Gases exert pressure on the surrounding limiting surfaces. Pressure force, directed perpendicularly to the surface, per one area unit is called pressure. Pressure generated by the air column on the Earth surface is known as atmospheric pressure. Currently, the atmospheric pressure value is usually expressed in hectopascals (hPa), however, other units, e.g. conventional millimetre of mercury (mmHg) or millibars (mb) are sometimes in use as well. Proportions between these units are presented as follows:
1 hPa= 100 Pa= 100 N⁄m2 = 100 kg⁄(m∙s2)
1 atm = 1013 hPa = 1013 mb = 760 mmHg
The value expressed as mmHg means that the mercury column, which height is 760 mm, exerts the same pressure on a horizontal surface as the whole average height of the atmosphere (approximately 100 km). This proportion is used in pressure measurement devices – mercury barometers.
The value 1013 hPa, which is presented above, corresponds to the average atmospheric pressure at sea level, environmental temperature 0°C, latitude 45°.
Because the mass of air floating above decreases along with height, the pressure which is exerted on the Earth surface decreases as well. In order to compare the values from spots of different heights and to determine scopes of baric centre, values reduced to the sea level are being applied. To obtain this quantity, it is assumed that in the bottom troposphere the pressure drops along with the height at the rate of 8m/hPa. Pressure drop with the height is exponential, however, when it occurs on the Earth surface, it is linear, which enables us to use the approximation presented above. Weather forecasts usually display reduced pressure.
Elaborated by K. Wałaszek