Air Exerts Pressure

    When a place gets heated by the sun, the air above it also gets heated, when air is heated, it expands and becomes lighter. 
    The warmer, lighter air rises and cooler, denser air blows in from the neighbouring areas. The movement of air is what we call wind. When air moves gently (low speed). it is called a breeze. 
    When it moves violently (high speed), it is called a storm. Storms are classified in many ways and given different names. But they have one thing in common-high-speed winds.

Air pressure :
    The air surrounding the earth exerts a pressure on its surface. Like temperature and humidity, air pressure also determines the weather of a place on any particular day. It is measured by an instrument called a barometer and it determines whether a day is           going to be calm or stormy.
    There is a close link between (air) pressure and wind. We can say that winds blow from regions of high pressure to regions of low pressure. 
    The greater the difference in pressure, the greater is the speed of the wind.
    Note : 
    (i) High speed wind is accompanied by a reduced air pressure.
    (ii) The instrument used to measure atomspheric pressure is called as barometre.


Air Exerts Pressure

Chapter 8: Winds, Storms and Cyclones

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.

Air exerts pressure

Air is made up of molecules of different gases. The molecules are colliding with each other continuously with the walls of the container. When a molecule collides with the wall, they exert small force on the wall The pressure exerted by the gas is due to the sum of all these collision forces.

High-Speed Winds Are Accompanied by Reduced Air Pressure

High speed winds are accompanied by reduced air pressure

As the pressure of an area reduces, wind speed increases and vice versa. Higher the pressure difference, higher the speed of wind. Thus, the two major concepts are: High speed winds are accompanied by reduced air pressure.

Air Expands on Heating


Air expands on heating

Air expands on heating. Why does this happen? It is because the particles of air on becoming warm, move away from each other and take up more space. So we can say that air on becoming hot occupies more space

Wind Currents Are Generated Due to Uneven Heating on The Earth

Wind currents : 
    Wind currents are formed due to uneven heating  of the earth. Two situations may arise :
1.    Uneven heating between the equator and the pole
      The equator, which receives the direct rays of the sun throughout the year, is the hottest part of the earth. The air above the equator gets heated (hence, becomes lighter) and rises. 
      This makes cooler, denser air from the tropical belts blow in towards the equator from the north to south. These permanent winds are called the north-east and south-east trade winds. 
      The rotation of the earth makes these winds blow in an eastern direction rather than straight north and south. Wind flow pattern due to uneven heating between the equator and the pole.

2.    Uneven heating of land and water : 
      Originally, the seasonal winds that bring rain to India and the other countries of South Asia between June and September are called monsoon. 
      The word monsoon was derived from the Arabic word ‘mausam’ which means weather. In summer, the landmass of northern India becomes much hotter than the surrounding seas. 
      The hot air rises and moist air blows in from the Arabian Sea and Bay of Bengal. These moisture-laden winds bring heavy rainfall in most parts of India, are called the summer monsoon.
      In winter, the direction of the wind flow gets reversed, it flows from the land to the ocean. These winds are called the winter monsoon. They pick up moisture as they blow over the Bay of Bengal and bring rainfall to coastal Tamil Nadu.


Wind Currents Are Generated Due to Uneven Heating on The Earth

Wind currents are generated due to uneven heating on the earth

Wind currents are produced due to the non-uniform heating of the Earth. That means uneven heating at the equator and the poles. This warm air rises and the making up of cooler air from the regions with the 0-30 degrees latitude belt on either side of the equator moves in. This is how wind currents are being generated.

Current winds bring up air from one type of climate to the other. Suppose let warm winds that travel above water tend to collect moisture as they travel, then the water vapour in the air gets to be condensed as it moves into colder climates. This is why temperate coastal areas are frequently receiving heavy rainfalls.

Thunderstorms and Cyclones

Type of storms :
    There are different types of storms. Some arise over land, others develop over seas. Some are accompanied by thunder, lightning and heavy rain. Others are dry dust storms. We will discuss three broad types of storms-thunderstorms, tornadoes and               cyclones.

Thunderstorms :
    As the air near the land gets heated, it becomes light and rises. The pressure under the rising air column drops. 
    If the air carries enough moisture, the moisture condenses when it comes in contact with colder air, and eventually, the raindrops become heavy enough to start falling. Then the air at the top of the column cools and spreads out. 
    The pressure above the column is high, so the cold air flows downwards. The upward movement of hot air and the downward movement of cold air and raindrop leads to stormy winds accompanied by heavy rain, lightning and thunder, which are typical           characteristics of thunderstrom. 

