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. Millibars are commonly used in weather reporting and aviation meteorology to express barometric pressure.
Plain English
A unit used to measure how heavy the air is pressing down. Higher numbers mean higher pressure; lower numbers mean lower pressure, which often signals weather changes.
Context Anchor
Seen in aviation weather reports, weather charts, and discussions of atmospheric pressure.
Derivation
From the Latin 'mille' (thousand) and 'bar' (from the Greek 'baros' meaning weight). A millibar is one-thousandth of a bar, the original metric unit of pressure. Knowing the root 'baros' (weight) helps connect millibars to barometer and barometric pressure -- all describing the weight of the atmosphere.
Why Pilots Care
Accurate millibar readings let pilots set their altimeters correctly so altitude indications match true height above sea level.
Grounding Statement
When a weather report says the pressure is 1013 millibars, it is describing the current push of the air at that location.
Intuition Check
Millibars are not physical bars or lines on a chart. They are units for measuring pressure.
Example Sentence 1
The METAR reported a sea-level pressure of 1008 millibars, suggesting a low-pressure system was moving through the area.
Example Sentence 2
A reading above 1025 millibars usually means high pressure and stable flying conditions.