Bar (unit) mb (rather than the proper mbar) to denote the millibar. Between 1793 and 1795, the word bar was used for a unit of weight in an early version of the metric system. Atmospheric air pressure is often given in millibars, where standard atmospheric pressure at sea level is defined as 1013.25 mbar, 101.325 kPa, 1.01325 bar, which is about . 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
Pressure Pressure is distributed to solid boundaries or across arbitrary sections of fluid "normal to" these boundaries or sections at every point. It is a fundamental parameter in thermodynamics, and it is conjugate to volume. The SI unit for pressure is the pascal (Pa), equal to one newton per square metre (N/m, or kg·m·s). This name for the unit was added in 1971; before that, pressure in SI was expressed simply in newtons per square metre. Other units of pressure, such as pounds per square inch and bar, are also in common use. The CGS unit of pressure is the barye