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Atmospheric Pressure
The air in the atmosphere of the Earth exerts a pressure on the surface of the Earth. Areas of the atmosphere with greater density exert a higher pressure than areas with lower density. Air pressure is measured with barometers, leading to the term barometric pressure.
Highs and Lows
The higher-density air masses are associated with high barometric pressure (highs) while boundary areas between these air masses are associated with lower-barometric pressure (lows). The barometric pressure rises and drops with the approach and dissipation of low-pressure boundaries.
Measuring Air Pressure
Early barometers consisted of a J-shaped glass tube open at one end and closed at the other. These barometers were filled with mercury, and changes in air pressure produced a corresponding change in the height of the mercury in the tube. Aneroid barometers that use a bellows have replaced many mercury barometers but barometric pressure is often still given in inches or millimeters of mercury, though millibars and hectopascals are increasingly being used.