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10
Wind, Storm and Cyclones
Learning Objectives KNOW THESE POINTS BEFORE YOU START
The envelope of air surrounding our earth is called atmosphere.The weight of air acting on a surface causes air pressure.
High speed winds are
accompanied by reduced air pressure.Winds always move from a
region of high pressure to a
region of low pressure.The direction of wind depends on the temperature and air pressure. The greater
the difference in air pressure, the greater is the speed of the wind.
The students wil be able to
know that air exerts
pressure.
understand how winds
are caused.
learn that air expands on
heating.
understandthunderstormand cyclone. Warm air is lighter than cold air and, therefore, it rises up. The cool, dense airrushes to takes the place of warm air.
know how thunderstorm
A thunderstorm is a storm accompanied by lightning and thunder. The formationof a thunderstorm needs moisture, rapidly rising warm air and fronts like seabreeze or mountains that help in lifting the warm air upwards.
and cyclone are formed.
knowthe precautions
that can be taken duringa thunderstorm, cyclone
Cyclones are violent storms with an intense spiral. These are accompanied bystrong winds and heavy rains.
and tornado.
Now, study the chapter in detail.
AIR AND WIND
Air is all around us. The envelope of air surrounding our earth
called atmosphere. We cannot see, taste or smell air. Air movesfrom one place to another. We call moving air as wind. Even airhas weight. But can you feel the weight of air? The weight oair presses our bodies all the time. This weight of air acting On a
surface causes air
pressure.
Air exerts pressure
Let us understand more about air pressure
with the help of some interesting activities.
You will have fun doing these activities.
The air pressure above the newspaper
will make it difficult to move the
ruler up.
Yes. You are right. The pressure of
air is more above the newspaper. It
will make it difficult to lift the ruler
up. It's the air pressure that presses
the newspaper down on the ruler.
Activity
Take water up to the brim
of a glass. Place a piece of
cardboard over the top of
the glass.
Now, holding the cardboard
in place over a sink, turn
the glass upside down.
Remove your hand gently
away from the cardboard.
What do you notice? Does the water stay in the
glass? What holds the piece of cardboard against
the glass?
It is the air pressure. The air pushes against the
cardboard from below. This keeps the water in the
glass.
Activity
(To be demonstrated by the teacher.)
Boil an egg and deshell it. Take a clean and dry glass
bottle whose neck is slightly smaller than that of the
egg. Very carefully, place a burning crumbied piece
of paper inside the bottle with the help of tongs.
Push the egg into the neck of the bottle.
Does the bottle swallow the whole egg?
The burning paper uses up the oxygen present inside
the bottle. This reduces the air pressure inside the
bottle. The air pressure outside the bottle is more
than that inside the bottle which pushes the egg
inside the bottle. When the paper stops burning,
the pressure inside the bottle does not fall any
further. Does the egg move now? No, the egg then
stops moving.
Fig. 10.1 A windy scene
Activity
Place a ruler on a table
so that one of its ends
hangs about 8 cm out
of the edge. Spread a
newspaper sheet over
the ruler as shown in the All these activities confirm that air exerts
figure. Guess, what would
happen if you hit the part
of the ruler that is hanging
out with your hand?
pressure. Even, some of your own daily
experiences can confirm that air exerts
pressure. For example,
While boating, you find it easier to row
the boat when the wind is blowing from
behind you.
It is easier to fy a kite when the wind is
blowing from behind you.
Activity
Take two ping-pong balls and hang them about
10 cm away from each other with the help of a
thread (as shown in the figure). Blow in between
the balls and try to separate them. Try hard! What
do you observe?
ActivityShare those experiences with your friends whic
show that air exerts pressure and list them below.
S. No. Experiences that show air exerts pressure
Are you able to move the balls away from each
other by blowing in between them?
High speed winds are accompanied by
reduced air pressure
We can feel the air when it is moving. This
can be understood by the fact that when we
Activity
Take a thick piece of paper and arrange it in the
shape of a bridge. Blow air below the bridge. Does
the bridge get blown away?
switch the fan o, we feel the movement ot
air inside the room. We can see the curtains
swaying, paper kept on a table may fly away,
the pages of an open book may flip, and so on.
