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The bar is a metric (but not SI) unit of pressure exactly equal to 100000 Pa. It is about equal to the atmospheric pressure on Earth at sea level.
The bar and the millibar were introduced by the British meteorologist William Napier Shaw in 1909, while he was the director of the Meteorological Office in London.
Units derived from the bar are the megabar (symbol: Mbar), kilobar (symbol: kbar), decibar (symbol: dbar), centibar (symbol: cbar), and millibar (symbol: mbar or mb). These are not SI or cgs units, but they are accepted by the BIPM for use with the SI. The bar is legally recognized in countries of the European Union.
The bar unit is considered deprecated by some entities. While the BIPM includes it under the class "Non-SI units accepted for use with the SI", the NIST includes it in the list of units to avoid and recommends the use of kilopascals (kPa) and megapascals (MPa) instead. The IAU also lists it under "Non-SI units and symbols whose continued use is deprecated."
Bar(g) is a unit of gauge pressure, i.e., pressure in bars above ambient or atmospheric pressure; see absolute pressure and gauge pressure below.