NCERT Textbook: Winds, Storms & Cyclones | General Science(Prelims) by IRS Divey Sethi - UPSC PDF Download

 Page 1


SCIENCE 80
Winds, Storms
and Cyclones
8
O
rissa was hit by a cyclone with
wind speed of 200 km/h on
18 October 1999. The cyclone
smashed 45,000 houses making
7,00,000 people homeless. On
29 October the same year, a second
cyclone with wind speed of 260 km/h
hit Orissa again. It was accompanied by
water waves about 9 m high.  Thousands
of people lost their lives.  Property worth
crores of rupees was destroyed. The
cyclone affected agriculture, transport,
communication, and electricity supply.
But, what are cyclones?  How are
they formed? Why are they so
destructive? In this chapter we shall
seek answers to some of these questions.
We begin with some activities
involving air. These activities will clarify
some basic features concerning a
cyclone. Before we begin, remember that
the moving air is called the wind.
8.1 AIR EXERTS PRESSURE
Activity 8.1
Fig. 8.1   Image taken by a satellite of a cyclone
approaching the coast of Orissa
Courtesy: India Meteorological Department,
New Delhi
Whenever an activity involves heating,
be very careful.  It is advised that such
activities are performed in the
presence of an elderly person from
your family. Or, carry out these
activities in the presence of your
teacher.
You need to boil water in the following
activity.
Take a tin can with a lid. Fill it
approximately half with water. Heat the
can on a candle flame till the water boils.
Let the water boil for a few minutes. Blow
out the candle. Immediately put the lid
tightly on the can. Be careful in handling
the hot can. Put the can carefully in a
shallow metallic vessel or a washbasin.
Pour fresh water over the can.
What happens to the shape of the can?
2022-23
Page 2


SCIENCE 80
Winds, Storms
and Cyclones
8
O
rissa was hit by a cyclone with
wind speed of 200 km/h on
18 October 1999. The cyclone
smashed 45,000 houses making
7,00,000 people homeless. On
29 October the same year, a second
cyclone with wind speed of 260 km/h
hit Orissa again. It was accompanied by
water waves about 9 m high.  Thousands
of people lost their lives.  Property worth
crores of rupees was destroyed. The
cyclone affected agriculture, transport,
communication, and electricity supply.
But, what are cyclones?  How are
they formed? Why are they so
destructive? In this chapter we shall
seek answers to some of these questions.
We begin with some activities
involving air. These activities will clarify
some basic features concerning a
cyclone. Before we begin, remember that
the moving air is called the wind.
8.1 AIR EXERTS PRESSURE
Activity 8.1
Fig. 8.1   Image taken by a satellite of a cyclone
approaching the coast of Orissa
Courtesy: India Meteorological Department,
New Delhi
Whenever an activity involves heating,
be very careful.  It is advised that such
activities are performed in the
presence of an elderly person from
your family. Or, carry out these
activities in the presence of your
teacher.
You need to boil water in the following
activity.
Take a tin can with a lid. Fill it
approximately half with water. Heat the
can on a candle flame till the water boils.
Let the water boil for a few minutes. Blow
out the candle. Immediately put the lid
tightly on the can. Be careful in handling
the hot can. Put the can carefully in a
shallow metallic vessel or a washbasin.
Pour fresh water over the can.
What happens to the shape of the can?
2022-23
WINDS, STORMS AND CYCLONES 81
Can you guess why the shape of the
can gets distorted?
If you cannot get a tin can, take a
soft plastic bottle. Fill it with hot water.
Empty the bottle and immediately cap
it tightly. Place the bottle under running
water.
Recall now some of your experiences.
When you fly a kite, does the wind
coming from your back help?
If you are in a boat, is it easier to
row it if there is wind coming from
behind you?
Do you find it difficult to ride a bicycle
against the direction of the wind.
You know that we have to fill air into
the bicycle tube to keep it tight. Also,
you know that a bicycle tube overfilled
with air may burst. What is the air doing
inside the tube?
Discuss with your friends how the
air in the bicycle tube keeps it in shape.
All these experiences show that the
air exerts pressure. It is due to this
pressure that the leaves of trees,
banners, or flags flutter when the wind
is blowing. You can list some more
experiences which show that the air has
pressure.
