Air Pressure

Air pressure is the result of the weight of a column of air pushing down on an area. The column of air extends upward through the entire atmosphere. The atmosphere is heavy. The weight of the column of air above your desk is about the same as the weight of a large schoolbus. So why doesn't air pressure crush your desk? The reason is that the molecules in air push in all directions - down, up, and sideways. The air pushing down on top of your desk is balanced by the air pushing up on the bottom of your desk. Air pressure can change from day to day. A denser substance has more mass per unit volume than a less dense one. So denser air exerts more pressure than less dense air.

 

A mercury barometer consists of a glass tube open at the bottom end and partially filled with mercury. The space in the tube above the mercury is almost a vacuum - it contains very little air. The open end of the tube rests in a dish of mercury. The air pressure pushing down on the surface of the mercury in the dish is equal to the pressure exerted by the weight of the column of mercury in the tube. When the air pressure increases, it presses down more on the surface of the mercury. Greater air pressure forces the column of mercury higher. At sea level the mercury column is about 76 centimeters high, on average.

 

 

 

 

An aneroid barometer has an airtight metal chamber. The metal chamber is sensitive to changes in air pressure. When air pressure increases, the thin walls of the chamber are pushed in. When the pressure drops, the walls bulge out. The chamber is connected to a dial by a series of springs and levers. As the shape of the chamber changes, the needle on the dial moves.