Definition
A unit of atmospheric pressure equal to one-thousandth of a bar, or 100 pascals. Standard sea-level atmospheric pressure is 1013.2 mb, which corresponds to 29.92 inches of mercury ("Hg). Millibars are the pressure unit used on weather charts, in METARs and TAFs internationally, and in many meteorological discussions of pressure systems and altimeter settings.
Plain English
A unit used to measure how much the air is pressing on something. The higher the number, the more pressure the air is exerting. Standard sea-level air pressure is 1013.2 in this unit.
Context Anchor
Seen in atmospheric pressure discussions, aviation weather material, and some weather charts or reports that give pressure in millibars instead of inches of mercury.
Derivation
From milli- (Latin, one-thousandth) and bar (from the Greek baros, meaning weight). A bar is roughly the weight of the atmosphere at sea level, so a millibar is one-thousandth of that weight. The name fits — it is literally a small unit of atmospheric weight.
Why Pilots Care
Correct interpretation of millibar readings ensures accurate altimeter settings and helps predict weather changes affecting flight.
Grounding Statement
On a standard day at sea level, the weight of the air above you creates about 1013 mb of pressure.
Intuition Check
Do not read mb as computer storage, such as megabyte, in this aviation context. Here it means millibar, a unit of air pressure.
Example Sentence 1
The surface analysis chart showed a low-pressure center of 998 mb moving across the region.
Example Sentence 2
Pilots convert millibars to inches of mercury when needed for altimeter adjustments.