Let us now study the effect of moving air
with the help of some activities.
Activity
Place a book on a table as shown in the picture. Fix
a paper strip above the book as shown. Blow over
the top of the paper. What do you observe?
Let us now understand what is happening in
these activities.
S In activity 5, when you blew over the top of
the paper, the air moves faster. When the
96
in the upward direction. This lifts the paper are these differences in air pressure created
ping-pong balls, the air
pressure between Let us understand this with the help of the
fast, the pressure becomes less.
air moves
The greater is the difference in air pressure,Butthe airRut the air below the paper exerts
pressure the greater is the speed of wind. But how
upwards.
Tn activity 6, when you blew between the
in nature? This is because air expands on
heating
hem gets reduced. The air pressure outside following activities.
he balls is higher. This pushes the ballsthe balls
towards each other.
In activity 7, when you blew below the
paper bridge, the air pressure below the paper
Air expands on heating
paperbric
Activityridge gets reduced. The harder you blow,
the harder the table holds the bridge. Again,
the air pressure above the bridge is more
compared to the air pressure below the
Take a small bottle. Tie a balloon tightly on the neck
of the bottle. Place this bottle in a container having
hot water for some time. What do you observe?
Does the balloon blow up? Why?
Now, remove the bottle from the container. Alliow it
bridge. to cool down to room temperature. Now, place this
bottle in a container having ice-cold water for some
time. What do you observe now? Does the balloon
What do we conclude from these activities?
Moving air lowers the pressure of the area
it occupies. get deflated? Why?
How winds are produced
Moving air is called wind. Winds are caused
due to change in temperature and pressure
in the air. Winds always move from a region
Empty bottles
Inflated
balloon
Deflated
balloon
of high pressure to a region of low pressure.
This can be understood by a very simple
Hot Cold
water- water
activity.
Activity
Blow a balloon and hold its neck tightly. The air
present inside the balloon is at a higher pressure
In the frst case, the balloon blows up as
the air in the bottle gets heated by the
hot water and expands the particles of air
inside the bottle. The particles on becoming
warm, move away trom each other and take
up more space. The air becomes less dense
he air rushes out of the balloon like a gust and also lighter. As a result, the warm air
as compared to the air outside the balloon.
Now, release the neck of the balloon.
What do you find?
OWind? This is exactly how winds occur rises.
Our atmosphere. The direction of wind On cooling, the particles of air contract and
pends on the temperature and the air
take up less space.
pressure.
Activity Wind is caused due to uneven heatiof the earthTake a
long, thin wooden stick. Mark its centre. Tie a
piece of thread in the middle of the stick. Hang two
identical paper bags at each end of the stick. Withthe help of the thread, hold the stick and balance the
bags. Now, place a candle below one of the paperbags (taking care that the bag does not catch fire).What do you notice? Do you find any disturbancein the balance of the bags? Why does this happen?
Different parts of the earth receive differeamounts of heat. The equator receives dit
rays from the sun. Air gets heated and risesleaving low pressure areas behind. The cooair between 30 latitude and the equator(either side) moves in to take the placewarm air. These winds blow from either sid-
north or south towards the
equator. Thezat the poles is much cooler than that at 6latitudes. Here also, the warm air from 6latitudes rises up and cooler air from the pola
regions moves in. This sets up circulation oiwind from the
polar regions to the
latitudes
90
Polar front, warmer air
60 /, / rises, loW pressure
Air descends,30
high pressure
When the air becomes warm it becomes
//77//lighter and rises up. Then, the bag above thecandle is pushed up. This disturbs the balanceof the paper bag. This shows that warm air is
lighter than the cold air.
You are, by now, familiar with land breeze andsea breeze. The warm air expands, becomes
light and rises up. This reduces the air
pressure at this place. The cooler air is denserand it rushes in to take the place vacated by Wind is caused due to uneven heating
warm air. This is how winds are
caused, and of land and waterthe movement of air is due to the setting of
During summer, the land near the equato
Warm air rises,0°
low pressure
Air descends,
high pressure
30°
60
Polar front, warmerairrises, low pressure
90
Fig. 10.2 The Earth showing different winds
of land and water
During summer, the land near the equatorbecomes warm faster than the water in the
convection current.
Check PelNT 1.
