What does an altimeter measure?

An altimeter measures atmospheric pressure, which is a fundamental principle behind its function. It operates based on the relationship between altitude and pressure; as an aircraft ascends or descends, the ambient pressure changes and the altimeter reflects these changes in terms of altitude. By calibrating to a specific pressure setting (usually sea level pressure), the altimeter can provide a readout that corresponds to the aircraft's height above that reference point.

Understanding that the altimeter is intrinsically connected to pressure helps clarify why altitude above ground level is not directly measured; rather, it provides the altitude based on a standard pressure setting. Therefore, an altimeter is crucial for pilots to determine their altitude while accounting for variations in atmospheric pressure due to weather conditions and elevation changes.