CBSE Class 7th Science

Winds, Storms and Cyclones

Air pressure

  • The air around us exerts pressure. This is seen in the following activity:
  • A tin can is filled with water heated over a flame. As soon as the water comes to a boil, the lid is put and it is kept under running cold water. We find that the shape of the can gets distorted.
  • Inference: When cold water is poured over the can, some of the steam in the can condenses, reducing the amount air inside the can. The air from outside compresses the can from outside, thereby distorting it. This shows that air exerts pressure.


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.


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.

Storms and Cyclones

Wind Currents Due to Uneven Heating between the Equator and the Poles

  • The regions in the proximity to the equator receive the highest amount of heat from the sun, and the warm air rises up. The air from latitudes 0-300 moves in from north and south to take its place.
  • Similarly, the air at the poles is much cooler than the air around latitudes of 600 which is comparatively warmer. This warmer air rises up while the cool air from the poles races inside to occupy its place. Subsequently, wind currents move from poles to warmer latitudes due to uneven heating of the earth’s surface.


Thunderstorms develop in areas that are very hot and humid like India. The high temperatures create strong upward currents that carry water vapour to high altitudes where they condense and fall down again. Due to such drastic pressure variations, they are accompanied by lightning and strong winds.

Thunderstorm to Cyclone Conversion

  • During a thunderstorm, water releases heat when it changes from vapour to rain in the atmosphere. This warms the air around and creates a drop in pressure.
  • This causes air to rush in towards the centre of the storm. This cycle causes large low-pressure systems accompanied by strong high-speed winds that swirl around it. This is the formation of a 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.

Types of Cyclones

Cyclones are given different names in different parts of the world. They are called as hurricanes in the American continent and as typhoons in Japan.


  • A tornado occurs within a cyclone. It is in shape of a rotating funnel that sucks in debris, dust and everything at the bottom and throws them out at the top.
  • Its diameter ranges from a metre to a kilometre with speeds of up to 300 km/hr.

Coriolis Effect

The force due to earth’s rotation that tries to deflect winds towards left or right is known as Coriolis effect.

Safety Measures and Role of Advanced Technology

Steps to follow if a storm is followed by lightning

  • Avoid taking refuge under isolated trees.
  • If in a forest, seek shelter under a small tree
  • Do not lie on the ground
  • Do not try taking cover under an umbrella
  • Avoid sitting near metal sheds or open garages
  • If in water, rush out and go inside a building
  • A car or a bus is a safe place to take cover.

Measures of safety from tornados

  • Cyclone forecast and warning system.
  • Rapid warning systems to Governments and fishermen and people in prone zones.
  • Construction of cyclone shelters and rapid evacuation schemes.
  • Avoiding fallen power lines.
  • Cooperation with others and rescue teams.
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