Let us now try to explain why the
can (or the bottle) gets distorted. As
water is poured over the can, some
steam in the can condenses into water,
reducing the amount of air inside. The
pressure of air inside the can decreases
than the pressure exerted by the air
from outside the can. As a result the
can gets compressed.
This activity again confirms that air
exerts pressure.
8.2 HIGH SPEED WINDS ARE
ACCOMPANIED BY REDUCED AIR
PRESSURE
Activity 8.2
Crumple a small piece of paper into
a ball of size smaller than the mouth of
an empty bottle. Hold the empty bottle
on its side and place the paper ball just
inside its mouth. Now try to blow on
the ball to force it into the bottle. Try
the activity with bottles of different sizes.
Challenge your friends if they can force
Fig. 8.3  Blowing into the bottle
Fig. 8.2   Can with hot water being cooled
2022-23
Page 3


SCIENCE 80
Winds, Storms
and Cyclones
8
O
rissa was hit by a cyclone with
wind speed of 200 km/h on
18 October 1999. The cyclone
smashed 45,000 houses making
7,00,000 people homeless. On
29 October the same year, a second
cyclone with wind speed of 260 km/h
hit Orissa again. It was accompanied by
water waves about 9 m high.  Thousands
of people lost their lives.  Property worth
crores of rupees was destroyed. The
cyclone affected agriculture, transport,
communication, and electricity supply.
But, what are cyclones?  How are
they formed? Why are they so
destructive? In this chapter we shall
seek answers to some of these questions.
We begin with some activities
involving air. These activities will clarify
some basic features concerning a
cyclone. Before we begin, remember that
the moving air is called the wind.
8.1 AIR EXERTS PRESSURE
Activity 8.1
Fig. 8.1   Image taken by a satellite of a cyclone
approaching the coast of Orissa
Courtesy: India Meteorological Department,
New Delhi
Whenever an activity involves heating,
be very careful.  It is advised that such
activities are performed in the
presence of an elderly person from
your family. Or, carry out these
activities in the presence of your
teacher.
You need to boil water in the following
activity.
Take a tin can with a lid. Fill it
approximately half with water. Heat the
can on a candle flame till the water boils.
Let the water boil for a few minutes. Blow
out the candle. Immediately put the lid
tightly on the can. Be careful in handling
the hot can. Put the can carefully in a
shallow metallic vessel or a washbasin.
Pour fresh water over the can.
What happens to the shape of the can?
2022-23
WINDS, STORMS AND CYCLONES 81
Can you guess why the shape of the
can gets distorted?
If you cannot get a tin can, take a
soft plastic bottle. Fill it with hot water.
Empty the bottle and immediately cap
it tightly. Place the bottle under running
water.
Recall now some of your experiences.
When you fly a kite, does the wind
coming from your back help?
If you are in a boat, is it easier to
row it if there is wind coming from
behind you?
Do you find it difficult to ride a bicycle
against the direction of the wind.
You know that we have to fill air into
the bicycle tube to keep it tight. Also,
you know that a bicycle tube overfilled
with air may burst. What is the air doing
inside the tube?
Discuss with your friends how the
air in the bicycle tube keeps it in shape.
All these experiences show that the
air exerts pressure. It is due to this
pressure that the leaves of trees,
banners, or flags flutter when the wind
is blowing. You can list some more
experiences which show that the air has
pressure.
Let us now try to explain why the
can (or the bottle) gets distorted. As
water is poured over the can, some
steam in the can condenses into water,
reducing the amount of air inside. The
pressure of air inside the can decreases
than the pressure exerted by the air
from outside the can. As a result the
can gets compressed.
This activity again confirms that air
exerts pressure.
8.2 HIGH SPEED WINDS ARE
ACCOMPANIED BY REDUCED AIR
PRESSURE
Activity 8.2
Crumple a small piece of paper into
a ball of size smaller than the mouth of
an empty bottle. Hold the empty bottle
on its side and place the paper ball just
inside its mouth. Now try to blow on
the ball to force it into the bottle. Try
the activity with bottles of different sizes.
