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1. Winds, Storms
and Cyclones
8

2. Development of the lesson
Introduction
• Air: The mixture of gases that makes up the Earth’s atmosphere.
• Wind: The moving air is called wind.
1. Since air is a mixture of gases and wind is the flow of gases on a large
scale.
2. Winds are caused by the heating difference between different parts
of the world and the movement of the earth.
• Winds can be of classified on the basis of:
1. Duration: Long and Short
2. Strength: High, medium and low
3. Speed: Slow and Fast

3. • High strength winds for a longer duration, like cyclone or hurricanes,
may cause hazardous effects.
• Applications of wind include Windmills (to generate electricity), fan,
flying kite, Hair Dryer etc.

4. 1. Air Exerts Pressure
For Example:
• The air around us exerts pressure. This is seen in the following activity:
• A tin can is filled with water heated over a flame. As soon as the water comes to a boil, the
lid is put and it is kept under running cold water. We find that the shape of the can gets
distorted.
(This is the activity)

5. 2. High-Speed Winds
• High-speed winds are accompanied by reduced air pressure.
Air moves from the region of higher pressure to the lower pressure. The greater the
difference in pressure, the faster the air moves. It is due to the heat of the sun from
which air becomes warm producing a low pressure. So, the warm air rises up and the
cool air from the surroundings moves towards the sea.
• The fast-moving air blowing over an object helps in lifting the object up by producing a
region of low pressure above it. This can be shown by the following activity.
• Air pressure is caused due to the constant bombardment by moving air molecules on the
surface of the paper strip. We have learnt that when high-speed air is blown over a strip
of paper, it is lifted up. Similarly, if the high wind blows over the roof of houses, it will
reduce the air pressure above the roofs and if the roofs of houses are weak then higher
air pressure from below will lift up the roofs which can then be blown away by the fast
winds. Therefore, weak roofs of houses can be lifted and blown away by the high-speed
wind that is called a storm.

6. This is seen in the following activities:
• 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, is it difficult to force the paper ball into the bottle?
• 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. When you blew between the balloons, they moved
towards each other.

7. 3. Air expands on heating
• Gases or air expand more than solid and liquid on heating. Air is a mixture of gases. On heating,
the molecules of air gain kinetic energy. It moves away from each other, thus occupying more
space. Therefore, the air becomes lighter. Warm air rises up whereas comparatively cool air tends
to sink towards the earth’s surface.
• The term ‘expansion of air’ means the increase in the volume of air. When the air is heated, its
volume increases and occupies a bigger space.
• When the air is heated, it becomes lighter and rises up in the sky. This is the reason that when we
burn woods, the smoke rises along with the hot air near the place of fire. This can be
demonstrated by the following activities.
• This fact of air is utilized in launching hot air balloons. 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.
Hot air rising up

8. 4.Wind currents are generated due to uneven heating on the earth
(a) Uneven heating between the equator and the poles
• The 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 regions in the 0–30 degrees latitude belt on either side of the
equator moves in. These winds blow from the north and the south
towards the equator.
• At the poles, the air is colder than that at latitudes about 60 degrees.
The warm air at these latitudes rises up and the cold wind from the
polar regions rushes in, to take its place. In this way, wind circulation is
set up from the poles to the warmer latitudes.

9. * Uneven Heating of the Earth

10. (b) Uneven heating of land and water
• In summer, near the equator the land warms up faster and most of the
time the temperature of the land is higher than that of water in the
oceans. The air over the land gets heated and rises. This causes the winds
to flow from the oceans towards the land. These are monsoon winds. The
word monsoon is derived from the Arabic word ‘Mausam’, which means
‘season’.
• In winter, the direction of the wind flow gets reversed; it flows from the
land to the ocean.
• The winds from the oceans carry water and bring rain. It is a part of the
water cycle.

11. Uneven heating of land
especially the Rajasthan
desert generates
monsoon winds from
southwest direction in
summer. These winds
carry lots of water from
the Indian Ocean……

12. Uneven heating of land
and water in winter
generate winds from the
northwest colder land.
These colder winds carry
little water, hence bring
small amount of rain in
winter……

13. * The monsoon winds carry water and it rains.
• Clouds bring rain and give us happiness. Farmers in our country depend
mainly on rains for their harvests.
• However, it is not always a happy ending. Rains often create problems.
Heavy rainfall can lead to numerous hazards,
• For example: flooding, including risk to human life, damage to buildings
and infrastructure, and loss of crops and livestock. landslides, which can
threaten human life, disrupt transport and communication ,etc.
• In nature itself there are certain situations that can sometimes create
disasters and pose threat to humans, animals and plant life.

14. 5. Thunderstorms
• Thunderstorms develop in hot, humid tropical areas like India very frequently.
• The rising temperatures produce strong upward rising winds. These winds
carry water droplets upwards, where they freeze, and fall down again. The
swift movement of the falling water droplets along with the rising air create
lightning and sound. This event we call it as thunderstorm.

