Bar (unit)

For other uses, see Bar (disambiguation).
bar
A pressure of 700 bar flattened this length of aluminium tubing of wall thickness 5 millimetres (0.20 in).
General information
Unit systemMetric system
Unit ofpressure
Symbolbar
Conversions
1 bar in ...... is equal to ...
   SI units   100 kPa
   CGS units   106 Ba
   US customary units   14.50377 psi
   Atmospheres   0.986923 atm

The bar is a metric unit of pressure defined as 100,000 Pa (100 kPa), though not part of the International System of Units (SI). A pressure of 1 bar is slightly less than the current average atmospheric pressure on Earth at sea level (approximately 1.013 bar).[1][2] 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.[3]

The SI brochure, despite previously mentioning the bar,[citation needed] now omits any mention of it.[1] The bar has been legally recognised in countries of the European Union since 2004.[2] 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".[4] The International Astronomical Union (IAU) also lists it under "Non-SI units and symbols whose continued use is deprecated".[5]

Units derived from the bar include the megabar (symbol: Mbar), kilobar (symbol: kbar), decibar (symbol: dbar), centibar (symbol: cbar), and millibar (symbol: mbar).

  1. ^ a b The International System of Units (PDF) (9th ed.), International Bureau of Weights and Measures, Dec 2022, ISBN 978-92-822-2272-0.
  2. ^ a b British Standard BS 350:2004 Conversion Factors for Units.
  3. ^ "Nomenclature of the unit of absolute pressure, Charles F. Marvin, 1918" (PDF). noaa.gov. Archived (PDF) from the original on 29 April 2017. Retrieved 6 May 2018.
  4. ^ NIST Special Publication 1038 Archived 2016-03-19 at the Wayback Machine, Sec. 4.3.2; NIST Special Publication 811, 2008 edition Archived 2016-06-03 at the Wayback Machine, Sec. 5.2
  5. ^ International Astronomical Union Style Manual. Comm. 5 in IAU Transactions XXB, 1989, Table 6