How can an aircraft instrument estimate altitude from air pressure?

A pressure altimeter can indicate that an aircraft is climbing without looking at the ground. Its direct measurement is the surrounding air pressure. Pressure generally decreases with height because there is less atmosphere above, so an instrument can use an assumed pressure–height relationship to estimate altitude.

In a traditional mechanical altimeter, local outside pressure reaches the instrument case through a static-pressure connection. Inside are sealed flexible capsules, often called aneroid capsules. A reduction in the surrounding pressure allows them to expand; higher surrounding pressure compresses them. Linkages magnify this small movement and turn the display pointers.

The scale translates capsule movement into height using an atmospheric model. The mechanism therefore contains two distinct steps: pressure changes a physical shape, and a calibrated relationship turns that change into an altitude indication. Electronic instruments use pressure sensors and calculations in place of the capsule-and-pointer mechanism.

Weather can change pressure without moving the aircraft. An appropriate reference-pressure setting relates the estimate to the intended reference level. Temperature also matters because warm and cold air columns do not have the same pressure–height relationship. The same measured pressure can therefore occur at different true heights under different conditions.

A barometric altitude is not a direct measurement of the gap between the aircraft and the terrain. A mountain can rise below it while the measured pressure changes little. The instrument is useful because it connects an accessible quantity, air pressure, with an otherwise less accessible one, height; its meaning remains tied to the reference and assumptions used for that connection.