Notes

Air pressure is a measure of the weight of air pushing down on the Earth. The pressure varies from place to place and from time to time. The average pressure at sea level is about 14.7 pounds per square inch. This average pressure means that the weight of a column of air over an area of one square inch is about 14.7 pounds. To understand how pressure is related to weather, we must look at the instruments used to measure pressure and at how high pressure and low pressure affect each other.

Air pressure is measured with an instrument called a barometer. Barometers are of two types, the mercury barometer and the aneroid barometer. The mercury barometer is made of a long glass tube filled with mercury and sealed at one end. The open end of the tube is inverted into a bowl of mercury. Some of the mercury in the tube will flow into the bowl as gravity pulls on the mercury. Air pressure on the mercury in the bowl tries to push mercury back up the tube. At some point the pull of gravity and the push of the air are equal.