Notes of Ch 8 Winds, Storms and Cyclones Class 7th Science

Study Material and Notes of Ch 8 Winds, Storms and Cyclones Class 7th Science

Topics in the chapter
  • Air exerts Pressure
  • Air expands on Heating
  • Wind currents generated due to uneven heating on Earth
  • Monsoon Winds
  • Thunderstorms
  • Cyclones
  • Destruction caused by Cyclones
  • Tornadoes
  • Safety Measures For Cyclones
  • Precautions for Cyclone prone areas


→ Moving air is called wind.

Air exerts Pressure

→ A tin can filled with hot water gets distorted when placed under running water because steam condenses back into water.

→ The pressure of air inside the can decreases than the pressure exerted by the air from outside the can. As a result, the can gets compressed.

→ In an anemometer, moving air exerts pressure on the bowls. Hence, this indicates the direction of wind.

→ High speed winds are always accompanied by reduced air pressure.

→ A paper ball acquires zigzag motion and does not go into a bottle when air is blown on it to force it into the bottle.

→ The balloons move towards each other when air is blown between them.

→ The strip of paper lifts when air is blown over it.

→ Air moves from high air pressure region to low air pressure region. The greater the difference in air pressure, the faster the air moves.

Air expands on Heating

→ Take a boiling tube. Stretch a balloon tightly over the neck of the tube.

→ Pour some hot water in a beaker. Insert the boiling tube with the balloon in the hot water. Observe for 2–3 minutes for any change in shape of the balloon.

→ The balloon will expand and changes its shape. This happened due to expansion of air inside the balloon and it occupies more space.

→ The warm air is lighter than the cold air because it occupies more space. Due to this reason the smoke goes up. 

→ When 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.

Wind currents generated due to uneven heating on the Earth

There are two situation for this phenomenon.

(i) 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.

(ii) Uneven heating of land and water

• 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.

The monsoon winds carry water and it rains.

• Uneven heating of land especially the Rajasthan desert generates monsoon winds from
southwest direction in summer. These winds carry lots of water from the Indian Ocean.

• Uneven heating of land and water in winter generate winds from the northwest colder land. These colder winds carry little water, hence bring small amount of rain in winter.


• 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. This event is called thunderstorm.


• 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’

• 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.

Destruction caused by Cyclones

• Strong winds push water towards the shore even if the storm is hundreds of kilometres away.

• 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.

• 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.


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

• 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.

Effective Safety Measures for Cyclones

• A cyclone forecast and warning service.

• Rapid communication of warnings to the Government agencies, the ports, fishermen, ships and to the general public.

• Construction of cyclone shelters in the cyclone prone areas, and Administrative arrangements for moving people fast to safer places.

Action taken by the people

• We should not ignore the warnings issued by the meteorological department through TV, radio, or newspapers.

• 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.

Precautions for Cyclone prone areas

• 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.

Help from Advanced Technology

• With the help of satellites and radars, 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.

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