Challenge your friends if they can force
Fig. 8.3  Blowing into the bottle
Fig. 8.2   Can with hot water being cooled
2022-23
SCIENCE 82
Paheli thinks that the strip will be
lifted up. Boojho thinks that the strip
will bend down.
Fig. 8.5   Blowing over a strip of paper
Fig. 8.4   Blowing between the balloons
the paper ball in by blowing into the
bottle.
Paheli and Boojho are thinking about
the following question:
Why is it difficult to force the paper
ball into the bottle?
Activity 8.3
Blow the balloons
Take two balloons of approximately
equal size. Put a little water into the
balloons. Blow up both the balloons and
tie each one to a string. Hang the
balloons 8–10 cm apart on a cycle spoke
or a stick. Blow in the space between
the balloons.
What did you expect? What
happens?
Try different ways of blowing on the
balloons to see what happens.
Activity 8.4
Can you blow and lift?
Hold a strip of paper, 20 cm long and
3 cm wide, between your thumb and
forefinger as shown in the Fig. 8.5. Now
blow over the paper.
What do you think will happen to
the paper?
Let us try to understand the
observations in Activities 8.2, 8.3 and 8.4.
Were the observations along the lines
you thought? Do you get the feeling
that the increased wind speed is
accompanied by a reduced air pressure?
When we blow into the mouth of the
bottle, the air near the mouth has higher
speed. This decreases the pressure
there. The air pressure inside the bottle
is higher than near the mouth. The air
inside the bottle pushes the ball out.
In Activity 8.3 you saw that when you
blew between the balloons, they moved
2022-23
Page 4


SCIENCE 80
Winds, Storms
and Cyclones
8
O
rissa was hit by a cyclone with
wind speed of 200 km/h on
18 October 1999. The cyclone
smashed 45,000 houses making
7,00,000 people homeless. On
29 October the same year, a second
cyclone with wind speed of 260 km/h
hit Orissa again. It was accompanied by
water waves about 9 m high.  Thousands
of people lost their lives.  Property worth
crores of rupees was destroyed. The
cyclone affected agriculture, transport,
communication, and electricity supply.
But, what are cyclones?  How are
they formed? Why are they so
destructive? In this chapter we shall
seek answers to some of these questions.
We begin with some activities
involving air. These activities will clarify
some basic features concerning a
cyclone. Before we begin, remember that
the moving air is called the wind.
8.1 AIR EXERTS PRESSURE
Activity 8.1
Fig. 8.1   Image taken by a satellite of a cyclone
approaching the coast of Orissa
Courtesy: India Meteorological Department,
New Delhi
Whenever an activity involves heating,
be very careful.  It is advised that such
activities are performed in the
presence of an elderly person from
your family. Or, carry out these
activities in the presence of your
teacher.
You need to boil water in the following
activity.
Take a tin can with a lid. Fill it
approximately half with water. Heat the
can on a candle flame till the water boils.
Let the water boil for a few minutes. Blow
out the candle. Immediately put the lid
tightly on the can. Be careful in handling
the hot can. Put the can carefully in a
shallow metallic vessel or a washbasin.
Pour fresh water over the can.
What happens to the shape of the can?
2022-23
WINDS, STORMS AND CYCLONES 81
Can you guess why the shape of the
can gets distorted?
If you cannot get a tin can, take a
soft plastic bottle. Fill it with hot water.
Empty the bottle and immediately cap
it tightly. Place the bottle under running
water.
Recall now some of your experiences.
When you fly a kite, does the wind
coming from your back help?
If you are in a boat, is it easier to
row it if there is wind coming from
behind you?
Do you find it difficult to ride a bicycle
against the direction of the wind.
You know that we have to fill air into
the bicycle tube to keep it tight. Also,
you know that a bicycle tube overfilled
with air may burst. What is the air doing
inside the tube?
Discuss with your friends how the
air in the bicycle tube keeps it in shape.
All these experiences show that the
air exerts pressure. It is due to this
pressure that the leaves of trees,
banners, or flags flutter when the wind
is blowing. You can list some more
experiences which show that the air has
pressure.
Let us now try to explain why the
can (or the bottle) gets distorted. As
water is poured over the can, some
steam in the can condenses into water,
reducing the amount of air inside. The
pressure of air inside the can decreases
than the pressure exerted by the air
from outside the can. As a result the
can gets compressed.