Tornadoes :
    A tornado is a dark funnel shaped cloud that reaches from the sky to the ground. The funnel sucks dust, debris  and everything near it at the base due to low pressure and throws them out near the top. 
    The diameter of tornado can be as small as few kilometers a violent tornado can travel at a speed of about 300 km/hr. Tornadoes, are also called twisters or whirl winds. A tornado over the sea is called a waterspout because it sucks up a huge column of         water.

Cyclones :

A cyclone is a storm that develops over the sea. The distinctive feature about a cyclone is high-speed winds swirling around a low-pressure centre. This low-pressure centre is a region of calm, called the eye of the storm or storm centre.
Cyclones develop over tropical seas. Air heated by the warm sun rises, creating a region of low pressure. Cold air rushes in, forcing up more hot air. This brings in more cold air and so on, setting up a cycle or current of air. The rotation of the earth drags this air current around the region of low pressure. 

Note: This is how the winds whirl around the centre. They whirl anticlockwise in the northern hemisphere and clockwise in the southern hemisphere.

Cyclones in the western Atlantic and eastern Pacific are called hurricanes. While those in the western Pacific are called typhoons. 
Those developing over the Indian Ocean, Bay of Bengal and Arabian sea are simply called cyclones.

Thunderstorms and Cyclones

Thunderstorm and cyclones

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.

Destruction Caused by Cyclones

Impact of storms :
    Every year, storms cause great damage across the world. Cyclones are the worst in this respect because they last longer and affect a wider area. The wind speeds are also greater, reaching up to 300 km/h.

    As they travel over the sea, they gather a lot of moisture. So when they hit a coast, they bring heavy rainfall. High waves hit the coast and flood vast areas.

    Towns and villages get submerged. Trees get uprooted. Telephone and electric lines snap. Buildings, bridges and other man-made structures get damaged.

    The extent of damage is usually much less in a thunderstorm. However, when counted over the whole world, more people are killed by lighting (accompanying thunderstorms) than by cyclones.

    Tornadoes are far more destructive than thunderstorms. The core of low pressure creates a suction which can blow off roofs and make houses collapse.

    All storms uproot trees and kill wildlife. Cyclones also affect marine life, and coral reefs, in particular. The rushing in of sea water has another harmful effect. It turns soil saline and unfit for cultivation. (i.e. cylclone reduces the fertility of soil)

Destruction Caused by Cyclones

Destruction caused by cyclones

The destruction caused by cyclones depends on its intensity, location, and size. In forest regions, the trees get uprooted and canopies get affected. In coastal regions, the banks and embankments get eroded. In deserts, the sand dunes get reshaped whereas in mountainous regions mudslides and landslides occur.

Effective Safety Measures

Effective safety measures : 
1.    A cyclone forecast and warning service
2.    Rapid communication of warnings to the government agencies, fishermen, ships and to the general public.
3.    Construction of cyclone shelters in the cyclone prone areas and administrative arrangements for moving people fast to safer places.

Action on the part of the people :
1.    People living in cyclone-prone areas should always listen to the weather forecast.
2.    If there is a warning, they should make arrangements to shift the essential household goods, domestic animals, vehicles etc. to safer places.

Advanced technology for forecasting cyclone :
    Using satellites and radar technology, a cyclone alert or Cyclone warning is issued 24 hrs in advance.

Effective Safety Measures

Effective safety measures

Keep your emergency kits ready and handy.

Make sure you have kept your phones and other necessary electronic devices charged.

Keep candles ready in case the electrical power is cut off.

Repair your roof shed or tiles. …

Tie loose poles and rods with ropes before the storm hits

Advanced Technology Has Helped

                   Extended Learning - Activities and projects 

1.    To show that air exerts pressure :
      Take a tin can with a lid and fill it two-thirds with water.
    • Heat the water on a burner till the water starts boiling.
    • Now put off the burner, cover the mouth of the can with its lid tightly and pour cold water on the hot can as shown in figure.
     You will observe that the tin loses its shape.
     When the water in the can is heated, it changes into vapour form. When cold water runs over the tin can containtng hot water, some of the steam in the can turns back to its liquid state, reducing the amount of air inside. This reduces the air pressure                inside the can compared to the air pressure outside. As a result, the can gets compressed or deshaped.

2.    Pumping of 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.

3.    Wind Speed is accompanied by reduced air pressure : 
       Fold the piece 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. 

4.    Air Expands on Heating : 
      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 lighter than the cold air.

      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.

