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.
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, i.e. wind
Wind currents are generated due to Uneven Heating on the Earth. These situations are :
(a)Uneven heating between the equator and the poles.
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.
The winds would have flown in the north-south direction from north to south, or from south to north.
A change in direction is however, caused by the rotation of the earth.
(b) Uneven heating of land and water.
regions 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.
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.
Activity 1:
Tin can crushed by atmospheric pressure
Take a tin can with a tight fitting stopper. First pour a little water into the tin can and heat it over a burner.
Keep on heating until steam starts forming so that it drives out most of the air from the tin can.
Remove the tin can from over the burner and fit the stopper at its mouth tightly.
What happens to the tin can after sometime? We find that after sometime the tin can gets badly crushed.
When the tin gets cooled, the steam inside the tin can condenses. Now, there is nothing inside the tin can to
balance the outside atmospheric pressure. Therefore, the atmospheric pressure acting from outside crushes the tin can.
Caution: Do this activity only under the supervision of your teacher.
Activity 2:
Pumping the air: we use air pump to fill the air into cycle tires and some times football. In pumping,air inside the pump
cylinder get pressed by the moving the piston down and subsequently forced inside the cycle tire tube or football through a valve.
If we just close the out let of pump with a finger, then it is very difficult to move the piston rod further down to other end as pressed
air has no where to go and exert pressure on the piston thus preventing its further down movement.
Activity 1:
Fold the plece of the paper in half. Then place it on the very edge of a table, as shown. 
Stick your face down near
the opening to paper tunnel. Start to blow in the centre of paper tunnel. Your will observe the paper is bending down towards the table.
What is happening here ? When you blow air though the paper tunnel. You are changing the air pressure inside the tunnel.
The air pressure inside and outside the tunnel previously the same. But when you blow, it becomes lower than outside air pressure.
The outside air pressure pushes down on the paper and the paper flattens. We see that, the increased wind speed is, indeed,
accompanied by a reduced air pressure.
Activity 2:
Blow the balloons: Take two balloons of approximately equal size. Put a little water into Ihe balloons.
Blow up both the balloons and tie each one 10 a string. Hang the balloons 8-10 cm apart on a cycle spoke or a stick.
Blow in the space between the balloons.
you will observe that When you blew between the balloons, they moved towards each other. This happens if the pressure of air
between the balloons were somehow reduced. The pressure outside the balloons would then push them towards each other.
Activity:
Take two paper bags or empty paper cups of the same size. Hang the two bags on the rod which is balanced as shown in the figure
Then glow a candle below one of the bags. As the warm air rises up, it pushes the bag above the candle
The disturbance of the balance suggest that the warm air is lig hter than the cold air.
Also, it is important to remember that on heating the air expands and occuples 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.
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. It is this event that we call a thunderstorm.
HOW A THUNDERSTORM BECOMES 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.
A cyclone is known by different names in different parts of the world. It is called a ‘hurricane’ in the American continent. In Philippines and Japan it is called a ‘typhoon’
DESTRUCTION CAUSED BY CYCLONES
Cyclones can be very destructive. Strong winds push water towards the shore even if the storm is hundeds of kilometres 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.
TORNADOES
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.
SAFETY MEASURES
1. A cyclone forecast and warning service must be there.
2. Rapid communication of warnings to the construction of cyclone shelters in the cyclone prone areas,
and Administrative arrangements for moving people fast to safer places.
3. We should not ignore the warnings issued by the meteorological department through TV, radio, or newspapers.
4. 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.
5. For do not drink water that could be contaminated. Always store drinking water for emergencies.
6. Do not touch wet switches and fallen power lines.
7. Do not go out just for the sake of fun.
8. Do not pressurise the rescue force by making undue demands.
9. Cooperate and help your neighbours and friends.
ROLE OF ADVANCED TECHNOLOGY
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. Due 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 hrs in advance.
The message is broadcast every hour or half hour when a cyclone is nearer the coast. Several national and international organisations
cooperate to monitor the cyclone-related disasters.
UNSOLVED PROBLEMS
Q.1 What is air pressure ? How does it determine the kind of weather a place will have ?
Q.2 What does a barometer measure ?
Q.3 What is a storm surge ?
Q.7 What causes winds ?
Q.8 How does a thunderstorm develop ?
Q.9 What are cyclones in the Atlantic and Pacific called ? Why are cyclones given names ?
Q.10 What are the trade winds ? How are they caused?
Q.11 Explain why roofs get blown off during a storm.
Q.12 What is a tornado ? How does it form and what impact does it have ?
Q.13 How does a cyclone develop ? Describe its impact briefly.
Q.14 What safety measures can one take to
protect oneself from a cyclone ?
Q.15 Write briefly about the winds that carry rain to most parts of our country between June and September.
Q.16 Suggest two methods to find out wind
direction at a given place.
Q.17 Explain why holes are made in hanging banners and hoardings.
