AskDefine | Define millibar

Dictionary Definition

millibar n : a unit of atmospheric pressure equal to one thousandth of a bar; "atmospheric pressure at sea level is 1013 millibars"

User Contributed Dictionary

English

Etymology

milli- + bar

Noun

millibar (plural millibars)
  1. A unit of pressure equal to one-thousandth of a bar or 100 pascals, most commonly used to measure atmospheric pressure.

Synonyms

Translations

Extensive Definition

The bar (symbol bar), decibar (symbol dbar) and the millibar (symbol mbar, also mb) are units of pressure. They are not SI units, nor are they cgs units, but they are accepted for use with the SI. The bar is still widely used in descriptions of pressure because it is about the same as atmospheric pressure.

Definition

The bar, decibar and millibar are defined as:
  • 1 bar = 100 kPa (kilopascals) = 1,000,000 dynes per square centimeter (baryes)
  • 1 dbar = 0.1 bar = 10 kPa = 100,000 dyn/cm²
  • 1 mbar = 0.001 bar = 0.1 kPa = 1 hPa = 1,000 dyn/cm²
(A pascal is one newton per square meter.)

Origin

The word bar has its origin in the Greek word βάρος (baros), meaning weight. Its official symbol is "bar"; the earlier "b" is now deprecated, but still often seen especially in "mb" rather than the proper "mbar" for millibars.
The bar and millibar were introduced by Sir Napier Shaw in 1909 and internationally adopted in 1929.

Discussion

Atmospheric air pressure is often given in millibars where "standard" sea level pressure is defined as 1013.25 mbar (hPa), equal to 1.01325 bar. Despite millibars not being an SI unit, they are still used locally in meteorology in some countries to describe atmospheric pressure. 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 millibars. After the introduction of SI units, others use hectopascals (which are equivalent to millibars) so they could stick to the same numeric scale. Similar pressures are given in kilopascals in practically all other fields, where the hecto prefix is hardly ever used. In particular, Canadian weather reports use kilopascals (which could also be called centibars).
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.
In some countries, pressure is measured with reference to atmospheric pressure. This is gauge pressure and denoted by barg, often written with no spaces, spoken "bar gauge", and sometimes using symbols such as 'bar(g)'. For example, if someone says that their car tyres are pressurised to 2.3 bar they actually mean bar gauge: the pressure in the tyre is really 3.3 bar, but only 2.3 bar above atmospheric, which is the scale a tyre gauge would read. When absolute pressure is desired, it is sometimes denoted 'bara' or 'bar(a)' for "bar absolute". The alteration of units of measure for this purpose is now deprecated, with qualification of the physical property being preferred, e.g., "The gauge pressure is 2.3 bar; the absolute pressure is 3.3 bar".
In water, there is an approximate numerical correspondence between the change in pressure in decibars and the change in depth from the sea surface in meters. Specifically, an increase of 1 decibar occurs for every 1.019716 m increase in depth close to the surface. As a result, decibars are commonly used in oceanography.
Unicode has a character for "mb": (), but exists only for compatibility with legacy Asian encodings. There is also a character "bar": .

See also

millibar in Afrikaans: Bar
millibar in Arabic: بار
millibar in Belarusian (Tarashkevitsa): Бар (адзінка вымярэньня)
millibar in Bosnian: Bar (jedinica)
millibar in Bulgarian: Бар (единица)
millibar in Catalan: Bar (unitat de pressió)
millibar in Czech: Bar (jednotka)
millibar in German: Bar (Einheit)
millibar in Modern Greek (1453-): Βαρομετρική μονάδα
millibar in Spanish: Bar (unidad de presión)
millibar in Esperanto: Baro
millibar in Basque: Bar
millibar in French: Bar (unité)
millibar in Galician: Bar (unidade)
millibar in Korean: 바 (단위)
millibar in Croatian: Bar (jedinica)
millibar in Indonesian: Bar
millibar in Italian: Bar (unità di misura)
millibar in Hebrew: בר (מידה)
millibar in Georgian: ბარი (ერთეული)
millibar in Hungarian: Bar (mértékegység)
millibar in Dutch: Bar (druk)
millibar in Japanese: バール (単位)
millibar in Norwegian Nynorsk: Eininga bar
millibar in Low German: Bar (Eenheit)
millibar in Polish: Bar (jednostka)
millibar in Portuguese: Bar (unidade)
millibar in Romanian: Bar
millibar in Russian: Бар (единица измерения)
millibar in Simple English: Bar (unit)
millibar in Slovak: Bar (tlak)
millibar in Slovenian: Bar (enota)
millibar in Finnish: Baari
millibar in Swedish: Bar (måttenhet)
millibar in Tamil: பார் (அளவை)
millibar in Turkish: Bar (birim)
millibar in Ukrainian: Бар (одиниця тиску)
millibar in Chinese: 巴
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