Fill in the blanks. oceans. The air above the land becomes verhot and rises up. Cooler and denser wind'1. The envelope of air
surrounding the earth is
from the oceans flow towards the land. Thestcalled
2. Moving air lowersthe
it occupies.
Winds carry a lot of moisture and are calk
monsoon winds.
of the area
on heating
Twonder what happens during winteDoes the direction of the wind gereversed?
3. Air
4. The movement of air takes place due to
You have read
ve read about water cycle. The water falling droplets of water creates lightning and
he oceans, seas, lakes, etc. evaporate
to th sun's heat and form water vapour.
us
r a i Excessive rains can cause floods and
that what thunderstorms and cyclones are,
t r o m
sound.
d u e
sis water vapour condenses to form small
d r o p l e t s
e s which form clouds. Clouds bring
Are all thunderstorms dangerous?
create many problems.
e shall now discuss in the following sections
Yes! All thunderstorms are
dangerous. Lightning occurs with all
thunderstorms. This can kill many
people. It can cause damage to
forests, tall structures, etc.
Weshall n o
and how they are caused.
WHAT 1SA THUNDERSTORM
ActivityA thunderstorm i1S a storm accompanied
vith lightning and thunder. Formation of a
To make a thunderstorm in a container
Take a plastic container about the size of a
shoebox. Fill it two-thirds with lukewarm water. Allow
thunderstorm needs:
Moisture: This is required to form clouds
and eventually rain.
Front ike sea breeze or mountains that
help in lifting the warm air upwards.
Rapidly risingwarm air: Due to the sun's
heat, the temperature of a place rises. The
rise in temperature warms the air and
creates strong winds which rise in the
upward direction.
the water to stand for a while. At one end of the
container, place a red-coloured ice cube (made by
mixing red food colour in water). At the other end of
the container, put 2-3 drops of blue food colouring.
Observe for some time. What happens? The
red-coloured cold water sinks while the blue-coloured
warm water rises. This happens due to convection.
The red colour here represents the cold air and
the blue colour represents warm air. During a
thunderstorm, warm air is forced to rise up by cold
air. Convection plays a major role in the formation
of a thunderstorm.
Water droplets are also carried upwards What precautions must be taken
movemen of rapidly rising warm air and the
Fig. 10.3 Thunderstorm
ong with these winds. These water dropletstnen freeze and fall back on the earth. This
during a thunderstorm
We must take the following precautionsduring a thunderstorm:
99
If you are outside:
Avoid taking shelterunder tall trees or tall
towers.
Try to get into a car or a bus, or take shelter
inside a building.
Stay away from water bodies. Ifyou are in
water, move out to a safe shelter.
It you are inside:
Close the windows and doors tightly.
Do not touch electrical equipments or
telephones.
Fig. 10.4 Cyclone
The centre of the cyclone is a cloudless, calm
area. This is called the eye. In this area, the
Keep listening to a battery operated radio
winds are not strong and there is no rain.Bufor the latest information. the winds aroundthe eye can have speeds of
upto 200 km/hr.
WHAT ARE CYCLONES
Cyclones are violent storms with an intense Are cyclones dangerous
There are many dangers associated with
cyclones. Some of them are as follows:
Cyclones are accompanied by very strong
winds which can uproot trees, collapse
spiral. Cyclones are accompanied by strong
winds and heavy rains.
How are cyclones formed?
houses and buildings and toppleelectric
poles. Many people may be killed.
Cyclones are formed by strong winds blowing Fierce winds associated with cyclones can
around a central area having low atmospheric also cause storm surges. It is a condition
which causes an abnormal rise in the level
of sea water in the coastal areas. The sea
pressure.
The air above the seas and oceans gets heated
and becomes warm. The warm air rises
upwards and creates areas of low pressure.
The rising warm air gets cooler and loaded
water may enter low-lying areas causing
loss of property, lives of humans and
livestock and destruction of vegetation
This may also reduce soil fertility.with moisture. This condenses to form
massive thunder clouds. Large amount of
heatCyclones may also cause prolongedheais also released. Cooler air rushes to fill the
space created by rising warm air. This cycle
keeps repeating. This leads to the formation
of a very low pressure system with winds of
great force revolving around it. These strong
winds rotate faster and faster forming a very Even after a cyclone is over, rail and road
huge circle. This atmospheric condition is
called a cyclone.
rains. This may lead to floods causing
loss of life and property. Water becomes
contaminated and people may suffer from
waterborne diseases like gastroenteritis
typhoid, etc.
transport may remain blocked by tloou
waters.
