Definition
Flight instruments that determine altitude by measuring the surrounding atmospheric (static) pressure and converting it into a height reading, based on the principle that air pressure decreases predictably with increasing altitude. The pilot sets a local pressure setting (in inches of mercury or hectopascals) in the Kollsman window so the instrument references the correct pressure datum.
Plain English
An altimeter that figures out how high you are by sensing how much the air around it is pressing on it. Higher up means less pressure, so the instrument turns that pressure into an altitude reading on the dial.
Context Anchor
Seen on the flight instruments during instrument flying, inadvertent entry into clouds, altitude checks, approaches, and any situation where the pilot must hold a safe altitude without outside visual references.
Derivation
Barometric comes from the Greek baros, meaning weight, combined with metric, meaning to measure. So a barometric altimeter literally measures altitude using the weight of the air above the aircraft.
Why Pilots Care
In IMC, especially in helicopters operating at lower altitudes, these altimeters provide the primary reference for assigned altitudes and terrain clearance when visual references are lost.
Grounding Statement
As the helicopter climbs, outside air pressure decreases, and the barometric altimeters turn that pressure change into an altitude reading.
Intuition Check
Barometric altimeters do not directly measure distance above the ground. They measure outside air pressure and convert it into an altitude reading using the pressure setting selected by the pilot.
Example Sentence 1
Before takeoff, the pilot set the current altimeter setting in the barometric altimeter so the field elevation showed correctly on the ground.
Example Sentence 2
During inadvertent IMC the crew cross-checked both barometric altimeters against each other to confirm they were holding the assigned altitude.