conditions over the Indian Subcontinent in the Month of January
2015-16
CONTEMPORARY INDIA 30
conditions is known as the Southern
Oscillation or SO. The difference in pressure
over Tahiti (Pacific Ocean, 18°S/149°W) and
Darwin in northern Australia (Indian Ocean,
12°30’S/131°E) is computed to predict the
intensity of the monsoons. If the pressure
differences were negative, it would mean below
average and late monsoons. A feature connected
with the SO is the El Nino phenomenon in which
a warm ocean current that flows past the
Peruvian Coast, in place of the cold Peruvian
current, every 2 to 5 years. The changes in
pressure conditions are connected to the El Nino.
Hence, the phenomenon is referred to as ENSO
(El Nino Southern Oscillations).
El Nino: This is a name given to the periodic
development of a warm ocean current along the
coast of Peru as a temporary replacement of the
cold Peruvian current. ‘El Nino’ is a Spanish word
meaning ‘the child’, and refers to the baby Christ,
as this current starts flowing during Christmas.
The presence of the El Nino leads to an increase
in sea-surface temperatures and weakening of
the trade winds in the region.
THE ONSET OF THE MONSOON AND WITHDRAW AL
The Monsoon, unlike the trades, are not steady
winds but are pulsating in nature, affected by
different atmospheric conditions encountered
by it, on its way over the warm tropical seas.
The duration of the monsoon is between 100-
120 days from early June to mid-September.
Around the time of its arrival, the normal
rainfall increases suddenly and continues
constantly for several days. This is known as
the ‘burst’ of the monsoon, and can be
distinguished from the pre-monsoon showers.
The monsoon arrives at the southern tip of the
Indian peninsula generally by the first week of
June. Subsequently, it proceeds into two – the
Arabian Sea branch and the Bay of Bengal
branch. The Arabian Sea branch reaches
Mumbai about ten days later on
approximately the 10
th
 of June. This is a fairly
rapid advance. The Bay of Bengal branch also
advances rapidly and arrives in Assam in the
first week of June. The lofty mountains causes
the monsoon winds to deflect towards the west
The monsoons are experienced in the
tropical area roughly between 20° N and 20°
S. To understand the mechanism of the
monsoons, the following facts are important.
(a) The differential heating and cooling of
land and water creates low pressure on the
landmass of India while the seas around
experience comparatively high pressure.
(b) The shift of the position of Inter Tropical
Convergence Zone (ITCZ) in summer, over
the Ganga plain (this is the equatorial trough
normally positioned about 5°N of the
equator. It is also known as the monsoon-
trough during the monsoon season).
(c) The presence of the high-pressure area,
east of Madagascar, approximately at
20°S over the Indian Ocean. The intensity
and position of this high-pressure area
affects the Indian Monsoon.
(d) The Tibetan plateau gets intensely
heated during summer, which results in
strong vertical air currents and the
formation of low pressure over the plateau
at about 9 km above sea level.
(e) The movement of the westerly jet stream
to the north of the Himalayas and the
presence of the tropical easterly jet
stream over the Indian peninsula during
summer.
Inter Tropical Convergence Zone
The Inter Tropical Convergence Zone (ITCZ,) is a
broad trough of low pressure in equatorial
latitudes. This is where the northeast and the
southeast trade winds converge. This
convergence zone lies more or less parallel to
the equator but moves north or south with the
apparent movement of the sun.
Apart from this, it has also been noticed that
changes in the pressure conditions over the
southern oceans also affect the monsoons.
Normally when the tropical eastern south Pacific
Ocean experiences high pressure, the tropical
eastern Indian Ocean experiences low pressure.
But in certain years, there is a reversal in the
pressure conditions and the eastern Pacific has
lower pressure in comparison to the eastern
Indian Ocean. This periodic change in pressure
2015-16
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