Under which condition will pressure altitude be equal to true altitude?

• A. When standard atmospheric conditions exist.
• B. When temperature is extremely cold.
• C. When humidity is high.
• D. When altitude is near sea level.

Answer: (A)

Under standard atmospheric conditions the pressure surface for a given height matches the geometric height, so pressure altitude and true altitude are the same. Pressure altitude relies on the ambient pressure relative to the standard 29.92 inHg reference, while true altitude is the actual height above mean sea level. When the atmosphere differs from the standard (temperature or humidity deviations from ISA), the pressure distribution shifts with height, causing pressure altitude to diverge from true altitude. Humidity mostly affects density altitude, not pressure altitude, so it isn’t the factor that keeps the two heights equal. Therefore, the two altitudes coincide only when the atmosphere behaves like the standard atmosphere.