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CLASS 7-Winds, Storms and Cyclones

 Air Exerts Pressure

·         The Force exerted on an object or against an object when something comes in contact with it, is called pressure.

·         Air exerts pressure in different ways, different situations and on all objects and in all directions.

·         Air pressure helps leaves of trees, banners or flags to flutter when the wind is blowing. It causes the tyres, balloons to inflate.

Importance of Air pressure

·         It makes wind the difference in atmospheric pressure gives rise to the wind on the earth.

·         It influences weather Air movement, i.e. up or down movement of air caused due to differences in pressure creates the cloudy and clear sky, brings rain or fine weather.

·         Weather forecasting the changes in air pressure gives important clues for weather forecasting. barometer is an instrument  that measures air pressure.


·         High-speed winds are accompanied by reduced air pressure.

·         Air moves from high pressure to the low pressure.

·         The greater the difference of air pressure, the faster the air moves.  The warm air rises up and the cool air from the surroundings moves towards the sea.

The properties of air are:

·         Air takes up space.

·         Air has mass.

·         Air is affected by heat.(expands)

·         Air exerts pressure.

·         Air can be compressed.

·         Air is affected by altitude.


Air Expands on Heating:-

·         The  molecules  of air expand on heating and contract on cooling.

·          Cold air molecules take less space than hot air molecules.

       Cool air heaver than hot air.

·         Warm air rises up and cool air settles down.

·         And  when we stop giving it heat then it starts to contract. 

Wind currents are generated from uneven heating on earth

(i)Uneven heating between the equator and the poles

·         Wind currents are made due to the non-uniform heating of the Earth.

·         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.

·         The poles have air colder than that at latitudes about 60 degrees.

·         The warm air at  latitudes about 60 degrees rise up and the cold wind from the polar Regions rushes in, to take its place.

·         Wind circulation is set up from the poles to the warmer latitudes, as shown in.

·         The winds flown in the north-south direction from north to south, or from south to north.

·         A change in direction is caused by the rotation of the earth.


(ii) Uneven heating of land and water

·         In the summer, air near the equator  warms up faster and most of the time the temperature of the air is higher than that of water in the oceans.

·         This is  the main reason  why 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 oceans carry water and bring rain.

·       Monsoon winds carry with it water and cause rain.


·         Thunderstorms develop in humid tropical areas like India.

·         The rising temperature produce strong upward winds.

·         These winds carry water vapours upwards, where they freeze, and fall down again.

·         The swift movement of the falling water vapours along with the rising air create lightning and sound. This event is called  thunderstorm.

How a thunderstorm becomes a cyclone

·         Before cloud formation, water takes up heat from the water body to change into vapour.

·         When water vapour changes to liquid back this is known as raindrops, this heat is released to the atmosphere.

·         The heat released to the atmosphere warms the air.

·         The air temperature  rise and causes a drop in pressure.

·         More air rushes to the eye of the storm.

·         This cycle is repeated. 

       This chain is called storm.



Destruction caused by cyclones

·         Cyclones can be very destructive.

·        Wind push are that strong water towards the shore even if the storm is hundreds of kilometers away. This is the first step to define a cyclone.

·         The water waves produced by the wind are much powerful that a person cannot overcome them.

·         The low pressure in the eye of the cyclone lifts water surface into the centre.

·         The rising water may be  3–12 metres in height.

·         It looks like a water-wall moving towards the shore.

·          As a result of, the seawater enters the low-lying coastal areas, causing severe loss of property as well as life.

·         It also impacts adversely the fertility of the soil.

·         Continuous heavy rainfall may develop flood situation.

·          High-speed winds along with a cyclone can damage houses, telephones and other communication systems, trees, etc., and resulting tremendous loss of life and property.

·         Cyclone is given different names in different parts of the world. It is termed as ‘hurricane’ in the American continent.  it is called a ‘typhoon’ in  Philippines and Japan.


·         The tornado is a dark funnel shaped cloud that reaches from the sky to the ground .

·         Generally the tornadoes are weak.

·          A severe tornado can travel at speeds of about 300 km/h. It may form within cyclones.

·         The whole coastline of India is vulnerable to cyclones, specially the east coast.

·         The west coast of India is less vulnerable to cyclonic storms both in terms of intensity and frequency of the cyclones.

Some precautions if you are 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.


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