 The air that surrounds the earth is constantly placing pressure on the earth's surface. The pressure in the earth's atmosphere is measured using an instrument called a barometer. There are two general types of barometer: a mercury barometer and an aneroid barometer. Barometric pressure is most often recorded in inches of mercury, millibars, or kilopascals.
Changes in air pressure are critical in forecasting the weather. As the air masses move across the earth, the air pressure changes in a fairly reliable manner. When the air pressure is low, warm, moist air is rising and moving counterclockwise toward the center, creating less pressure. Because of this, a barometer usually shows falling pressure as a storm system approaches. In contrast, when the air pressure is high, cold, dry air is falling and moving clockwise toward the center, creating greater pressure. Because of this, rising pressure usually indicates cooler weather and clear skies. |
