Information

Pressure

Atmospheric pressure

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 0 °C and latitude 45°. Under normal conditions the following is valid 1 mmHg = 1.3332 hPa.
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 8 m equals 1 hPa of pressure change. To have comparable readings of the atmospheric pressure for the weather observation all readings are given at sea level. Therefore the influence of the altitude to the atmospheric pressure is to be corrected.
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.

Equation pressure

pabs = absolute atmospheric pressure [hPa]
pred = atmospheric pressure with altitude reduced to sea level [hPa]
Δp = pressure difference [hPa]
h = altitude [m above sea level]
 
h [m NN] Δ p [hPa] h [m NN] Δ p [hPa]
100 12,0 1100 125,4
200 23,8 1200 136,1
300 35,5 1300 146,8
400 47,2 1400 157,3
500 58,7 1500 167,7
600 70,1 1600 178,1
700 81,3 1700 188,3
800 92,5 1800 198,4
900 103,6 1900 208,4
1000 114,5 2000 218,4

 
 

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