Air exerts a pressure on any surface in the atmosphere which corresponds to
the weight of the column of air that it supports. Every surface in the
neighborhood of sea level carries a load of about 1 kg per cm², or 1 ton per
ft². As the altitude increases above sea level, the atmosphere below no longer
contributes to the pressure, which is correspondingly reduced.
Using this concept, atmospheric pressure can be expressed as the height of a
column of mercury in a barometer, in millimeters or inches, with the pressure at
sea level being 760 mm or 29.9 inches of mercury at a standard temperature of 0
°C (32 °F). The barometer reading must be corrected for the temperature of the
mercury as well as for the latitude.
The bar is the unit of pressure in an absolute system of measurement adopted
for scientific use to replace the arbitrarily chosen column of mercury.
Atmospheric-pressure measurements in meteorological work are normally expressed
in units of one millibar. One bar corresponds very nearly to 750 mm or 29.5
inches of mercury at 0 °C (32 °F), or 1019 cm or 401 inches of water, which is
the atmospheric pressure a little above sea