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Winds - Lesson Summary

Air expands on heating and contracts on cooling. To prove this we can perform the following activity. Take two deep pans - one filled with hot water and the other with cold water. 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 gets 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 containing cold water. The balloon deflates and shrinks. This is because the air inside the bottle gets cooled and hence contracts.

Warm air, being lighter than cool air, rises up, whereas relatively cooler air, being heavier than the warm air, sinks towards the earth's surface. As warm air rises up, air pressure at that place reduces and cool air moves in to take the place of warm air. Moving air is called wind. Air moves from the region of high air pressure to the region of low air pressure. The greater the difference in air pressure, the faster the air moves. The increased wind speed is accompanied by a reduced air pressure.

Wind Currents

Wind currents occur due to (a) uneven heating between the regions near the equator and the poles and the rotation of the earth and (b) uneven heating of land and water in coastal areas. The equatorial and tropical regions get hotter than the polar regions as they get maximum heat from the sun. 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 the circulation of wind across the globe. But the wind flow is not in the exact north-south direction because a change in direction is caused by the rotation of the earth.

Monsoon Winds

The word monsoon is derived from the Arabic word "mausam," which means season. The winds from the ocean carry water in the form of water vapours, resulting in rains over land. These are called monsoon winds.


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