Definition
A unit of atmospheric pressure equal to one-thousandth of a bar, or 100 pascals. Standard sea level pressure is 1013.2 millibars, which corresponds to 29.92 inches of mercury. Millibars are used on weather charts and in aviation weather products to report station pressure, sea level pressure, and altimeter settings outside the United States.
Plain English
A way of measuring how heavy the air is pressing down on the ground. Higher numbers mean higher pressure; lower numbers mean lower pressure.
Context Anchor
Seen when discussing altimeter settings, weather pressure, and international pressure reports.
Derivation
From Latin 'mille' meaning thousand, plus 'bar' from Greek 'baros' meaning weight. So a millibar is literally one-thousandth of a 'weight' unit — fitting, because pressure is the weight of the atmosphere pressing down.
Why Pilots Care
Altimeter settings are often given in millibars; using the correct value ensures the altimeter reads true altitude above sea level or the chosen reference.
Grounding Statement
At sea level on a standard day, the air column above you weighs in at 1013.2 millibars — that's the baseline pressure all weather is measured against.
Intuition Check
Millibars are not a measure of height. They are a measure of air pressure used to help determine the altitude shown on the altimeter.
Example Sentence 1
The surface chart showed a low-pressure center of 996 millibars over the coast, with tightly packed isobars indicating strong winds.
Example Sentence 2
In many countries outside the United States, pressure is given in millibars instead of inches of mercury.