Definition
A small opening on the face of an altimeter that displays the current altimeter setting, expressed in inches of mercury or hectopascals/millibars. The pilot adjusts a knob to dial the local sea-level pressure into this window, which calibrates the altimeter to read correct altitude for the current atmospheric conditions.
Plain English
A little numbered window on the altimeter where the pilot sets the local air pressure so the altimeter shows the right height.
Context Anchor
Seen when setting or checking the altimeter, especially before takeoff, during descent, or when studying how nonstandard pressure affects indicated altitude.
Derivation
‘Barometric’ comes from the Greek baros (weight) and metron (measure) — literally ‘weight measure,’ referring to the weight of the atmosphere. The window shows the barometric pressure setting because the altimeter is, at its core, a barometer that has been re-labeled to read in feet instead of pressure units.
Why Pilots Care
Setting it correctly prevents altitude errors that could result in terrain conflicts or incorrect flight levels.
Intuition Check
Do not picture a cockpit window or an outside view. In this term, window means a small display area on the altimeter that shows the selected pressure setting.
Example Sentence 1
Before takeoff, the pilot tuned ATIS, heard the altimeter setting was 30.12, and dialed it into the barometric pressure window.
Example Sentence 2
After receiving the updated altimeter setting from ATC, she turned the knob to change the barometric pressure window.