Calculation of atmospheric pressure in various units
For ordinary laboratory purposes, atmospheric pressure is measured in mmHg by a Fortin barometer (page 113). The average value of the barometeric height at sea-level over a long period is 760 mmHg, and this has been taken as a unit of pressure and called standard atmospheric pressure or one atmosphere. The unit of pressure used for meteorological purposes is called the bar and is defined as a pressure of 105 Njm2. The more commonly used sub-unit is the millibar or one-thousandth of a bar, thus:
Note. As a matter of interest, atmospheric pressures over the British Isles rarely go below 960 mbar or above 1040 mbar.