NCERT Notes for Class 8 | NCERT Quiz | NCERT Standard 8 - Science


Chapter 8. Wind, Storm and Cyclones

Wind : 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 greater the difference in pressure, the faster the air moves.

Why smoke always rises up : 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. 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.

Uneven heating in Equator and Polar regions : 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. Uneven heating of land especially the Rajasthan desert generates monsoon winds from 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 colder land. These colder winds carry little water, hence bring small amount of rain in winter.

Thunderstorms : 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. It is this event that we call a thunderstorm.  

Cyclone : 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 ith the formation of a verylow-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.
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.  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. The whole coast line 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.

Tornadoes :  In our country they are not very frequent. 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. 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. Tornadoes may form within cyclones also.

Anemometer : All storms are lowpressure systems. 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.

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