Rajasthan Board RBSE Class 8 Science Notes Chapter 15 Natural Phenomena
Air Exerts Pressure:
Take a plastic bottle, fill it half with hot water, empty it, and immediately tightly close its with lid/cap. Pour cold water on bottle on observation it is seen that bottle sticks. Why is this happen ? It is so because some water vapour inside the bottle cools and converts into water. It results that inner air pressure of bottle decreases compared to outer air pressure. Due to differences in air pressure bottle sticks. Flying of kite, bursting of balloon, hosting of flag, blowing of dust and rustling leaves etc occur due to air pressure. All these activities proves that air exerts pressure.
Effect of Velocity of Air on Air Pressure:
Take an empty bottle, fold a piece of paper and make a ball of it smaller than the mouth of the bottle. Keep it near the mouth of the bottle, blow air with mouth and try to push it inside the bottle. Repeat this process with different size of mouth bottles.
Why is it difficult to push ball inside the bottle?
Due to blow of air, velocity of air increased at the mouth which results in decreasing air pressure. The air pressure inside the bottle is more than near the mouth of the bottle. Hence inside air of the bottle pushes the ball out of the bottle.
Take two same size balloon and fill both with little water, inflate air in both balloons and tie their mouth with thread. Hang the balloons on a wooden rod approx 10 cm distance. Blow between balloons both the balloons come close to each other, why this happens? Due to blow of air pressure between the balloons decreases. While more air pressure on the other side of the balloons pushes them towards each other. These two activities proves that when velocity of air increases air pressure decreases.
Why air blows?
Non – Uniform Heating of Earth:
Earth is not heated uniformly. Parts near the equator get more heat of the sun. Hence surrounding air becomes hot. It rises upwards to take their place. Cool breeze from polar region blows towards equator. Due to inequality of hotness air blows from colder to hotter regions.
Non – Uniform heating of Land and Water:
Land becomes hotter than water. In summers land becomes hot. Temperature of surface of the earth is more than oceanic water. The hot air surrounding the earth rises up and cool sea breeze. It starts blowing towards land. They are called monsoon winds also. Speedy air is wind or moving air is called breeze.
When temperature rises, then hot air rises upward, it takes water vapour along it. Due to low temperature water vapour condenses in the form of water. Droplets of water comes down with great speed, due to interaction between air and water droplets, lightning occurs. It creates sound. It is called thunder lightning.
Measures to protect from thunder lightning:
- Don’t stay in open, go inside a building.
- Don’t come out of the houses.
- Don’t do any activity like washing clothes, bathing etc.
- Take shelter in cars or buses.
- Don’t stand under tall and single trees.
- Don’t lay down in open land.
On 12 October 2013, 9° clock at night, wind of 220 km/hr speed striked with great momentum to the sea coast of Odisha, started with heavy rains, very high oceanic waves originated at shores, oceanic water entered in the coastal region. It destroyed building, lacs of hectare land submerged in water, crops were destroyed, electric poles were rooted, darkness spread everywhere, many lost their lives, such natural phenomena is called as cyclone. In America it is called as Hurricane and in Japan ‘Typhoon’.
How do cyclone originate?:
Water converts into vapour in atmosphere after taking heat. When this vapour condenses and again convert into water droplets. Heat is released in the atmosphere, it makes the surrounding air hot. This hot air rises up, result decrease of air pressure, to take place of hot air more speedy air started moving towards the in direction of hot air. This cycle is repeated many times, this repetition stops when a low air pressure area is evolved. Around this area many layer of air starts moving in spiral form such condition of weather is called cyclone. Its centre is calm and called eye of the cyclone.
Safety measures at social and government level:
- Forecasting of cyclone by the meteorological department of government.
- Speedy communication system to give warning to the fishermen, steamers, government, institutions, people living in the coastal regions.
- Speedy rescue operation system to shift people at safe places.
Activities done by people:
- We should not ignore governmental warnings transmitted on television, radio, newspapers, meteorological department etc.
- We should do arrangement of shifting essential domestic goods, domestic animals and vehicles at safer places.
- Avoid driving on roads submerged under water.
- We should keep the list of telephone numbers of emergency services like police, fire brigade, hospitals, health centres with us.
People living in cyclone prone areas should keep following precautions:
- Store drinking water in adverse situation.
- Don’t touch broken electricity wires, wet electric switches.
- Give co-operation and help to neighbors and friends as far as possible.
It is dark colored funnel shaped clouds. They look coming down towards the surface of the earth from sky. They move with less than 180 km/hr speed. They are weak. The eastern coast of India is more prone or sensitive specially to cyclones. In our country tornado are rare.