Definition
A millibar (mb) is a unit of atmospheric pressure equal to one-thousandth of a bar, or 100 pascals. Standard sea-level pressure is 1013.2 mb, which corresponds to 29.92 inches of mercury. Millibars (also called hectopascals, hPa) are the standard pressure unit used in aviation weather reports outside the United States and in surface analysis charts worldwide.
Plain English
Millibars are a way of measuring how much the air is pressing down. The higher the number, the higher the air pressure. Most of the world reports air pressure in millibars; the U.S. usually reports it in inches of mercury instead.
Context Anchor
Seen when setting or reading an altimeter, especially in references that use millibars or hectopascals instead of inches of mercury.
Derivation
From the Latin 'milli-' meaning one-thousandth, and 'bar' from the Greek 'baros' meaning weight or pressure. So a millibar is literally one-thousandth of a 'bar' of pressure — a small, convenient unit for reporting everyday atmospheric pressure changes.
Why Pilots Care
An incorrect millibar setting causes the altimeter to show the wrong altitude, creating a direct safety risk.
Grounding Statement
As weather systems move through an area, air pressure changes, and the mb number used for the altimeter setting changes with it.
Intuition Check
In aviation, mb does not mean megabytes. Here, mb means millibars, a unit of air pressure.
Example Sentence 1
Approaching Heathrow, the controller passed an altimeter setting of 1008 mb, and the pilot dialed it into the Kollsman window.
Example Sentence 2
The pilot set the altimeter to 998 mb to match the current local pressure.