Definition
A small adjustable scale on an altimeter, visible through a window in the instrument face, that allows the pilot to set the current local atmospheric pressure (in inches of mercury or millibars/hectopascals) so the altimeter reads accurate altitude relative to that pressure setting.
Plain English
The little number window on the altimeter where you dial in the current air pressure setting given by ATC or the weather report. Setting it correctly makes the altimeter show the right altitude.
Context Anchor
Seen on the altimeter face or in the small setting window when a pilot sets the reported pressure before takeoff, during flight, or before landing.
Derivation
From 'barometer' (Greek baros = weight, of air) plus 'scale.' A barometer measures air pressure, and the scale is the marked dial used to set it. The name reflects that the pilot is adjusting the altimeter to match a barometric pressure reading.
Why Pilots Care
A correct setting produces accurate altitude readings needed for terrain clearance, traffic separation, and regulatory compliance.
Intuition Check
Do not read “scale” here as the main altitude scale on the altimeter. The barometric scale is the pressure-setting scale that helps the altimeter calculate the altitude it displays.
Example Sentence 1
Before takeoff, the pilot turned the knob to set 30.12 on the barometric scale to match the current altimeter setting from ATIS.
Example Sentence 2
During the flight the changing weather required the pilot to reset the barometric scale for continued accurate altitude.