00

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Wind, Storm and Cyclone

  • 1. 10 Wind, Storm and Cyclones Learning Objectives KNOW THESE POINTS BEFORE YOU START The envelope of air surrounding our earth is called atmosphere.The weight of air acting on a surface causes air pressure. High speed winds are accompanied by reduced air pressure.Winds always move from a region of high pressure to a region of low pressure.The direction of wind depends on the temperature and air pressure. The greater the difference in air pressure, the greater is the speed of the wind. The students wil be able to know that air exerts pressure. understand how winds are caused. learn that air expands on heating. understandthunderstormand cyclone. Warm air is lighter than cold air and, therefore, it rises up. The cool, dense airrushes to takes the place of warm air. know how thunderstorm A thunderstorm is a storm accompanied by lightning and thunder. The formationof a thunderstorm needs moisture, rapidly rising warm air and fronts like seabreeze or mountains that help in lifting the warm air upwards. and cyclone are formed. knowthe precautions that can be taken duringa thunderstorm, cyclone Cyclones are violent storms with an intense spiral. These are accompanied bystrong winds and heavy rains. and tornado. Now, study the chapter in detail. AIR AND WIND Air is all around us. The envelope of air surrounding our earth called atmosphere. We cannot see, taste or smell air. Air movesfrom one place to another. We call moving air as wind. Even airhas weight. But can you feel the weight of air? The weight oair presses our bodies all the time. This weight of air acting On a surface causes air pressure.
  • 2. Air exerts pressure Let us understand more about air pressure with the help of some interesting activities. You will have fun doing these activities. The air pressure above the newspaper will make it difficult to move the ruler up. Yes. You are right. The pressure of air is more above the newspaper. It will make it difficult to lift the ruler up. It's the air pressure that presses the newspaper down on the ruler. Activity Take water up to the brim of a glass. Place a piece of cardboard over the top of the glass. Now, holding the cardboard in place over a sink, turn the glass upside down. Remove your hand gently away from the cardboard. What do you notice? Does the water stay in the glass? What holds the piece of cardboard against the glass? It is the air pressure. The air pushes against the cardboard from below. This keeps the water in the glass. Activity (To be demonstrated by the teacher.) Boil an egg and deshell it. Take a clean and dry glass bottle whose neck is slightly smaller than that of the egg. Very carefully, place a burning crumbied piece of paper inside the bottle with the help of tongs. Push the egg into the neck of the bottle. Does the bottle swallow the whole egg? The burning paper uses up the oxygen present inside the bottle. This reduces the air pressure inside the bottle. The air pressure outside the bottle is more than that inside the bottle which pushes the egg inside the bottle. When the paper stops burning, the pressure inside the bottle does not fall any further. Does the egg move now? No, the egg then stops moving. Fig. 10.1 A windy scene Activity Place a ruler on a table so that one of its ends hangs about 8 cm out of the edge. Spread a newspaper sheet over the ruler as shown in the All these activities confirm that air exerts figure. Guess, what would happen if you hit the part of the ruler that is hanging out with your hand? pressure. Even, some of your own daily experiences can confirm that air exerts pressure. For example,
  • 3. While boating, you find it easier to row the boat when the wind is blowing from behind you. It is easier to fy a kite when the wind is blowing from behind you. Activity Take two ping-pong balls and hang them about 10 cm away from each other with the help of a thread (as shown in the figure). Blow in between the balls and try to separate them. Try hard! What do you observe? ActivityShare those experiences with your friends whic show that air exerts pressure and list them below. S. No. Experiences that show air exerts pressure Are you able to move the balls away from each other by blowing in between them? High speed winds are accompanied by reduced air pressure We can feel the air when it is moving. This can be understood by the fact that when we Activity Take a thick piece of paper and arrange it in the shape of a bridge. Blow air below the bridge. Does the bridge get blown away? switch the fan o, we feel the movement ot air inside the room. We can see the curtains swaying, paper kept on a table may fly away, the pages of an open book may flip, and so on. Let us now study the effect of moving air with the help of some activities. Activity Place a book on a table as shown in the picture. Fix a paper strip above the book as shown. Blow over the top of the paper. What do you observe? Let us now understand what is happening in these activities. S In activity 5, when you blew over the top of the paper, the air moves faster. When the 96