Q.18 What planning is required in advance to deal with the situation created by a cyclone?
Q.19 Explain how wind currents are generated due to uneven heating on the earth.
Q.20 Explain why a hot can (or bottle) gets
distorted when water is oured over it?
Q.1 Which one of the following place is unlikely to be affected by a cyclone ?
(A) Chennai (B) Manglore
(C) Amritsar (D) Puri
Q.2 Which of the statements given below is correct?
(A) In winter the winds flow from the land to the ocean
(B) In summer the winds flow from the land towards the ocean.
(C) A cyclone is formed by a very high-pressure system with very high-speed winds revolving around it
(D) The coastline of India is not vulnerable to cyclones
Q.3 Wind is …………… air
(A) Moving
(B) Resting
(C) Moving or resting
(D) None of these
Q.4 Winds are generated due to …………… heating on the earth .
(A) Direct
(B) Indirect
(C) Direct or Indirect
(D) One of these
Q.5 Near the earth’s surface …………… air rises up whereas …………… air comes down :
(A) Hot, cold (B) Cold, hot
(C) Cold, cold (D) Hot, hot
Q.6 The energy possessed by wind is
(A) Kinetic energy
(B) Potential energy
(C) Thermal energy
(D) Sound energy
Q.7 The cause of formation of cyclones is
(A) Air rotates at low pressure with high speed
(B) Air rotates at high pressure with high speed
(C) Air rotates at high pressure with low speed
(D) None of these
Q.8 Air moves from a region of ——— pressure to a region of——— pressure.
(A) High , low (B) Low, high
(C) High, high (D) Low, low
Q.9 We know that material bodies
(A) contract on heating and expand on cooling
(B) neither expand nor contract on heating
(C) expand on heating and contract on cooling
(D) expand on cooling and remain unaffaced on heating
Q.10 When water changes its state from liquid state to vapour state.
(A) heat is givne out
(B) heat is absorbed
(C) heat is given out and then absorbed
(D) there is no exchange of heat between the system and the surroundings
Q.11 Consequences of cyclones are they
(A) do not cause severe loss of life and property
(B) increase the fertility of soil by bringing in natural fertilizers
(C) reduce the fertility of soil
(D) help in the
ANSWER KEY
1. A 2. A 3. A 4. A
5. A 6. A 7. A 8. A
Exercise-I
A. Fill the missing words in the blank spaces
1. Wind is ____________ air.
2. Winds are generated due to ____________ heating on the earth
3. Near the earth’s surface ____________ air rises up whereas ____________ air comes down.
4. Air moves from a region of ........... pressure to a region of .............. pressure.
5. Twister and whirl wind are alternative names for a ____________.
6. An anemometer measures ____________.
7. High pressure indicates ____________ weather.
8. A tornado over the sea is called a ____________.
9. Scientists who study weather are called ____________.
10. The best place to be in during a tornado is the ____________.
11. The cold, dry winds that blow over northern India in winter are called the ____________.
B. True/False
1. Anemometer is used to measure atomospheric pressure.
2. Cyclones increases the fertility of the soil.
3. Monsoon winds carry lots of water from the Indian ocean.
4. Air expands on cooling.
5. High speed winds and air pressure difference can cause cyclone.
6. The centre of cyclone is called the eye of the storm.
7. Cyclone is unpredictable
8. Increased wind speed is accompanied by reduced air pressure.
9. Uneven heating of land and water in winter generates winds from southwest direction.
10. The coastline of India is not vulnerable to cyclones.
Exercise-II
(i) Very short questions (1 mark)
1. What is the meaning of ‘monsoon’?
2. How does it rain?
3. What happens to air pressure when wind-speed is increased?
4. What is known as the eye of the storm?
5. In which areas (regions), thunderstorms develop very frequently?
6. What is the name of cyclone, in American continent?
7. In which countries, cyclone is known as typhoon?
8. What is the main cause of wind movements?
9. What happens to air when it gets heated?
10. How is cyclone caused ?
(ii) Short questions (2 mark)
1. Explain why holes are made in hanging banners and hoardings?
2. A “Kachcha’ House is more comfortable in summer than a “Pucca” house. Why?
3. How does thunderstorm occur?
4. Write two precautions, if you are staying in a cyclone hit area.
5. What are the factors which contribute to the development of cylcones?
6. Why does not the wind flow exactly in the north-south direction due to uneven heating between the equator and the poles?
(iii) Long questions (3 mark)
1. What planning is required in advance to deal with the situation created by a cyclone ?
2. Write a short note on tornado.
3. Why is it difficult to force a crushed paper ball placed at the mouth of bottle by blowing into it ?
(Size of ball is smaller than the mouth of the empty bottle)
4. What are the actions which should be taken by the people if there is a possibility of occurrence of cyclone?
5. You want to buy a house. Would you like to buy a house having windows but no ventilators ? Explain
your answer.
