What does the altimeter measure?

The altimeter measures the aircraft's altitude, which is the height of the aircraft above a specified reference point, typically sea level. This instrument works by measuring the atmospheric pressure outside the aircraft. As the aircraft climbs, the air pressure decreases, and the altimeter converts this change in pressure into an altitude reading. Pilots rely on the altimeter to maintain the correct altitude and for safe navigation during flight, especially when flying in instruments and adverse weather conditions where visual references may be limited.

Other options, such as aircraft speed, direction, and weight, are measured by different instruments. The airspeed indicator measures speed, the compass or heading indicator provides direction, and the weighing scales (used on the ground) determine weight. Each of these instruments serves a distinct purpose in the cockpit, but the altimeter's primary function is unequivocally to provide altitude information.