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Air Pressure

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Air Pressure - Lesson Summary

Air Pressure

Air exerts pressure in all directions.  The pressure exerted by air is called the air pressure, which is the thrust exerted by air per unit area.  The weight of the air acting on a unit surface area is called the atmospheric pressure.  

To show that air exerts pressure, take a glass and fill it with water. Cover the mouth of the glass with an index card. Now hold the card in place and invert the glass over a sink and remove your hand from the card. The card sticks to the glass. This is because air exerts pressure on the card  from below to keep the card in place. Here the pressure exerted by air upwards is more than the pressure exerted by water downwards. 


Air expands on heating and contracts on cooling. 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. This is due to convection, a mode of transfer of heat. As warm air rises up, reducing air pressure at that place, cool air moves in to take the place of warm air. Moving air is called wind. Winds are caused by variations in air pressure. A wind blows from a region of high pressure to a region of low pressure. The speed of the wind mainly depends on the difference between the pressures of the air in the two regions. When air moves at high speeds, it creates a low pressure area. High speed winds are known to blow away thatched and tiled roofs in rural and semi-urban areas. Strong winds can uproot trees and electric poles, and even snap cables.


The instrument used to measure the speed of wind is called an anemometer. It is usually fixed on top of a building.


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