Wind is the flow of gases on a large scale. On the surface of the Earth, wind consists of the bulk movement of air.Air expands on heating, and the expanded air pushes for more space.
1) Take two deep pans - one filled with hot water and the other with cold water. To prove that air expands when heated,
- Stretch a balloon across the mouth of a milk bottle and seal it with tape.
- Keep this bottle in the hot water pan
- The balloon is slowly inflated. This is because the heat from the hot water in the pan heats up the air inside the bottle, which makes the air inside the bottle expand.
Now place the bottle in the pan with cold water. The balloon deflates and shrinks. This is because the air inside the bottle gets cooled and so contracts.
2) Observe a pressure cooker where the steam comes out of the nozzle and escapes upwards. This is because steam escaping from a pressure cooker is lighter than the relative cool air surrounding it. Thus, hot air rises.
Circulation of Wind
The equatorial and tropical regions get hotter than the polar regions. The warm air at the equator rises, and the cold air moves in from the polar regions. The air moves due to uneven heating of the earth’s surface between the equator and poles, which results in circulation of wind across the globe.
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Direction of Wind
Winds are also formed due to uneven heating of land and water in coastal areas. As the earth rotates on its axis from west to east, these wind currents are not exact. There is a difference in the temperatures of the air over sea and over land. Cold winds from the South Pole move north towards the equator, and whereas winds from the North Pole move south towards the equator.
The word monsoon is derived from the Arabic word "mausam," which means season. The winds from the ocean carry water in the form of vapours, resulting rains over land. These are called monsoon winds.