Which condition allows indicated altitude to equal true altitude?

• A. Being at sea level under standard conditions
• B. Being at high altitude with standard temperature
• C. Being at sea level with nonstandard temperature
• D. Being at sea level with humidity

Answer: (A)

Indicated altitude represents height above a standard pressure surface, using the current pressure and an assumed standard atmosphere. It will match true altitude only when the actual atmosphere behaves like that standard model and the pressure setting is correct. At sea level under standard conditions, the ambient pressure equals the standard sea‑level pressure, and true altitude is zero. With the altimeter properly set, it reads zero feet, exactly the true altitude. In other scenarios, deviations in temperature or humidity alter the pressure–height relationship, so a given pressure does not correspond to the same height as the actual mean sea level height, causing the indicated and true altitudes to differ. Humidity has only a minor effect, while nonstandard temperature (warmer or cooler air) shifts the relationship more noticeably.