  • 4. in the upward direction. This lifts the paper are these differences in air pressure created ping-pong balls, the air pressure between Let us understand this with the help of the fast, the pressure becomes less. air moves The greater is the difference in air pressure,Butthe airRut the air below the paper exerts pressure the greater is the speed of wind. But how upwards. Tn activity 6, when you blew between the in nature? This is because air expands on heating hem gets reduced. The air pressure outside following activities. he balls is higher. This pushes the ballsthe balls towards each other. In activity 7, when you blew below the paper bridge, the air pressure below the paper Air expands on heating paperbric Activityridge gets reduced. The harder you blow, the harder the table holds the bridge. Again, the air pressure above the bridge is more compared to the air pressure below the Take a small bottle. Tie a balloon tightly on the neck of the bottle. Place this bottle in a container having hot water for some time. What do you observe? Does the balloon blow up? Why? Now, remove the bottle from the container. Alliow it bridge. to cool down to room temperature. Now, place this bottle in a container having ice-cold water for some time. What do you observe now? Does the balloon What do we conclude from these activities? Moving air lowers the pressure of the area it occupies. get deflated? Why? How winds are produced Moving air is called wind. Winds are caused due to change in temperature and pressure in the air. Winds always move from a region Empty bottles Inflated balloon Deflated balloon of high pressure to a region of low pressure. This can be understood by a very simple Hot Cold water- water activity. Activity Blow a balloon and hold its neck tightly. The air present inside the balloon is at a higher pressure In the frst case, the balloon blows up as the air in the bottle gets heated by the hot water and expands the particles of air inside the bottle. The particles on becoming warm, move away trom each other and take up more space. The air becomes less dense he air rushes out of the balloon like a gust and also lighter. As a result, the warm air as compared to the air outside the balloon. Now, release the neck of the balloon. What do you find? OWind? This is exactly how winds occur rises. Our atmosphere. The direction of wind On cooling, the particles of air contract and pends on the temperature and the air take up less space. pressure.
  • 5. Activity Wind is caused due to uneven heatiof the earthTake a long, thin wooden stick. Mark its centre. Tie a piece of thread in the middle of the stick. Hang two identical paper bags at each end of the stick. Withthe help of the thread, hold the stick and balance the bags. Now, place a candle below one of the paperbags (taking care that the bag does not catch fire).What do you notice? Do you find any disturbancein the balance of the bags? Why does this happen? Different parts of the earth receive differeamounts of heat. The equator receives dit rays from the sun. Air gets heated and risesleaving low pressure areas behind. The cooair between 30 latitude and the equator(either side) moves in to take the placewarm air. These winds blow from either sid- north or south towards the equator. Thezat the poles is much cooler than that at 6latitudes. Here also, the warm air from 6latitudes rises up and cooler air from the pola regions moves in. This sets up circulation oiwind from the polar regions to the latitudes 90 Polar front, warmer air 60 /, / rises, loW pressure Air descends,30 high pressure When the air becomes warm it becomes //77//lighter and rises up. Then, the bag above thecandle is pushed up. This disturbs the balanceof the paper bag. This shows that warm air is lighter than the cold air. You are, by now, familiar with land breeze andsea breeze. The warm air expands, becomes light and rises up. This reduces the air pressure at this place. The cooler air is denserand it rushes in to take the place vacated by Wind is caused due to uneven heating warm air. This is how winds are caused, and of land and waterthe movement of air is due to the setting of During summer, the land near the equato Warm air rises,0° low pressure Air descends, high pressure 30° 60 Polar front, warmerairrises, low pressure 90 Fig. 10.2 The Earth showing different winds of land and water During summer, the land near the equatorbecomes warm faster than the water in the convection current. Check PelNT 1. Fill in the blanks. oceans. The air above the land becomes verhot and rises up. Cooler and denser wind'1. The envelope of air surrounding the earth is from the oceans flow towards the land. Thestcalled 2. Moving air lowersthe it occupies. Winds carry a lot of moisture and are calk monsoon winds. of the area on heating Twonder what happens during winteDoes the direction of the wind gereversed? 3. Air 4. The movement of air takes place due to