This activity again confirms that air
exerts pressure.
8.2 HIGH SPEED WINDS ARE
ACCOMPANIED BY REDUCED AIR
PRESSURE
Activity 8.2
Crumple a small piece of paper into
a ball of size smaller than the mouth of
an empty bottle. Hold the empty bottle
on its side and place the paper ball just
inside its mouth. Now try to blow on
the ball to force it into the bottle. Try
the activity with bottles of different sizes.
Challenge your friends if they can force
Fig. 8.3  Blowing into the bottle
Fig. 8.2   Can with hot water being cooled
2022-23
SCIENCE 82
Paheli thinks that the strip will be
lifted up. Boojho thinks that the strip
will bend down.
Fig. 8.5   Blowing over a strip of paper
Fig. 8.4   Blowing between the balloons
the paper ball in by blowing into the
bottle.
Paheli and Boojho are thinking about
the following question:
Why is it difficult to force the paper
ball into the bottle?
Activity 8.3
Blow the balloons
Take two balloons of approximately
equal size. Put a little water into the
balloons. Blow up both the balloons and
tie each one to a string. Hang the
balloons 8–10 cm apart on a cycle spoke
or a stick. Blow in the space between
the balloons.
What did you expect? What
happens?
Try different ways of blowing on the
balloons to see what happens.
Activity 8.4
Can you blow and lift?
Hold a strip of paper, 20 cm long and
3 cm wide, between your thumb and
forefinger as shown in the Fig. 8.5. Now
blow over the paper.
What do you think will happen to
the paper?
Let us try to understand the
observations in Activities 8.2, 8.3 and 8.4.
Were the observations along the lines
you thought? Do you get the feeling
that the increased wind speed is
accompanied by a reduced air pressure?
When we blow into the mouth of the
bottle, the air near the mouth has higher
speed. This decreases the pressure
there. The air pressure inside the bottle
is higher than near the mouth. The air
inside the bottle pushes the ball out.
In Activity 8.3 you saw that when you
blew between the balloons, they moved
2022-23
WINDS, STORMS AND CYCLONES 83
towards each other. How could this
happen? This could happen if the
pressure of air between the balloons
were somehow reduced. The pressure
outside the balloons would then push
them towards each other.
In Activity 8.4 you saw that when you
blew over the paper strip, it went
upwards. Again, this could happen if
blowing over the paper reduced the air
pressure above the strip.
We see that the  increased wind
speed is, indeed, accompanied by a
reduced air pressure.
Can you imagine what would happen
if high-speed winds blew over the roofs
of buildings? If the roofs were weak, they
could be lifted and blown away. If you
have any such experience, share it with
your friends.
Let us try to understand how winds
are produced, how they bring rain and
how they can be destructive sometimes.
You already know that when  air
moves, it is called wind.  Air moves from
the region where the air pressure is
high to the region where the pressure
is low. The greater the difference in
pressure, the faster the air moves. But
how are the pressure differences created
in nature? Is the difference in
temperature involved? The following
activities will help you to understand
this.
8.3 AIR EXPANDS ON HEATING
Activity 8.5
Take a boiling tube. Stretch a balloon
tightly over the neck of the tube. You
can use a tape to make it tight. Pour
some hot water in a beaker.  Insert the
boiling tube with the balloon in the hot
water. Observe for 2–3 minutes for any
change in shape of the balloon. Take the
tube out, let it cool down to the room
temperature. Take some ice-cold water
in another beaker and place the tube
with the balloon in cold water for 2–3
minutes. Observe the change in the
shape of the balloon.
Fig. 8.6  The shape of the balloon in hot and cold water
Think and try to answer:
What makes the balloon
inflated when the boiling tube
is placed in hot water?
Why is the same balloon
deflated when the tube is kept
in cold water?
Can we infer from the first
observation that air expands
on heating? Can you now state
what happens to the air in the
boiling tube when it cools
down?
