What does an altimeter measure?

An altimeter measures the altitude of the aircraft above sea level by determining the atmospheric pressure at the aircraft's location. The altimeter operates on the principle that air pressure decreases as altitude increases; therefore, it can accurately reflect changes in altitude based on the pressure changes. When an aircraft ascends, the pressure surrounding it decreases, prompting the altimeter to indicate a higher altitude. Conversely, when descending, the increase in pressure results in a lower altitude reading. This is crucial for pilots to maintain safe flying levels, especially when navigating in varied terrain and controlled airspace.