CLASS 7 SCIENCE CHAPTER 8 WINDS, STORMS AND CYCLONES NCERT EXAM NOTES

Winds, Storms and Cyclones:

SHORT NOTES:

Historical accidents and detriments:

• Orissa was hit by a cyclone with wind speed of 200 km/h on 18 October 1999.
• The cyclone smashed 45,000 houses making 7,00,000 people homeless.
• On 29 October the same year, a second cyclone with wind speed of 260 km/h hit Orissa again.
• It was accompanied by water waves about 9 m high. Thousands of people lost their lives.
• Property worth crores of rupees was destroyed.
• The cyclone affected agriculture, transport, communication, and electricity supply.

Air Exerts Pressure:

• Air has weight, and pressure. 

• The particles of air push in all directions and the force that is exerted is called air pressure.

Examples:

• 1. The balloon expands as we fill air into it. The balloon expands due to air pressure. When it blown more and more, it bursts eventually due to air pressure. The air pressure inside the balloon is equal to the atmospheric pressure.
• 2. When we fill mouth by air and close it. And then press the cheeks with the fingers. That air inside the mouth exerts pressure on the fingers.
• 3. Cycling against the wind.

Wind

• The moving air is called the wind.
• Air moves from the higher air pressure region to the lower air pressure region.
• The greater the difference in pressure, the faster the air moves.
• Heating the air expands and occupies more space.
• When the same thing occupies more space, becomes lighter.
• Therefore, the warm air is lighter than the cold air. That’s why the smoke goes up.
• In nature, there are many situations of warm air rise at a place.
• The air pressure at that place is lowered.
• The cold air from the surrounding areas fill its place and sets up convection in air.

Wind Currents are Generated Due to Uneven Heating on the Earth.

These situations are:

(a) Uneven heating between the equator and the poles:

• The regions close to the equator get maximum heat from the Sun.
• The air in these regions gets warm and rises.
• And the cooler air moves in from the regions in the 0–30 degrees latitude belt on either side of the equator.
• 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.
• The winds would have flown in the north-south direction from north to south, or from south to north.
• A change in direction is however, caused by the rotation of the earth.

(b) Uneven heating of land and water:

• In summer, near the equator the land warms up faster.
• 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.
• The word monsoon is derived from the Arabic word ‘mausam’, which means ‘season’.
• In winter, the wind direction flow gets reversed(it flows from the land to the ocean).
• The winds from the oceans carry water and bring rain. It is a part of the water cycle.
• The monsoon winds carry water and it rains.
• Clouds bring rain and give us happiness.
• Farmers depend mainly on rains for their harvests.
• Uneven heating of land especially the Rajasthan desert generates monsoon winds from southwest 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 northwest colder land.
• These colder winds carry little water, hence bring small amount of rain in winter.

Thunderstorms and cyclones

Certain situation can create disasters and pose threat to humans, animals and plant life.

Thunderstorms

• Thunderstorms develop in hot, humid tropical areas like India very frequently.
• The rising temperatures produce strong upward rising winds carry water droplets upwards, where they freeze, and fall down again.
• The swift movement of the falling water droplets with the rising air create lightning and sound called thunderstorm.

Cyclones

Structure of a cyclone:

• The centre of a cyclone is a calm and clear area 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 the 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.

How a thunderstorm becomes a cyclone?

• Before cloud formation, water takes up heat 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 with the formation of a very low-pressure system with very high-speed winds revolving around it.
• This weather condition is called cyclone.
• Factors like wind speed, wind direction, temperature and humidity helps to the development of cyclones.

Destruction caused by cyclones:

• Cyclones can be very destructive.
• Strong winds push water towards the shore even if the storm is hundreds of kilometres away.
• These are the first indications of an approaching cyclone.
• The water waves formed by the wind are so powerful (person can’t overcome them).
• The low pressure in the eye lifts water surface in the centre.
• The rising water may be as high as 3–12 metres.
• It appears like a water-wall moving towards the shore.
• As a result, the seawater enters the low-lying coastal areas, causing severe loss of life and property, reduces the fertility of the soil.
• Continuous heavy rainfall may further worsen the flood situation.
• High-speed winds accompanying a cyclone can damage houses, telephones and other communication systems, trees, etc.,
• That means, causing tremendous loss of life and property.
• 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’

Tornadoes:

• Very rare in our country.
• It is a dark funnel shaped cloud reaches from the sky to the ground.
• Most of the tornadoes are weak.
• A violent tornado can travel at speeds of about 300 km/h. Tornadoes may form within cyclones.
• The whole coastline of India is vulnerable to cyclones, particularly the east coast.
• The west coast of India is less vulnerable to cyclonic storms(In terms of intensity and frequency of the cyclones).
• All storms are low-pressure 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.

Structure of tornado:

The diameter of a tornado can be so small in metre and so large in km, or wider. It’s funnel sucks dust, debris and everything near it at the base (due to low pressure) and throws them out near the top.

Effective safety measures:

• 1. A cyclone forecast and warning service.
• 2. Rapid communication of warnings to the Government agencies, the ports, fishermen, ships and to the general public.
• 3. Construction of cyclone shelters in the cyclone prone areas, and Administrative arrangements for moving people fast to safer places.

Actions should be taken by us:

• 1. We shouldn’t ignore the warnings issued by the meteorological department through TV, radio, or newspapers.
• 2. We should —Make necessary arrangements to shift the essential household goods, domestic animals and vehicles, etc. to safer places.
• 3. Avoid driving on roads through standing water, as floods may have damaged the roads.
• 4. Keep ready the phone numbers of all emergency services like police, fire brigade, and medical centres.

Some other precautions, if staying in a cyclone hit area:

• 1. Do not drink water that could be contaminated. Always store drinking water for emergencies.
• 2. Do not touch wet switches and fallen power lines.
• 3. Do not go out just for the sake of fun.
• 4. Do not pressurise the rescue force by making undue demands.
• 5. Cooperate and help your neighbours and friends.

Advanced technology has Helped:

• We are thankful to satellites and radars, a 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.