5.    To show that high speed wind lifts and blows off articles :
    • Take a strip of paper about 15 cm long and 3 cm wide.
    • Hold it firmly with your thumb and finger as shown in figure.
    • Now blow hard over the paper strip.
    You will observe that the other end of the paper strip rises up which was earlier hanging down. When you blew over the paper strip, the increased wind speed reduced the air pressure on top. The pressure below the paper strip was higher and lifted it up.


6.    To show that air expands on heating :
    • Take a test tube and stretch a balloon over its open end tightly with the help of cellotape.
    • Now take a beaker containing hot water and put the test tube into it [figure (a)].
    • Observe the balloon after a few minutes. Why has it inflated a bit?
    • Now take out the test tube from the boiling hot water beaker. Cool it a bit and put it into a beaker containing crushed ice [figure(b)]. Do you observe any change in the balloon after a few minutes? Why has it deflated? • Now again take out the test tube           from the beaker containing crushed ice and put it in a beaker containing water at room temperature [figure (c)].
    The size of the balloon remains the same when the test tube is kept at normal room temperature [figure (c)] whereas it gets inflated at high temperature [figure (a)] and deflated at low temperature [figure (b)]. The inference we draw from this activity is that       heating expands air, while cooling contracts it.

                 (a) Hot water                          (b) Ice-cold water                (c) Water at normal temperature      

                                                                                                                                                                       Figure Change in the shape of ballon

7.    To prove that hot air is light and rises :
    • Take two empty paper cups of the same size and hang them in an inverted position on the two ends of a stick.
    • Tie a piece of thread in the middle of the stick and hang it so that the two cups are in balanced position.
    • Now put a burning candle just below one of the hanging cups and observe the change.
    You will observe that the balancing of the two cups is disturbed. The cup under which the candle is lit moves up. The lig-hted candle warms the surrounding air. As a result, the air becomes light and rises and pushes the paper cup up.


1.    Wind: Moving air is called the wind. Winds that move from east to west are called trade winds. Winds that move from west to east are called westerlies. Horse latitudes are the areas of calm or no wind.
2.    Air exerts pressure. It is the pressure which helps the leaves of trees, banners or flags to flutter when the wind is blowing.
3.    Increased wind speed is accompanied by a reduced air pressure.
4.    Air moves from the region where the air pressure is high to the region where the air pressure is low.
5.    Greater the difference in pressure, the faster the air moves.
6.    On heating, the air expands and become lighter.
7.    Warm air rises up, where as the cool air tends to ink toward the earth's surface. 
8.    Wind currents are generated due to uneven heating of the earth, for example:
      (i) uneven heating between the equator and the poles.
      (ii) uneven heating of land and water.
9.   The winds from ocean carry water and bring rain. These are monsoon winds.
10.  Formation of clouds, storms and cyclone is shown in the following flow chart:

11.    A man-made object that orbits around the earth, the moon or any other celestial body is called satellite. Cyclone detecting radar can locate and track the approaching tropical cyclone within a range of 400 km. Satellites are also useful for monitoring the         cyclones.

12.     Effective safety measures: (Low pressure region with high speed winds around it). A cyclone can be very destructive in its effect. It causes tremendous loss of life and property. Therefore, we should have effective safety measures.
         (i)     It has become easier to monitor cyclones with the help of advance technology like satellites and radars.
        (ii)     Self help is the best help. Therefore, it is better to plan in advance and be ready with defence against any approaching cyclone.



1.     Anemometer: The instrument that measures the speed of wind is called as -anemometer.

2.     Cyclone: A very low pressure system with a very high speed winds that revolves around is called a cyclone.

3.     Tropical cyclones are called hurricanes in American continent and Typhoon in Japan and Philippines.

4.     Lightning: When two clouds with unlike charges approach each other, charges start moving with high speed through the air in between. When this happens lightning strikes and intense spark of electricity travels in air.

5.     Low pressure: Increased wind-speed, is accompanied by a reduced pressure. Also warm air rises up causing low pressure.

6.     Monsoon winds: The winds from the oceans carry water and bring rain known as monsoon winds.

7.    Thunderstorm: In hot and humid area, the rising temperature produces 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 is called a thunderstorm.

8.     Tornadoes: A tornado is a dark funnel shaped cloud that reaches from the sky to the ground.

9.     Wind flow pattern: In summers winds flow from the oceans towards the land. In winters winds flow from land to oceans.

Advanced Technology Has Helped

Advance technology has helped

Advancements in satellite and radar has made it easier to see storms that may or are producing a tornado. With the new addition of dual polarization to the national WSR-88D doppler radars, the radar is able to scan a storm twice.

Related Chapter Name