millibar

The millibar is a unit of pressure equal to one thousandth of a bar. Specifically, 1 millibar equals 0.001 bar and 100 pascals. It is a non-SI unit, though it remains widely used in meteorology and aviation. The symbol mb is traditional, and the term mbar is also seen; in practice, many contexts treat mb and hPa as interchangeable.

In the international system of units, the preferred unit for pressure is the pascal (Pa). The bar is defined as 100,000 Pa, so 1 bar equals 1000 millibars. Because 1 hectopascal (hPa) is defined as 100 Pa, 1 mb and 1 hPa are numerically equal, though hPa is the SI-derived unit commonly used in meteorology alongside mb in older literature.

Atmospheric pressure at sea level is typically expressed in millibars or hectopascals. A standard reference value is 1013.25 mb (which is the same as 1013.25 hPa or 101,325 Pa). In everyday meteorology, pressures commonly range from about 980 to 1050 mb, with higher values associated with high-pressure systems and lower values with low-pressure systems. Isobar maps often use mb or hPa to indicate regions of equal atmospheric pressure.

Measurement and usage: millibars are measured with barometers and used in weather forecasts, aviation, and scientific discussions of atmospheric pressure. While the millibar remains common in practical meteorology, the SI-preferrered unit is the hectopascal, and modern scientific literature often reports pressure in pascals or kilopascals.