CLASS 7 WINDS STORMS AND CYCLONES.

 NOTES

Air pressure

• The air around us exerts pressure.

Wind

The natural motion of air, due to a current in a particular direction is called as wind.

Winds and pressure difference

• Increased wind speed is accompanied by reduced air pressure.
• Air moves from a region of high pressure to a region of low pressure.

Warm air and cool air

• On heating, air expands and rises up. Since it expands it takes up more space and therefore becomes lighter.
• Warm air is lighter than the cool air. That is why smoke always rises up.
• Once warm air rises up, air pressure is lowered. Then the cold air from the surroundings rushes in to fill its place.

Wind Currents

Wind current is a current of air, sometimes of considerable force, moving generally horizontally from areas of high pressure to areas of low pressure.

Wind currents due to uneven heating of land and water

• Land gains and loses heat much faster than water. During summer winds move from cooler seas towards the land which is hotter. These winds carry moisture and cause monsoons.
• During winter this direction of wind flow is reversed and it moves from land towards the oceans.

Rain

Rains are caused when winds from the oceans and seas flow towards land carrying moisture. The moisture in the clouds get saturated after a point and pour down as showers, which we call as monsoons.

Cyclone

• A cyclone is a large mass of air that rotates violently about 1015 km high in the air.
• The centre of the cyclone is a calm area called as the eye of the storm.

Structure of a Cyclone

• The centre of a cyclone is a calm area called the eye. It has a diameter of about 10 – 30 km.
• Around the eye is a region of clouds of about 150 km. This also has winds with speeds of up to 150-250 km/hr.
• The speed of the wind gradually decreases as it moves away from the eye.

NCERT EXCERCISE 

Q.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.
Ans.(a) moving (b) uneven
(c) warm, cooler (d) high, low

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

Q.3.State two experiences that made you think that air exerts pressure.
Ans.(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.

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

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

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

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

Q.8. Which one of the following places is unlikely to be affected by cyclone?
(i) Chennai (ii) Mangalore
(iii) Amritsar (iv) Puri
Ans.(iv) Amritsar

Q.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.
Ans.(i) In winter the winds flow from the land to the ocean.