(iv) Very long questions (4 mark)
1. Draw a labelled diagram of formation of cyclone.
2. Describe two activities to show that increased wind speed reduces air pressure.
3. Suggest two methods to find out wind direction at a given place.
4. Make a flowchart which explain the formation of clouds and forming of rain and creation of storms and cyclone.
5. What precautions will you take if you are staying in a cyclone hit area ?
Exercise-III
(i) MCQ (one correct option)
1. Which one of the following places is unlikely to be affected by a cyclone ?
(A) Chennai (B) Mangaluru (Manglore) C) Amritsar (D) Puri
2. Which of the statements given below is correct ?
(A) In winter, the winds flow from the land to the ocean
(B) In summer, the winds flow from the land towards the ocean
(C) A cyclone is formed by a high pressure system with very high-speed winds revolving around it.
(D) The coast line of India is not vulnerable to cyclones.
3. Winds carrying water vapour brings
(A) hotness (B) rain (C) flood (D) both (a) and (b)
4. Cyclone watch is issued
(A) 24 hours in advance (B) 48 hours in advance
(C) 36 hours in advance (D) 72 hours in advance
5. Cyclone warning is issued
(A) 24 hours in advance (B) 48 hours in advance (C) 36 hours in advance (D) 72 hours in advance
6. A violent tornado can travel at a speed of
(A) 100 km/h (B) 300 km/h (C) 500 km/h (D) 400 km/h
7. Winds are generated due to____________ heating on the earth
(A) exessive (B) slow (C) rapid (D) uneven
8. The term “hurricane” indicates to a tropical storm in northem hemisphere.
What is the name of this storm in southern hemisphere ?
(A) Depression (B) Typhoon (C) Disturbance (D) Cyclone
9. A tropical storm which occurs in the north west pacific ocean west of the dateline, is called
(A) Hurricane (B) Severe cyclonic storm (C) Typhoon (D) Severe tropical cyclone
10. Worldwide, tropical cyclone activity peaks towards the end of which season and why ?
(A) spring, due to rapid warming of water
(B) Winter, due to the difference in air and water temperature
(C) Rainy season, as the temperature falls, is water remains warm.
(D) Summmer, because water temperature warmest
11. Anemometers, from the Greek word “Anemos” meaning wind, can be used to measure
(A) wind’s pressure and velocity (B) wind’s velocity only
(C) wind’s moisture and velocity (D) wind’s pressure only
12. A drop in pressure indicates the possibility of
(A) flood (B) earth quarke (C) storm (D) all of these
13. An instrument to measure the air pressure is
(A) anemometer (B) manometer (C) barometer (D) hygrometer
14. Air pressure falls when temperature
(A) falls slowly (B) rises (C) falls rapidly (D) both (a) and (b)
15. Roofs get blown up during a storm because
(A) pressure above the roof becomes less than that below it
(B) pressure above the roof becomes more than that below it
(C) increased wind speed is associated with increased air pressure
(D) none of these
16. Wind currents are generated due to
(A) uneven heating of land and water only
(B) uneven heating between the equator and the poles only
(C) Both (a) and (b)
(D) None of these
17. The centre of a cyclone is
(A) a calm area (B) very turbulent
(C) called the eye of the storm (D) both (a) and (b)
18. Factor contributing to the development of cyclones are
(A) wind speed and wind direction
(B) temperature and humidity
(C) tectonic movements of earth’s plate or Volcanic eruptions
(D) Both (a) and (b)
19. Orissa was hit by cyclone twice in the year 1999 - first on 18 october and second on 29 october. The wind speed was
(A) 100 km/hr and 200 km/hr respectively (B) 150 km/hr and 250 km/hr respectively
(C) 100 km/hr and 250 km/hr respectively (D) 200 km/hr and 260 km/hr respectively
(ii) One word or one sentence questions
1. What is the effect of cyclone on soil fertility ?
2. What causes storm ?
3. How is the speed of wind associated with the air pressure ?
4. Which instrument measures the speed of wind ?
5. What is meant by thunderstorm ?
6. What is lighter, warm air or cold air ?
7. Which part of the earth’s surface get maximum heat from the sun ?
8. Which part of India is vulnerable to cyclones ?
9. What can be the diameter of a tornado at the lowest level ?
10. What is the name of cyclone, in American continent ?
Answer Key
Exercise-I
A. Fill in the blanks
1. moving 2. uneven 3. hot, cold 4. high, low
5. tornado 6. wind speed 7. calm 8. water sport
9. meteorologists 10. basement of a building 11. winter monsoon
B. True/False
1. F 2. F 3. T 4. F 5. T 6. T
7. F 8. T 9. F 10. F
Exercise-III
Multiple Choice Questions
1. C 2. A 3. B 4. B 5. A 6. B
7. D 8. D 9. C 10. D 11. B 12. C
13. C 14. B 15. A 16. C 17. A 18. D
19. D