Boiling tube
immersed in
hot water
Boiling tube
immersed in
ice-cold water
Balloon tied over
the neck of the
boiling tube
2022-23
Page 5


SCIENCE 80
Winds, Storms
and Cyclones
8
O
rissa was hit by a cyclone with
wind speed of 200 km/h on
18 October 1999. The cyclone
smashed 45,000 houses making
7,00,000 people homeless. On
29 October the same year, a second
cyclone with wind speed of 260 km/h
hit Orissa again. It was accompanied by
water waves about 9 m high.  Thousands
of people lost their lives.  Property worth
crores of rupees was destroyed. The
cyclone affected agriculture, transport,
communication, and electricity supply.
But, what are cyclones?  How are
they formed? Why are they so
destructive? In this chapter we shall
seek answers to some of these questions.
We begin with some activities
involving air. These activities will clarify
some basic features concerning a
cyclone. Before we begin, remember that
the moving air is called the wind.
8.1 AIR EXERTS PRESSURE
Activity 8.1
Fig. 8.1   Image taken by a satellite of a cyclone
approaching the coast of Orissa
Courtesy: India Meteorological Department,
New Delhi
Whenever an activity involves heating,
be very careful.  It is advised that such
activities are performed in the
presence of an elderly person from
your family. Or, carry out these
activities in the presence of your
teacher.
You need to boil water in the following
activity.
Take a tin can with a lid. Fill it
approximately half with water. Heat the
can on a candle flame till the water boils.
Let the water boil for a few minutes. Blow
out the candle. Immediately put the lid
tightly on the can. Be careful in handling
the hot can. Put the can carefully in a
shallow metallic vessel or a washbasin.
Pour fresh water over the can.
What happens to the shape of the can?
2022-23
WINDS, STORMS AND CYCLONES 81
Can you guess why the shape of the
can gets distorted?
If you cannot get a tin can, take a
soft plastic bottle. Fill it with hot water.
Empty the bottle and immediately cap
it tightly. Place the bottle under running
water.
Recall now some of your experiences.
When you fly a kite, does the wind
coming from your back help?
If you are in a boat, is it easier to
row it if there is wind coming from
behind you?
Do you find it difficult to ride a bicycle
against the direction of the wind.
You know that we have to fill air into
the bicycle tube to keep it tight. Also,
you know that a bicycle tube overfilled
with air may burst. What is the air doing
inside the tube?
Discuss with your friends how the
air in the bicycle tube keeps it in shape.
All these experiences show that the
air exerts pressure. It is due to this
pressure that the leaves of trees,
banners, or flags flutter when the wind
is blowing. You can list some more
experiences which show that the air has
pressure.
Let us now try to explain why the
can (or the bottle) gets distorted. As
water is poured over the can, some
steam in the can condenses into water,
reducing the amount of air inside. The
pressure of air inside the can decreases
than the pressure exerted by the air
from outside the can. As a result the
can gets compressed.
This activity again confirms that air
exerts pressure.
8.2 HIGH SPEED WINDS ARE
ACCOMPANIED BY REDUCED AIR
PRESSURE
Activity 8.2
Crumple a small piece of paper into
a ball of size smaller than the mouth of
an empty bottle. Hold the empty bottle
on its side and place the paper ball just
inside its mouth. Now try to blow on
the ball to force it into the bottle. Try
the activity with bottles of different sizes.
Challenge your friends if they can force
Fig. 8.3  Blowing into the bottle
Fig. 8.2   Can with hot water being cooled
2022-23
SCIENCE 82
Paheli thinks that the strip will be
lifted up. Boojho thinks that the strip
will bend down.
Fig. 8.5   Blowing over a strip of paper
Fig. 8.4   Blowing between the balloons
the paper ball in by blowing into the
bottle.
Paheli and Boojho are thinking about
the following question:
Why is it difficult to force the paper
ball into the bottle?
Activity 8.3
Blow the balloons
Take two balloons of approximately
equal size. Put a little water into the
balloons. Blow up both the balloons and
tie each one to a string. Hang the
balloons 8–10 cm apart on a cycle spoke
or a stick. Blow in the space between
the balloons.
What did you expect? What
happens?
Try different ways of blowing on the
balloons to see what happens.
Activity 8.4
Can you blow and lift?
