What does pressure altitude refer to?

Pressure altitude refers to the height above the standard datum plane, which is a hypothetical reference level set at a standard atmospheric pressure of 29.92 inches of mercury (Hg) or 1013.25 hPa. This measurement is useful in aviation and meteorology because it allows for consistent calculations regarding altitude, regardless of local atmospheric conditions such as temperature and humidity.

When pilots use pressure altitude, they can effectively determine their altitude in relation to standard conditions, simplifying navigation and enhancing safety. It is calculated by adjusting the indicated altitude on an altimeter, typically set to the local altimeter setting, back to a baseline standard pressure.

Other options, while they touch on aspects related to altitude measurements, do not define pressure altitude accurately. The actual height of the aircraft may vary due to temperature changes and atmospheric pressure, and is not a fixed representation like pressure altitude. The correction for humidity would pertain to density altitude rather than pressure altitude. Lastly, the altitude relative to mean sea level (MSL) considers local atmospheric conditions and is not necessarily aligned with the fixed standard pressure level that defines pressure altitude.