Class 7 Science Winds Storms and Cyclones Chapter Notes

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Winds, Storms and Cyclones

2.1 Introduction
Moving air is called Wind. If the speed of wind is low, it is pleasant. If the speed is high, it becomes destructive.

2.2 Air Exerts Pressure
Take a tin can with a lid. Fill it approximately half with water. Heat the can on a candle flame till the water boils. Blow out the candle. Immediately put the lid tightly on the can. Put the can carefully in a shallow metallic vessel or a washbasin. Pour fresh water over the can. As water is poured over the can, some steam in the can condenses into water, reducing the amount of air inside. The pressure of air inside the can decreases than the pressure exerted by the air from outside the can. As a result the can gets compressed.

Activity 1

Crumple a small piece of paper into a ball of size smaller than the mouth of an empty bottle. Hold the empty bottle on its side and place the paper ball just inside its mouth. Now try to blow on the ball to force it into the bottle. When we blow into the mouth of the bottle, the air near the mouth has higher speed. This decreases the pressure there. The air pressure inside the bottle is higher than near the mouth. The air inside the bottle pushes the ball out.

Activity 2

When you blow between the balloons, they moved towards each other. This happens because pressure of air between the balloons were reduced. The pressure outside the balloons push them towards each other.

Activity 3

You saw that when you blew over the paper strip, it went upwards. This could happen if blowing over the paper reduced the air pressure above the strip.

We see that the increased wind speed is, accompanied by a reduced air pressure.

2.3 Movement of Air
When air moves, it is called wind. Air moves because of difference in the air pressure between two region. Air moves from the region where the air pressure is high to the region where the pressure is low. The greater the difference in pressure, the faster the air moves. On heating, the air expands and occupies more space. When the same thing occupies more space, it becomes less denser. The warm air is, therefore, lighter than the cold air. In nature there are several situations, where warm air rises at a place. The air pressure at that place is lowered. The cold air from the surrounding areas rushes in to fill its place. This sets up convention in air.

2.4 Wind Currents
The uneven heating between the equator and the poles generate wind currents. The air near equator regions gets warm. The warm air rises up, and the cooler air from the regions in the 0 – 30° latitude belt on either side of the equator moves in. In this way, wind circulation is set up from the poles to the
warmer latitudes.

The uneven heating of land and water also generate wind currents. 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. These are monsoon winds.

In winter, the direction of the wind flow gets reversed, it flows from the land to the ocean.

2.5 Thunderstorms and Cyclones
Thunderstorms develop in hot, humidtropical areas like India. The rising temperature produces strong upward rising winds. The swift movement of the falling water droplets along with the rising air create lightning and sound. This is called thunderstorm.
When water vapour changes back to liquid form as raindrops, heat is released to the atmosphere which 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. 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.

2.6 Structure of Cyclone

The centre of a cyclone is a calm area. It is called the eye of the storm. A large cyclone is a violentlyrotating mass of air in the atmosphere, 10 to 15 km high. The diameter of the eye varies from 10 to 30 km.
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’.

2.7 Destruction Caused by Cyclones Some of the harmful effects of cyclones are as follows:

i) Sea water entered in low coastal areas can damage human and animal life and property.

ii) Cyclones can reduce the fertility of soil.

iii) Frequent rains caused by cyclones can result in floods.

iv) High speed winds caused by cyclones can damage houses, telephone lines and other communication systems.

2.8 Tornadoes

A tornado is a dark funnel shaped cloud that reaches from the sky to the ground. Most of the tornadoes are weak. A violent tornado can travel at a speed of about 300 km/h. Tornadoes may form within cyclones.

2.9 Effective Safety Measures

Some of the safety measures in case of cyclones are as follows:

• A cyclone forecast and warning service.

• Rapid communication of warnings to the Government agencies, the ports, fishermen, ships andto the general public.

• Construction of cyclone shelters in the cyclone prone areas, and Administrative arrangements for moving people fast to safer places.

Action on the part of the people

• We should not ignore the warnings issued by the meteorological department through TV, radio, or newspapers.

• We should make necessary arrangements to shift the essential household goods, domestic animals and vehicles, etc., to safer place.

• We should keep ready the phone numbers of all emergency services like police, fire brigade, and medical centres.

• Do not drink water that could be contaminated. Always store drinking water for emergencies.

• Do not touch wet switches and fallen power lines.

• Do not pressurise the rescue force by making undue demands.

• Cooperate and help your neighbours and frien

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