Definition
The knob on a sensitive altimeter used to set the local barometric pressure (in inches of mercury or hectopascals) into the altimeter's Kollsman window, so the instrument reads altitude relative to the correct pressure reference.
Plain English
It's the small knob on the altimeter you turn to dial in the current pressure setting given by ATC or ATIS, so the altimeter shows the right altitude.
Context Anchor
Seen on the face of a sensitive altimeter, especially when setting the altimeter before takeoff, during flight, or when given a new altimeter setting by air traffic control.
Derivation
Barometric' comes from the Greek 'baros' meaning weight, referring to the weight (pressure) of the atmosphere. The knob adjusts the scale that calibrates the altimeter to that pressure.
Why Pilots Care
Setting the correct reference pressure ensures the altimeter shows true altitude above mean sea level or field elevation, preventing altitude deviations and terrain conflicts.
Intuition Check
Do not think of this as a general adjustment knob for making the altimeter “look right.” It specifically sets the pressure reference used by the altimeter.
Example Sentence 1
Approaching the destination, the pilot turned the barometric scale adjustment knob to set the new altimeter setting of 30.12 given by ATIS.
Example Sentence 2
The controller issued the current altimeter setting of 29.92, which the pilot dialed into the barometric scale adjustment knob.