Notes On Thunderstorms and Cyclones - CBSE Class 7 Science
Thunderstorms Storms are severe atmospheric disturbances accompanied by very strong, high-speed winds. Thunderstorms are whether conditions characterised by thunder, lightning and heavy rain. They occur due to the convection of air in hot and humid tropical areas. As the air near the earth's surface warms up, it rises. This results in strong upward rising winds which carry moisture with them. The moisture condenses into water droplets in the cooler, upper regions of the atmosphere. The water droplets fall down as rain or freeze and fall as hail. The rising air and the rapidly falling water droplets and ice crystals collide with each other and produce static electric charges. As a result the clouds get charged. When electric discharge occurs between the clouds or between a cloud and the ground, lightning occurs. The lightning heats up the air in its path very quickly and this results in a loud sound which we hear as thunder.  Precautions During a Thunderstorm      •  Move away from open garages, metal sheds and water bodies.      •  You may sit inside a car, a bus or a closed vehicle, or inside a building.      •  Do not take shelter under an isolated tree. If in a forest, take shelter under a small tree.      •  Do not take shelter under an umbrella with a metallic end.      •  Do not sit near an open window.      •  If you are in water (swimming pool, river, etc.) get out and go inside a building. Cyclone A cyclone is a natural calamity caused by difference in air pressure in the atmosphere. It is a violently rotating wind storm. It develops over the sea and may move over the land causing great damage. Cyclones begin as thunderstorms and later develop into cyclones. They are associated with strong winds, heavy rains and tidal waves.  In tropical regions like India, thunderstorms are common, but very few thunderstorms become 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 in Philippines and Japan it is called 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. Humidity is the amount of water vapour in the atmosphere. As the humidity in a region increases, the difference in temperature also increases. A cyclone arises due to the difference in the temperatures between 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 rises, it cools, condenses and forms clouds. When the water vapour in the clouds turns into rain drops, 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. Falling water droplets and rising air move vigourously to produce thunderstorm, which develops into a cyclone under certain weather conditions. The calm and clear area at the centre of a cyclone is called the eye of the cyclone.  Tornado 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 of America. Tornadoes occur regularly in the regions around the Atlantic Ocean. Weak tornadoes travel with wind speeds of 50 to 60 km/h, while a violent tornado can travel at a speed of about 400 km/h. A tornado is a natural disaster, but not as dangerous as a cyclone. A tornado shelter is a room situated underground or in the interior of a building with no windows. If tornado shelter is not available, it is advisable to shut the windows and take shelter under a table.

#### Summary

Thunderstorms Storms are severe atmospheric disturbances accompanied by very strong, high-speed winds. Thunderstorms are whether conditions characterised by thunder, lightning and heavy rain. They occur due to the convection of air in hot and humid tropical areas. As the air near the earth's surface warms up, it rises. This results in strong upward rising winds which carry moisture with them. The moisture condenses into water droplets in the cooler, upper regions of the atmosphere. The water droplets fall down as rain or freeze and fall as hail. The rising air and the rapidly falling water droplets and ice crystals collide with each other and produce static electric charges. As a result the clouds get charged. When electric discharge occurs between the clouds or between a cloud and the ground, lightning occurs. The lightning heats up the air in its path very quickly and this results in a loud sound which we hear as thunder.  Precautions During a Thunderstorm      •  Move away from open garages, metal sheds and water bodies.      •  You may sit inside a car, a bus or a closed vehicle, or inside a building.      •  Do not take shelter under an isolated tree. If in a forest, take shelter under a small tree.      •  Do not take shelter under an umbrella with a metallic end.      •  Do not sit near an open window.      •  If you are in water (swimming pool, river, etc.) get out and go inside a building. Cyclone A cyclone is a natural calamity caused by difference in air pressure in the atmosphere. It is a violently rotating wind storm. It develops over the sea and may move over the land causing great damage. Cyclones begin as thunderstorms and later develop into cyclones. They are associated with strong winds, heavy rains and tidal waves.  In tropical regions like India, thunderstorms are common, but very few thunderstorms become 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 in Philippines and Japan it is called 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. Humidity is the amount of water vapour in the atmosphere. As the humidity in a region increases, the difference in temperature also increases. A cyclone arises due to the difference in the temperatures between 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 rises, it cools, condenses and forms clouds. When the water vapour in the clouds turns into rain drops, 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. Falling water droplets and rising air move vigourously to produce thunderstorm, which develops into a cyclone under certain weather conditions. The calm and clear area at the centre of a cyclone is called the eye of the cyclone.  Tornado 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 of America. Tornadoes occur regularly in the regions around the Atlantic Ocean. Weak tornadoes travel with wind speeds of 50 to 60 km/h, while a violent tornado can travel at a speed of about 400 km/h. A tornado is a natural disaster, but not as dangerous as a cyclone. A tornado shelter is a room situated underground or in the interior of a building with no windows. If tornado shelter is not available, it is advisable to shut the windows and take shelter under a table.

#### Activities

 Activity 1 Wiley.com has developed an animation to explain how thundersorms are formed. It narrates the conditions under which a thunderstorm develops, how hailstones are formed and so on. Go to Activity Activity 2 Nhc.noaa.gov has created an animation which depicts the intensity of damage produced by a cyclone (hurricane) as a function of the wind speed. There are five catagories in the increasing order of the wind speed. Go to Activity

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