  • 6. You have read ve read about water cycle. The water falling droplets of water creates lightning and he oceans, seas, lakes, etc. evaporate to th sun's heat and form water vapour. us r a i Excessive rains can cause floods and that what thunderstorms and cyclones are, t r o m sound. d u e sis water vapour condenses to form small d r o p l e t s e s which form clouds. Clouds bring Are all thunderstorms dangerous? create many problems. e shall now discuss in the following sections Yes! All thunderstorms are dangerous. Lightning occurs with all thunderstorms. This can kill many people. It can cause damage to forests, tall structures, etc. Weshall n o and how they are caused. WHAT 1SA THUNDERSTORM ActivityA thunderstorm i1S a storm accompanied vith lightning and thunder. Formation of a To make a thunderstorm in a container Take a plastic container about the size of a shoebox. Fill it two-thirds with lukewarm water. Allow thunderstorm needs: Moisture: This is required to form clouds and eventually rain. Front ike sea breeze or mountains that help in lifting the warm air upwards. Rapidly risingwarm air: Due to the sun's heat, the temperature of a place rises. The rise in temperature warms the air and creates strong winds which rise in the upward direction. the water to stand for a while. At one end of the container, place a red-coloured ice cube (made by mixing red food colour in water). At the other end of the container, put 2-3 drops of blue food colouring. Observe for some time. What happens? The red-coloured cold water sinks while the blue-coloured warm water rises. This happens due to convection. The red colour here represents the cold air and the blue colour represents warm air. During a thunderstorm, warm air is forced to rise up by cold air. Convection plays a major role in the formation of a thunderstorm. Water droplets are also carried upwards What precautions must be taken movemen of rapidly rising warm air and the Fig. 10.3 Thunderstorm ong with these winds. These water dropletstnen freeze and fall back on the earth. This during a thunderstorm We must take the following precautionsduring a thunderstorm: 99
  • 7. If you are outside: Avoid taking shelterunder tall trees or tall towers. Try to get into a car or a bus, or take shelter inside a building. Stay away from water bodies. Ifyou are in water, move out to a safe shelter. It you are inside: Close the windows and doors tightly. Do not touch electrical equipments or telephones. Fig. 10.4 Cyclone The centre of the cyclone is a cloudless, calm area. This is called the eye. In this area, the Keep listening to a battery operated radio winds are not strong and there is no rain.Bufor the latest information. the winds aroundthe eye can have speeds of upto 200 km/hr. WHAT ARE CYCLONES Cyclones are violent storms with an intense Are cyclones dangerous There are many dangers associated with cyclones. Some of them are as follows: Cyclones are accompanied by very strong winds which can uproot trees, collapse spiral. Cyclones are accompanied by strong winds and heavy rains. How are cyclones formed? houses and buildings and toppleelectric poles. Many people may be killed. Cyclones are formed by strong winds blowing Fierce winds associated with cyclones can around a central area having low atmospheric also cause storm surges. It is a condition which causes an abnormal rise in the level of sea water in the coastal areas. The sea pressure. The air above the seas and oceans gets heated and becomes warm. The warm air rises upwards and creates areas of low pressure. The rising warm air gets cooler and loaded water may enter low-lying areas causing loss of property, lives of humans and livestock and destruction of vegetation This may also reduce soil fertility.with moisture. This condenses to form massive thunder clouds. Large amount of heatCyclones may also cause prolongedheais also released. Cooler air rushes to fill the space created by rising warm air. This cycle keeps repeating. This leads to the formation of a very low pressure system with winds of great force revolving around it. These strong winds rotate faster and faster forming a very Even after a cyclone is over, rail and road huge circle. This atmospheric condition is called a cyclone. rains. This may lead to floods causing loss of life and property. Water becomes contaminated and people may suffer from waterborne diseases like gastroenteritis typhoid, etc. transport may remain blocked by tloou waters. 00