What type of altimeter measures atmospheric pressure to indicate altitude?

The correct answer is the pressure altimeter. This type of altimeter is specifically designed to measure atmospheric pressure and uses the principle that atmospheric pressure decreases as altitude increases. By sensing the surrounding air pressure, the pressure altimeter can provide a reading of altitude above sea level or above a specific pressure level.

Pressure altimeters operate based on standard atmospheric conditions, applying barometric formulas to interpret changes in air pressure and convert those readings into an altitude. This makes them essential for pilots, as they help maintain safe flying altitudes and comply with flight regulations.

The other options represent different types of altimeters or instruments that do not utilize atmospheric pressure for altitude measurements. For example, digital altimeters use sensors to provide altitude information, but they often rely on a pressure sensor within their systems, functioning similarly to pressure altimeters yet presenting data in a digital format. Radio altimeters measure the distance between the aircraft and the ground using radio waves, which is a different method altogether. Visual altimeters, while useful for providing altitude in visual terms, do not measure atmospheric pressure. Therefore, the pressure altimeter is the instrument specifically tailored for measuring altitude based on atmospheric pressure changes.