The bar is a metric unit of pressure, but not part of the International System of Units (SI). It is exactly equal to 100000 Pa and is slightly less than the average atmospheric pressure on Earth at sea level.
The bar and the millibar were introduced by the Norwegian meteorologist Bjerknes, who was a founder of the modern practice of weather forecasting.
Use of the bar is deprecated by various bodies. The BIPM lists it as one of the "non-SI units [that authors] should have the freedom to use" but does not include it among the "Non-SI units accepted for use with the SI", and 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." As of 2004, the bar is legally recognized in countries of the European Union.
Units derived from the bar include the megabar (symbol: Mbar), kilobar (symbol: kbar), decibar (symbol: dbar), centibar (symbol:cbar), and millibar (symbol: mbar or mb). The notation bar(g), though deprecated by various bodies,represents gauge pressure, i.e., pressure in bars above ambient or atmospheric pressure.
Atmospheric air pressure is often given in millibars where standard sea level pressure is defined as 1013 mbar, 101.3 (kPa), or 1.013 bar. Despite the millibar not being an SI unit, meteorologists and weather reporters worldwide have long measured air pressure in millibars as the values are convenient. After the advent of SI units, some meteorologists began using hectopascals (symbol hPa) which are numerically equivalent to millibars; for the same reason, the hectopascal is now the standard unit used to express barometric pressures in aviation in most countries.
For example, the weather office of Environment Canada uses kilopascals and hectopascals on their weather maps. In contrast, Americans are familiar with the use of the millibar in US reports of hurricanes and other cyclonic storms.
From Wikipedia