True altitude will be lower than indicated altitude under which condition?

• A. Colder than standard air temperature.
• B. Warmer than standard air temperature.
• C. High humidity at sea level.
• D. Lower barometric pressure.

Answer: (A)

When the air is colder than standard, pressure surfaces are compressed toward the ground. An altimeter set to the standard pressure will still sense the same pressure change, so it will indicate a higher altitude than the aircraft’s actual height above the ground. In other words, the true altitude is lower than what the altimeter indicates in cold air. In warmer air, the opposite occurs: true altitude would be higher than indicated. Humidity has only a minor, indirect effect, and changes in surface pressure mainly shift the overall pressure field rather than the direct relationship described here, so the clearest, most consistent condition producing this difference is colder-than-standard air.