15. * Precautions to be Taken During a Thunderstorm
* When you are in open area :
• Do not take shelter under an isolated tree or building. If you are in a forest, take
shelter under a small tree. Do not lie on the ground.
• Do not take shelter under an umbrella having a metallic end.
• Never sit near a window, open garages, storage sheds, metal sheds, etc. to take
shelter.
• One can take shelter in a car or a bus.
• Do not go in the water. If you are in the water, get out and go inside a building.
* When you are inside :
• Do not sit near an open window. Close the doors and windows properly.
• Do not touch/operate any electrical devices or telephone.
• Do not watch TV

16. * Thunderstorm into a Cyclone
• Before cloud formation, water takes up heat from the atmosphere to change into
vapour. When water vapour changes back to liquid form as raindrops, this heat is
released to the atmosphere. The heat released to the atmosphere warms the air
around. The air tends to rise and causes a drop in pressure. More air rushes to
the Centre of the storm. This cycle is repeated. The chain of events ends with the
formation of a very low-pressure system with very high-speed winds revolving
around it. It is this weather condition that we call a cyclone. Factors like wind
speed, wind direction, temperature and humidity contribute to the development
of cyclones.

17. Formation of a cyclone

18. * Structure of a cyclone
• The Centre of a cyclone is a calm area. It is called the eye of the storm. A large
cyclone is a violently rotating mass of air in the atmosphere, 10 to 15 km high.
The diameter of the eye varies from 10 to 30 km. It is a region free of clouds and
has light winds. Around this calm and clear eye (Fig. 8.12), there is a cloud region
of about 150 km in size. In this region there are high-speed winds (150–250
km/h) and thick clouds with heavy rain. Away from this region the wind speed
gradually decreases. The formation of a cyclone is a very complex process.

19. 6. Destruction caused by Cyclones
• Cyclones can be very destructive. Strong winds push water towards the shore
even if the storm is hundreds of kilometers away.
• These are the first indications of an approaching cyclone. The water waves
produced by the wind are so powerful that a person cannot overcome them.
• The low pressure in the eye lifts water surface in the centre. The rising water may
be as high as 3–12 metres.
• It appears like a water-wall moving towards the shore. As a result, the seawater
enters the low-lying coastal areas, causing severe loss of life and property. It also
reduces the fertility of the soil.
• Continuous heavy rainfall may further worsen the flood situation.
• High-speed winds accompanying a cyclone can damage houses, telephones and
other communication systems, trees, etc., causing tremendous loss of life and
property.

20. House got destructed by Cyclone..

21. * Different names of cyclones in different parts of the world.
• ‘Hurricane’ in the American continent.
• ‘Typhoon’ in Philippines and Japan..
Figure showing Different names of cyclones in different parts
of the world.

22. * Tornadoes:
• In our country they are not very frequent. A tornado is a dark funnel shaped
cloud that reaches from the sky to the ground. Most of the tornadoes are weak. A
violent tornado can travel at speeds of about 300 km/h. Tornadoes may form
within cyclones. The whole coastline of India is vulnerable to cyclones,
particularly the east coast. The west coast of India is less vulnerable to cyclonic
storms both in terms of intensity and frequency of the cyclones.
Image of a tornado

23. * Protecting from a tornado
• The diameter of a tornado can be as small as a metre and as large as a km, or
even wider. The funnel of a tornado sucks dust, debris and everything near it at
the base (due to low pressure) and throws them out near the top.
• A tornado shelter is a room situated deep inside or underground having no
windows. Or otherwise it is better to shut windows and take shelter under a
table, workbench, where debris cannot reach. One has to bow down on knees
protecting head and neck using arms.
Protecting from a tornado

24. • A cyclone forecast and warning service.
• Rapid communication of warnings to the Government
agencies, the ports, fishermen, ships and to the general
public.
• Construction of cyclone shelters in the cyclone prone
areas, and Administrative arrangements for moving
people fast to safer places.
7. Effective Safety measures

25. * Anemometer
• All storms are low-pressure systems. Wind speed plays an important
role in the formation of storms. It is, therefore, important to measure
the wind speed. The instrument that measures the wind speed is
called an Anemometer.

26. * Action on the part of the people
• We should not ignore the warnings issued by the meteorological
department through TV, radio, or newspapers.
• We should make necessary arrangements to shift the essential
household goods, domestic animals and vehicles, etc. to safer places;
avoid driving on roads through standing water, as floods may have
damaged the roads; and keep ready the phone numbers of all
emergency services like police, fire brigade, and medical centres.

27. * Precautions while staying in a cyclone hit area
• Do not drink water that could be contaminated. Always store drinking
water for emergencies.
• Do not touch wet switches and fallen power lines.
• Do not go out just for the sake of fun.
• Do not pressurise the rescue force by making undue demands.
• Cooperate and help your neighbours and friends.