Hold a strip of paper, 20 cm long and
3 cm wide, between your thumb and
forefinger as shown in the Fig. 8.5. Now
blow over the paper.
What do you think will happen to
the paper?
Let us try to understand the
observations in Activities 8.2, 8.3 and 8.4.
Were the observations along the lines
you thought? Do you get the feeling
that the increased wind speed is
accompanied by a reduced air pressure?
When we blow into the mouth of the
bottle, the air near the mouth has higher
speed. This decreases the pressure
there. The air pressure inside the bottle
is higher than near the mouth. The air
inside the bottle pushes the ball out.
In Activity 8.3 you saw that when you
blew between the balloons, they moved
2022-23
WINDS, STORMS AND CYCLONES 83
towards each other. How could this
happen? This could happen if the
pressure of air between the balloons
were somehow reduced. The pressure
outside the balloons would then push
them towards each other.
In Activity 8.4 you saw that when you
blew over the paper strip, it went
upwards. Again, this could happen if
blowing over the paper reduced the air
pressure above the strip.
We see that the  increased wind
speed is, indeed, accompanied by a
reduced air pressure.
Can you imagine what would happen
if high-speed winds blew over the roofs
of buildings? If the roofs were weak, they
could be lifted and blown away. If you
have any such experience, share it with
your friends.
Let us try to understand how winds
are produced, how they bring rain and
how they can be destructive sometimes.
You already know that when  air
moves, it is called wind.  Air moves from
the region where the air pressure is
high to the region where the pressure
is low. The greater the difference in
pressure, the faster the air moves. But
how are the pressure differences created
in nature? Is the difference in
temperature involved? The following
activities will help you to understand
this.
8.3 AIR EXPANDS ON HEATING
Activity 8.5
Take a boiling tube. Stretch a balloon
tightly over the neck of the tube. You
can use a tape to make it tight. Pour
some hot water in a beaker.  Insert the
boiling tube with the balloon in the hot
water. Observe for 2–3 minutes for any
change in shape of the balloon. Take the
tube out, let it cool down to the room
temperature. Take some ice-cold water
in another beaker and place the tube
with the balloon in cold water for 2–3
minutes. Observe the change in the
shape of the balloon.
Fig. 8.6  The shape of the balloon in hot and cold water
Think and try to answer:
What makes the balloon
inflated when the boiling tube
is placed in hot water?
Why is the same balloon
deflated when the tube is kept
in cold water?
Can we infer from the first
observation that air expands
on heating? Can you now state
what happens to the air in the
boiling tube when it cools
down?
Boiling tube
immersed in
hot water
Boiling tube
immersed in
ice-cold water
Balloon tied over
the neck of the
boiling tube
2022-23
SCIENCE 84
The next activity is very interesting.
This will make you understand more
about hot air.
Activity 8.6
Take two paper bags or empty paper cups
of the same size. Hang the two bags in
candle below one of the bags as shown
in the figure. Observe what happens.
Why is the balance of the bags
disturbed?
Does this activity indicate that warm
air rises up? As the warm air rises up,
it pushes the bag above the candle. Does
the disturbance of the balance suggest
that the warm air is lighter than the cold
air?
Can you now explain why smoke
always rises up?
Also, it is important to remember that
on heating the air expands and occupies
more space. When  the same thing
occupies more space, it becomes lighter.
The warm air is, therefore, lighter than
the cold air. That is the reason that the
smoke goes up.
In nature there are several situations,
where warm air rises at a place. The air
pressure at that place is lowered. The
cold air from the surrounding areas
rushes in to fill its place. This sets up
convection in air, as you learnt in
Chapter 4.
8.4 WIND CURRENTS ARE GENERATED
DUE TO UNEVEN HEATING ON
THE EARTH
These situations are:
(a) Uneven heating between
the equator and the poles
You might have learnt in Geography that
regions close to the equator get
maximum heat from the Sun. The air
in these regions gets warm.  The warm
air rises, and the cooler air from the
Fig. 8.7   Hot air rising up
CAUTION
Handle the burning candle carefully.
the inverted position on the two ends of
a metal or wooden stick.
Tie a piece of thread in the middle of
the stick. Hold the stick by the thread
(Fig. 8.7) as in a balance.  Put a burning
2022-23