Definition
A unit of atmospheric pressure equal to one one-thousandth of a bar, or 100 pascals. Standard sea-level atmospheric pressure is 1013.2 millibars, equivalent to 29.92 inches of mercury.
Plain English
A small unit used to measure how much the air is pressing down. The higher the number, the higher the pressure.
Context Anchor
Seen in weather reports, weather charts, altimeter-setting discussions outside the United States, and some aircraft or engine performance data.
Derivation
From Latin 'mille' meaning 'thousand' and 'bar' from Greek 'baros' meaning 'weight.' A millibar is one-thousandth of a bar, where a bar represents roughly the weight of the atmosphere at sea level. Knowing this makes the unit easier to picture: it is a small slice of the total weight of the air above you.
Why Pilots Care
Correct millibar values let pilots set altimeters accurately so altitude readings stay reliable.
Analogy
Using millibars instead of another pressure unit is like using centimeters instead of inches for length. The thing being measured is the same; only the scale is different.
Grounding Statement
When a weather chart shows pressure in millibars, it is describing how hard the air is pressing at that place.
Intuition Check
A millibar is not a measure of height or engine power. It is a measure of pressure, usually air pressure.
Example Sentence 1
The METAR reported a sea-level pressure of 1008 millibars, slightly below standard.
Example Sentence 2
Engine manifold pressure read 950 millibars during the climb.