8 September 2014

Thunderstorms and Cyclones : Science CBSE Notes


A storm with lightning and thunder is called a thunderstorm. It occurs due to the convection of air in
hot and humid areas.A thunderstorm, also known as an electrical storm, a lightning storm, or a thundershower, is a type of storm characterized by the presence of lightning and its acoustic effect on the Earth's atmosphere known as thunder.


A natural calamity caused by difference in air pressure in the atmosphere is called a cyclone. A cyclone is a violently rotating windstorm. Cyclones are dangerous. Cyclones begin as thunderstorms.

Thunderstorms and Cyclones, Eye Of Cyclone, NCERT Notes, CBSE Notes

In tropical regions like India, thunderstorms are common, but very few thunderstorms convert into cyclones. In India, the eastern coast is more vulnerable to cyclones than the western coast.

In different parts of the world, cyclones are known differently. For example, in America, a cyclone is referred to as a hurricane, while the Filipinos and Japanese called it a typhoon.
The largest tropical cyclone recorded was Typhoon Tip that struck Japan in 1979. At its peak strength, the diameter of its eye wall was 2220 kilometres. The wind speeds were recorded at 305 kilometres per hour.

Formation of Cyclones

The formation of a cyclone depends on the speed and direction of the wind, temperature and humidity. A cyclone also arises:
  • Due to the difference in the temperatures of the two regions.
  • A low pressure is created as the air in the high temperature region becomes warm and rises.
  • The gap in the low pressure area is filled by cold air rushing in from the surrounding areas.
  • When the warm air arises, it cools, condenses and forms clouds.
  • When the water vapour in the clouds turns into raindrops, the heat possessed by the water vapour is released into the atmosphere.
  • The process repeats and the release of heat from the water vapour continues.


During a thunderstorm, move away from open garages, metal sheds and water bodies. During a thunderstorm, sit inside a car, a bus or a closed vehicle, or inside a building.
During a thunderstorm, do not:
  • Rest under a tree
  • Take shelter under an umbrella with a metallic end
  • Lie down flat, if in an open place
  • Sit near a window


Humidity is the amount of water vapour in the atmosphere. As the humidity in a region increases, the difference in temperature also increases. This results in the formation of a cyclone.


Winds moving at high speeds are known as gales.

Eye of a Cyclone

The calm and clear area at the centre of a cyclone is called the eye of the cyclone.


A rotating column of high speed winds that appears as a dark funnel-shaped cloud reaching from the ground to the sky is called a tornado. A tornado may form within a cyclone. A tornado forms due to the effect of low pressure in the eye of a cyclone. Objects near the base of a tornado, such as cars, dust, debris and even houses, are sucked into its funnel and thrown out at the top.
More than a thousand tornadoes occur every year across the world. Most occur in the United States. Tornadoes occur regularly in the regions around the Atlantic Ocean.
Weak tornadoes travel with wind speeds of 50 to 60 km/hr, while a violent tornado can travel at a speed of about 400 km/hr. A tornado is a natural disaster, but not as dangerous as a cyclone.