What does the altimeter compare to determine altitude?

The altimeter measures altitude by comparing the static pressure of the air around the aircraft to a reference pressure. The static pressure decreases as altitude increases due to the thinning atmosphere at higher elevations. The altimeter is calibrated to a standard reference pressure, which is the atmospheric pressure at sea level under standard conditions (29.92 inHg or 1013.25 hPa).

When the aircraft is at a certain altitude, the static pressure reading obtained by the altimeter will correspond to the pressure at that altitude according to the standard atmosphere. By taking this comparison, the altimeter can provide a readout that indicates the altitude at which the aircraft is flying. Understanding how an altimeter works is critical for pilots as it directly affects navigation and safety during flight.

The other options involve comparisons that do not directly pertain to how altitude is measured. Dynamic pressure relates more to airspeed and does not play a role in altitude measurement. Ground pressure and temperature comparisons also do not provide the necessary information to determine altitude relative to sea level or the surrounding atmosphere.