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What Is a Millibar?

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  • Written By: Mary McMahon
  • Edited By: O. Wallace
  • Last Modified Date: 19 April 2014
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A millibar is a unit of pressure which is most commonly seen in the context of meteorology, where atmospheric pressure is sometimes given in millibars. As the name would suggest, a millibar is equal to one thousandth of a bar. The bar is a unit of measurement introduced in the early 1900s, based on a system of measurement which uses dynes. Dynes are part of the now-outdated centimeter-gram-second (CGS) system of measurements.

Measurements can be a bit confusing. Several conflicting systems of measurement are in use currently, and sometimes nations which use one official system may periodically reference another, adding even further to the confusion. Millibars are widely accepted as a unit of measurement in meteorology, but are rarely seen outside this field. Bars and the related system of pascals are closely linked, and sometimes used interchangeably, with a single hectopascal being equivalent to a millibar.

At sea level, the standard pressure is around 1,013 millibars. Atmospheric pressure decreases as people climb in altitude, and becomes higher as people go below sea level. The rates of increase and decrease are quite stable, and there are calculations which can be used to trace the shifts in atmospheric pressure, and to determine the pressure at a known altitude, with the millibar being one unit of measurement which can be used in such calculations. As many people are aware, moving between areas of high and low pressure too quickly can be dangerous, as the body may not be able to adapt quickly enough.

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Local variations in atmospheric pressure occur in response to weather systems which move across the Earth's surface. When meteorologists give weather reports, they provide information about air pressure in millibars which people can use to learn more about the weather in the upcoming days. If the pressure is rising, the weather should be fair, while drops in pressure indicate that storm systems are on their way in. Rapid fluctuations in atmospheric pressure can be warning signs that a storm system is approaching.

One place where one might see millibars is in a weather report on the news or in the newspaper. Sometimes, the reporter gives the measurement in hectopascals, abbreviated as hPa, and it can be useful to remember that the terms "millibar" and "hectopascal" are equivalent. The term can also be seen in textbooks which cover weather issues, and it may crop up in safety classes for people who work outdoors and are therefore vulnerable to shifts in the weather. Boaters, for example, need to be aware of changes in air pressure and good at reading weather reports for safety reasons.

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