The bar is a measurement unit of pressure, equal to 1,000,000 dynes per square centimetre (baryes), or 100,000 newtons per square metre (pascals). The word bar is of Greek origin, báros meaning weight. Its official symbol is "bar"; the earlier "b" is now deprecated, but still often seen especially as "mb" rather than the proper "mbar" for millibars.
The preferred SI unit of pressure is the pascal (Pa), which is one newton per square metre. 1 bar is equal to 100,000 Pa. The bar is still widely used by the general public and in industry (in varying extents by geographic location and application) because 1 bar is so close to atmospheric pressure and for everyday purposes can be taken as equivalent.
Atmospheric air pressure is often given in millibars, where "standard" sea level pressure is equal to 1013.25 millibars (1.01325 bar). If the measure is given in SI units, the hectopascal (hPa) gives the same number as the millibar measure, i.e. 1000 mb = 1000 hPa. Thus the millibar and the hectopascal are directly interchangeable. In SI units the preferred multipliers are 1000's, hence the preferred unit is the kilopascal (kPa; 1 bar = 100 kPa) and it is occasionally used in meteorological publications, is normal usage for public weather reports in Canada, and is common in other sciences such as chemistry. However hPa remains more common than kPa in meteorology.
In everyday use pressure is often measured with reference to atmospheric pressure: this is gauge pressure and denoted by barg, spoken "bar gauge" and sometimes using symbols such as 'barg' or 'bar(g)'. For example if someone says that their car tyres are pressured up to 2.3 bar they actually mean bars gauge: the pressure in the tyre is really 3.3 bar, but only 2.3 bar above atmospheric. When absolute pressure is desired, then it is sometimes denoted 'bara' or 'bar(a)' for "bar absolute". The alteration of the symbols of units of measure for this purpose is now deprecated.
See also
