Winds, Storm and Cyclones
Science Class Seven
What is wind?
When air moves wind is created.
We perceive wind by observing fluttering of leaves, flags, etc. We feel wind when it strikes our body. We can sense wind by our skin when wind passes near to us.
Thus, it can be defined as "moving air is called wind".
Air Exerts Pressure
Air exerts pressure. It can be observed or proved by some of the activities.
Activity to show air exerts pressure
Activity (1)
• Take an empty tin can.
• Put some water into it.
• Heat the can over a gas stove or an electric heater.
• When water starts boiling, put off the gas stove, and close the lid carefully so that the hand could not be burnt.
• Allow it to cool. Or pour cold water over it.
• After cooling, the tin can gets distorted inward.
This happens because while boiling water, the air inside the can be expelled out with the steam produced by boiling water. And after getting cooled down, some of the steam present in the can gets condensed and changes into water. This reduces the air pressure inside the tin can, while the air in the surrounding exerts the same atmospheric pressure on the tin can. This resulted in the distortion of the tin can inwards.
Activity (2)
• Take a glass.
• Fill the glass with water.
• Take a postcard or rectangular or square-shaped cardboard that can cover the opening of the glass completely.
• Cover the glass with cardboard.
• Flip the glass vertically holding the cardboard with one hand.
• Now, slowly remove the hand from the cardboard.
After removing the hand below the cardboard, cardboard does not fall from the mouth of the glass, it gets stuck with the glass.
This happens because air present in the surroundings exerts pressure on the cardboard and prevents it from falling.
These activities show that air exerts pressure.
There are many other activities and phenomena that show air exerts pressure.
When air is blown in the cycle tyre, it gets inflated. This happens because air exerts pressure on the walls of tyre from inside.
When we blow air over the small pieces of paper, they get displaced from their original place. This happens because air exerts pressure.
What is pressure?
Pressure is the force which exerts per unit area.
How much pressure is exerted by the atmosphere over our head?
There are about 14 pounds of pressure exerted per square inch over our head at sea level. 14 pound is equal to 6.35 kg.
But we do not feel it.
We do not feel the pressure exerted over our bodies. This is because the pressure exerted by blood from inwards balances the pressure exerted by the atmosphere. And the force becomes nullified. And we do not feel any pressure.
Mountaineers usually suffer nose bleeding.
Some mountaineers suffer nose bleeding when going at high altitudes.
This happens because air pressure decreases with high altitude, while the pressure exerted by blood inside our body remains the same.
Thus, because of lesser pressure outside than the pressure inside the body, sometimes the blood vessels of mountaineers burst and their nose starts bleeding.
High-Speed Wind Are Accompanied By Reduced Air Pressure
We know that air exerts pressure, consequently, the high-speed wind would create more pressure and would be accompanied by more pressure. But actually, high-speed wind creates reduced air pressure.
This happens because air volume decreases with the high speed of wind which reduces the pressure. In other words, high-speed wind creates a vacuum because of this, the pressure reduces at the place of high-speed air.
This can be understood by blowing air in a bottle after keeping a small piece of paper scrambled inside near the mouth of a bottle.
When air is blown inside a bottle after keeping a small piece of scrambled paper inside the mouth, the paper does not go inside the bottle but rather sometimes expels out. This happens because high-speed air creates an area of low pressure.
Thus, increased wind speed creates a reduced pressure zone.
Flow of Air
Air moves from high-pressure regions to low-pressure regions.
In the low-pressure region, a vacuum is created thus in order to fulfill the vacuum, air from the high-pressure regions rushes toward the low-pressure region, and in this way air flows.
The greater the difference in pressure the speed of the wind will be greater and vice versa.
Why is the region of low pressure created?
Air Expands on Heating
Developing low-pressure region on the earth accompanied by the amount of sun shines received by a particular place.
Air expands on heating. When a particular place receives more sun shines, the air at that place becomes hotter. Particles of air expand after getting heat. On expansion, air occupies more space and becomes lighter, and rises up.
Rising air creates a vacuum consequently a low-pressure zone is created at that place. To fill up a vacuum or low-pressure zone at a particular place, air from surrounding areas rushes towards it. This causes the wind to blow.
The air blows because of the convection of air.
Thus, warmer air is lighter than colder.
Balloons work on this principle. In a balloon, the air is heated using a gas burner. On heating, air becomes lighter, which lifts the balloon into the air.
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