On a warm day, the indicated altitude is typically what relative to the true altitude?

• A. Lower
• B. Higher
• C. Equal
• D. Unreliable

Answer: (A)

Indicated altitude is what your altimeter shows, based on the current pressure and the standard atmosphere. On a warm day the air expands and the pressure surfaces rise. At a given true altitude, the actual ambient pressure is higher than the standard value for that height. Since higher pressure corresponds to being lower on the altimeter’s scale, the instrument reads a smaller, i.e., lower, altitude than you actually have. So the indicated altitude is lower than the true altitude on a warm day. This temperature-related error grows with height and is something pilots account for with temperature corrections, especially in high terrain or when precise altitude is critical.