NCERT Class 7 Science Winds Storms and Cyclones

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8 Winds, Storms and Cyclones

Orissa was hit by a cyclone with wind speed of 200 km/h on 18 October 1999. The cyclone smashed 45,000 houses making 7,00,000 people homeless. On 29 October the same year, a second cyclone with wind speed of 260 km/h hit Orissa again. It was accompanied by water waves about 9 m high. Thousands of people lost their lives. Property worth crores of rupees was destroyed. The cyclone affected agriculture, transport, communication, and electricity supply. 

But, what are cyclones? How are they formed? Why are they so destructive? In this chapter we shall seek answers to some of these questions. We begin with some activities involving air. These activities will clarify some basic features concerning a cyclone. Before we begin, remember that the moving air is called the wind.

8.1 AIR EXERTS PRESSURE

You need to boil water in the following Take a tin can with a lid. Fill it approximately half with water. Heat the can on a candle flame till the water boils. Let the water boil for a few minutes. Blow out the candle. Immediately put the lid tightly on the can. Be careful in handling the hot can. Put the can carefully in a shallow metallic vessel or a washbasin. Pour fresh water over the can.

What happens to the shape of the can? Can you guess why the shape of the can gets distorted? If you cannot get a tin can, take a soft plastic bottle. Fill it with hot water. Empty the bottle and immediately cap it tightly. Place the bottle under running water. Recall now some of your experiences. When you fly a kite, does the wind coming from your back help? If you are in a boat, is it easier to row it if there is wind coming from behind you?

Do you find it difficult to ride a bicycle against the direction of the wind. You know that we have to fill air into the bicycle tube to keep it tight. Also,you know that a bicycle tube overfilled with air may burst. What is the air doing inside the tube?

Exercises

1. Fill the missing word in the blank spaces in the following statements:

(a) Wind is——————air.

(b) Winds are generated due to——————heating on the earth.

(c) Near the earth’s surface __________air rises up whereas ___________ air comes down.

(d) Air moves from a region of ——— pressure to a region of——— pressure.

2. Suggest two methods to find out wind direction at a given place.

3. State two experiences that made you think that air exerts pressure(other than those given in the text).

4. You want to buy a house. Would you like to buy a house having windows but no ventilators? Explain your answer.

5. Explain why holes are made in hanging banners and hoardings.

6. How will you help your neighbours in case cyclone approaches your village/town?

7. What planning is required in advance to deal with the situation created by a cyclone?

8. Which one of the following place is unlikely to be affected by a cyclone.

(i) Chennai      (ii) Mangaluru (Mangalore)

(iii) Amritsar    (iv) Puri

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.


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