What is pressure altitude used for?

Pressure altitude is the altitude indicated by a barometric altimeter when it is set to a standard pressure setting of 29.92 inches of mercury (Hg) at sea level. This standardization allows aviators to use a common reference when operating at high altitudes. It is especially important for flight operations above the transition altitude, where pilots switch from using local barometric pressure to the standard setting.

Using pressure altitude as a reference is crucial for ensuring separation between aircraft flying at high altitudes, as it helps to provide a consistent frame of reference across different geographic locations and weather conditions. For high altitude flights, pressure altitude helps in air traffic control, navigation, and maintaining safe vertical separation between aircraft.

The other options—such as determining stall altitude, measuring altitude above ground level, and indicating temperature changes—do not accurately reflect the primary purpose of pressure altitude and its role in aviation safety and efficiency.