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Note that Mbar redirects here; if used with a capital M, it would be megabar. See SI prefix.

A millibar (mbar, also mb) is 1/1000th of a bar, a unit for measurement of pressure. It is equivalent to 1 hPa.

The millibar was introduced by Sir Napier Shaw in 1909, and internationally adopted in 1929.

Unicode has a symbol for "mb": (㏔).

The millibar is not an SI unit. The SI unit is the pascal (Pa), with 1 mbar = 100 Pa = 1 hPa = 0.1 kPa.

Meteorologists worldwide have long measured air pressure in a variety of units including millibars. It has taken some time after the introduction of SI units for people to change to pascals. The unit millibar is still used although official use is gradually changing to hPa which is the numerically equivalent SI unit. Similar pressures are given in kilopascals in practically all other fields where the hecto prefix is hardly ever used. In Canadian weather reports, the norm is kPa.

Americans are familiar with the millibar in US reports of hurricanes and other cyclonic storms, where lower central pressure generally means higher winds and a stronger storm.

Typical pressure

The average sea-level pressure is 1013.25 hPa (mbar) . Air pressure decreases exponentially with increased altitude, at a rate of about 12% per 1000 m.

See also

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