The air in its state of rest exerts a pressure directed perpendicular to each, any oriented surface. The static pressure acting on the surface of the earth is called atmospheric pressure. It is caused by the weight of a column of air reaching from the earth surface up to the top of the atmosphere. Atmospheric pressure is measured in hectopascals (hPa) or as the length of a column of mercury (mmHg) at
To measure atmospheric pressure, aneroid barometers use the deformation of a flat circular metal box exhausted of air with faces corrugated for strength (aneroid capsule). Variations of the external atmospheric pressure cause changes in the displacement of the walls of the box as long as a new equilibrium position is reached so that there is equilibrium between the elastic walls and the atmospheric pressure.
Absolute air pressure and air pressure corrected to sea level
With increasing altitude the column of air reaching from the earth surface up to the top of the atmosphere gets shorter and therefore the absolute atmospheric pressure drops. Near the earth surface a difference in altitude of about
The conversion from the absolute atmospheric pressure to the atmospheric pressure at sea level is done by adding a value of pressure in accordance to the altitude difference.
|h [m NN]||Δ p [hPa]||h [m NN]||Δ p [hPa]|