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Concept
Bar (unit)
Summary
The bar is a metric unit of pressure, but not part of the International System of Units (SI); it is defined as 100,000 Pa (100 kPa). A pressure of 1 bar is slightly less than the current average atmospheric pressure on Earth at sea level (approximately 1.013 bar). By the barometric formula, 1 bar is roughly the atmospheric pressure on Earth at an altitude of 111 metres at 15 °C.
The bar and the millibar were introduced by the Norwegian meteorologist Vilhelm Bjerknes, who was a founder of the modern practice of weather forecasting, with the bar defined as one megadyne per square centimeter.
The International System of Units, despite previously mentioning the bar, now omits any mention of it. The bar has been legally recognised in countries of the European Union since 2004. The US National Institute of Standards and Technology (NIST) deprecates its use except for "limited use in meteorology" and lists it as one of several units that "must not be introduced in fields where they are not presently used". The International Astronomical Union (IAU) also lists it under "Non-SI units and symbols whose continued use is deprecated".
Units derived from the bar include the megabar (symbol: Mbar), kilobar (symbol: kbar), decibar (symbol: dbar), centibar (symbol: cbar), and millibar (symbol: mbar).
The bar is defined using the SI derived unit, pascal: 1bar ≡ 100,000 Pa ≡ 100,000 N/m2.
Thus, 1bar is equal to:
1,000,000 Ba (barye) (in cgs units);
and 1 bar is approximately equal to:
1019.716 centimetres of water (cmH2O) (1 bar approximately corresponds to the gauge pressure of water (not absolute) at a depth of 10 meters).
Notes:
1 millibar (mbar) = 1 one-thousandth bar, or 1e-3bar
1 millibar = 1 hectopascal (1 hPa = 100 Pa).
The word bar has its origin in the Ancient Greek word βάρος (), meaning weight. The unit's official symbol is bar; the earlier symbol b is now deprecated and conflicts with the use of b denoting the unit barn, but it is still encountered, especially as mb (rather than the proper mbar) to denote the millibar.
Official source
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