Atmospheric Pressure force exerted by the weight of the air Atmospheric pressure is defined as the force per unit area exerted against a surface by the weight of the air molecules above that surface. The opposite is also true, where a reduction in the number of air molecules above a surface will result in a decrease in pressure. Atmospheric pressure is measured with an instrument called a barometer, which is why atmospheric pressure is also referred to as barometric pressure. The pressure applied by this air on the unit area would be 14.7 pounds per square inch.
Meteorologists use a metric unit for pressure called a millibar.
Average pressure at sea level is 1013.25 millibars. Terms for using data resources. Credits and Acknowledgments for WW2010.