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Atmospheric Pressure

An aneroid barograph and a mercury barometer are two ways to measure atmospheric pressure.

Atmospheric pressure is the amount of force exerted over a surface area, caused by the weight of air molecules above it. A elevation increases, fewer air molecules are present. Therefore, atmospheric pressure always decreases with increasing height. A column of air, 1 square inch in cross section, measured from sea level to the top of the atmosphere would weight approximately 14.7 pounds per square inch. The standard value for atmospheric pressure at sea level is: 29.92 inches or 760 millimeters of mercury.

Air pressure can be measured with a mercury barometer, an aneroid barometer, or an aneroid barograph. A standard mercury barometer has a glass column about 30 inches long, closed at one end, with a mercury-filled reservoir. Mercury in the tube adjusts until the weight of the mercury column balances the atmospheric force exerted on the reservoir. High atmospheric pressure forces the mercury higher in the column. Low pressure allows the mercury to drop in the column.

An aneroid barometer uses a small, flexible metal box called an aneroid cell. The box is tightly sealed after some of the air is removed, so that small changes in external air pressure cause the cell to expand or contract.

An aneroid barograph consists of a revolving drum on which the air pressure is recorded, a syphon cell that expands or contracts as air pressure rises and falls, and a lever that transfers the movement of the syphon cell to the recording pen that writes on the revolving drum. It is found in weather stations because it provides a permanent record of pressure readings.