What does the altimeter measure?

The altimeter measures altitude based on air pressure. It functions by comparing the atmospheric pressure at a given altitude to a pre-defined standard atmospheric pressure. As an aircraft ascends or descends, the air pressure changes, and the altimeter reflects these changes to provide a readout of the aircraft's height above sea level or above the ground, depending on the setting used.

This measurement is critical for maintaining proper altitude during flight, especially when flying in controlled airspace, and is essential for adherence to altitude regulations. Understanding this function is fundamental for pilots, as accurate altitude readings are vital for safety and navigation.

In contrast, the other choices relate to different aircraft instruments or functionalities. Airspeed is measured by the airspeed indicator, vertical speed is measured by a vertical speed indicator, and the angle of climb is often indicated by the aircraft's attitude indicator, not the altimeter.