milibares

Milibares, or millibars, are a unit of atmospheric pressure equal to one thousandth of a bar. Since one bar is 100,000 pascals, one milibar equals 100 pascals. The symbol historically used for the unit is mb or mbar. In Spanish, the unit is milibar and its plural is milibares.

Usage and context: Milibars have long been used in meteorology and aviation to report atmospheric pressure and to illustrate weather systems on synoptic charts. In modern practice, the hectopascal (hPa) has largely replaced millibars in official SI-compliant contexts, because 1 hPa equals 100 pascals, the same as 1 millibar. Consequently, values such as 1013 hPa are numerically identical to 1013 mb. The term “millibar” persists in older literature and in some regional weather products.

Relation to other units: The millibar is derived from the SI prefixes milli- (one thousandth) and bar, a non-SI unit of pressure. Although the millibar remains widely understood, meteorological data are increasingly presented in hectopascals, with the two units being interchangeable for practical purposes.

See also: Bar, pascal, hectopascal, atmospheric pressure.

Examples: Typical sea-level pressure hovers around 1013 mb or 1013 hPa, with variations due to weather systems such as high- and low-pressure areas.