28. 8. Advanced technology has helped
• These days we are better protected. In the early part of the last century, coastal
residents may have had less than a day to prepare or evacuate their homes from
an oncoming cyclone.
• The world today is very different. Thanks to satellites and radars, a Cyclone alert
or Cyclone watch is issued 48 hours in advance of any expected storm and a
Cyclone warning is issued 24 hours in advance.
• The message is broadcast every hour or half hour when a cyclone is nearer the
coast. Several national and international organizations cooperate to monitor the
cyclone-related disasters.
Satellites

29. * How hot is Lightening

30. Keywords
Anemometer - An Instrument that measures wind speed.
Cyclones - a weather condition consisting of a system of high-speed winds revolving around a central area of very
low pressure..
Hurricane - a tropical storm with winds that have reached a constant speed of 74 miles per hour or more.
Lightning - an electrical discharge caused by imbalances between storm clouds and the ground, or within the
clouds themselves.
Monsoon winds - The cool winds that blow from the surface of the sea towards the land which carries the rains
Pressure - the physical force exerted on an object.
Thunderstorms - a storm with sound and lightning and typically also heavy rain or hail.
Tornado - a violent windstorm circling around the centre of a low pressure area.
Typhoon - a tropical cyclone with winds faster than 74 miles per hour that occurs in the North Atlantic Ocean, the
Northeast Pacific Ocean east of the dateline, the South Pacific east of 160E and the Northwest Pacific Ocean west
of the date line.
Wind flow pattern - direction in which the wind is blowing.

31. What we have learnt….
• Air around us exerts pressure.
• Air expands on heating and contracts on cooling.
• Warm air rises up, whereas comparatively cooler air tends to sink towards the earth’s surface.
• As warm air rises, air pressure at that place is reduced and the cooler air moves to that place.
• The moving air is called wind.
• Uneven heating on the earth is the main cause of wind movements.
• Winds carrying water vapour bring rain.
• High-speed winds and air pressure difference can cause cyclones.
• It has become easier to monitor cyclones with the help of advance technology like satellites and
radars.
• Self-help is the best help. Therefore it is better to plan in advance and be ready with defence against
any approaching cyclone.

32. * The following flow chart will help you to understand the phenomena that
lead to the formation of clouds and falling of rain and creation of storms and
cyclones:

33. Exercises
1. Fill the missing word in the blank spaces in the following statements:
(a) Wind is moving air.
(b) Winds are generated due to uneven heating on the earth.
(c) Near the earth’s surface warm air rises up whereas cooler air comes down.
(d) Air moves from a region of high pressure to a region of low pressure.
2. Suggest two methods to find out wind direction at a given place.
Suggestion: (i) Take a piece of a paper in your hand. Allow it to fall from your
hand. It will flow in the direction in which wind is blowing.
(ii) You can also use a wind-pane which helps us to know accurate
wind direction.

34. 3. State two experiences that made you think that air exerts pressure (other
than those given in the text).
Statement: (i) Balloons and balls can be used only when they are inflated
with air. When balloon is overfilled with air it bursts due to
excessive air pressure.
(ii) Compressed air is used in the brake system for stopping
trains.
4. You want to buy a house. Would you like to buy a house having windows but
no ventilators? Explain your answer.
Explanation: No, a house which has no ventilators is not a safe or healthy
house to live in. The air circulation is not there in such a house.
So, it has no fresh air. Because warm air rises up and goes out
through ventilators and fresh air comes in through windows.

35. 5. Explain why holes are made in hanging banners and hoardings.
Explanation: Air exerts pressure. Due to this pressure banners and hoarding
flutter and torn when wind is blowing fast. Holes are made in
banners and hoardings so that wind passes through the holes and
they do not become loose and fall down.
6. How will you help your neighbours in case cyclone approaches your
village/town?
Answer: (i) I will make them aware of cyclone forecast and warning service.
(ii) Rapid communication of warning to the government agencies and
all the important places.
(iii) Construction of cyclone shelters in the cyclone prone areas.
(iv) Helping them to shift essential goods, domestic animals etc. to
safer places.

36. 7. What planning is required in advance to deal with the situation created by a
cyclone?
Answer: To deal with cyclone, it is important to follow the following
points :
(i) carefully listening the warnings transmitted on T.V. and radio.
(ii) moving to the safer places.
(iii) keeping an emergency kit ready.
(iv) store food in waterproof bags. .
(v) not venturing into sea.
(vi) keeping all the emergency numbers.
8. Which one of the following place is unlikely to be affected by a cyclone. (i)
Chennai (ii) Mangaluru (Mangalore) (iii) Amritsar (iv) Puri
Answer: (iv) Amritsar

37. 9. Which of the statements given below is correct?
(i)In winter the winds flow from the land to the ocean.
(ii)In summer the winds flow from the land towards the ocean.
(iii)A cyclone is formed by a very high-pressure system with very high-speed
winds revolving around it.
(iv)The coastline of India is not vulnerable to cyclones.
Answer: (i) In winter the winds flow from the land to the ocean.
*Did You Know ?
A bolt of lightning travels at a speed of more than 400,000 km/h. It can heat
the air around it to a temperature which is more than 4 times the
temperature of the surface of the sun. That is what makes lightning so
dangerous.