CBSE – 7th Standard Science

Chapter 8 – Winds, Storms and Cyclone

Chapter 8

Winds, storms and Cyclone

  • The moving air is called the wind.
  • The air exerts pressure. It is due to this pressure that the leaves of trees, banners, or flags flutter when the wind is blowing.
  • The increased wind speed is, indeed, accompanied by a reduced air pressure.


  • Air moves the region where the air pressure is high to the region where the pressure is low.
  • The greater the difference in pressure, the faster the air moves.
  • 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.
  • Wind currents are generated due to uneven heating on the Earth .
  • 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.


  • 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.
  • The monsoon winds carry water and it rains.
  • In winter, the direction of the wind flow gets reversed; it flows from the land to the ocean
  • 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 creates lightning and sound is called thunderstorm.


  • 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 a cyclone.


  • Factors like wind speed, wind direction, temperature and humidity contribute to the development of cyclones.
  • Cyclones can be very destructive. Strong winds push water towards the shore even if the storm is hundreds of kilometres away.
  • If a storm is accompanied by lightning, we must take the following precautions:
  • Do not take shelter under an isolated tree, in a forest take shelter under a small tree. Do not lie on the ground.
  • Do not take shelter under an umbrella with a metallic end.
  •  A car or a bus is a safe place to take shelter.


  • 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.
  • High-speed winds accompanying a cyclone can damage houses, telephones and other communication systems, trees causing tremendous loss of life and property.


  • In our country Tornadoes are not very frequent. A tornado is a dark funnel shaped cloud that reaches from the sky to the ground.
  •  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.
  • Wind speed plays an important role in the formation of storms.


  • It is, therefore, important to measure the wind speed.
  • The instrument that measures the wind speed is called an anemometer.


  • 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.
  • Air around us exerts pressure.
  • Air expands on heating and contracts on cooling.
  • Warm air rises up, whereas comparatively cooler air tends to sink towards the earth’s surface.
  • As warm air rises, air pressure at that place is reduced and the cooler air moves to that place.
  • The moving air is called wind.
  • Uneven heating on the earth is the main cause of wind movements.
  • Winds carrying water vapour bring rain.
  • High-speed winds and air pressure difference can cause cyclones.
  • It has become easier to monitor cyclones with the help of advance technology like satellites and radars.
  • Self-help is the best help. Therefore it is better to plan in advance and be ready with defence against any approaching cyclone.
  • The centre of a cyclone is a calm area.
  •  It is called the eye of the storm.
  • A large cyclone is a violently rotating mass of air in the atmosphere, 10 to 15 km high. The diameter of the eye varies from 10 to 30 km.
  • It is a region free of clouds and has light winds.
  • Around this calm and clear eye there is a cloud region of about 150 km in size.
  •  In this region there are high-speed winds (150–250 km/h) and thick clouds with heavy rain.
  • Away from this region the wind speed gradually decreases.
  • The formation of a cyclone is a very complex process.


  • The diameter of a tornado can be as small as a metre and as large as a km, or even wider.
  • The funnel of a tornado sucks dust, debris and everything near it at the base (due to low pressure) and throws them out near the top.
  • A tornado shelter is a room situated deep inside or underground having no windows.
  • Or otherwise it is better to shut windows and take shelter under a table, workbench, where debris cannot reach.
  • One has to bow down on knees protecting head and neck using arms