Air Pressure


The pressure exerted by air on all bodies at all times in all directions is called air pressure. When air moves at high speeds, it creates a low pressure area. 
The air inside a balloon exerts pressure in all directions, and makes it blow up.  Air opposes the motion of a moving object. This is called air resistance. That is why you have to exert yourself and pedal hard when you ride a bicycle. 

Another example in which air pressure can be observed: Take a glass and fill only 1/3 of 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 due to air pressure. The air outside exerts an upward pressure on the index card. This air pressure is more than the weight of the water in the glass.


Must Read : All About Air

Winds


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.
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. 

Anemometer


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


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