Under what condition will true altitude be lower than indicated altitude?

True altitude will be lower than indicated altitude when flying in colder than standard air temperature. This phenomenon occurs due to the influence of temperature on atmospheric pressure and altitude measurement.

Indicated altitude is based on the standard atmospheric model, which assumes a specific temperature at sea level (standard temperature of 15 degrees Celsius or 59 degrees Fahrenheit). When the air temperature is colder than standard, the air density increases, causing the altimeter to indicate a higher altitude than the actual true altitude.

The altimeter measures the atmospheric pressure and, based on the standard pressure lapse rate, calculates the altitude. However, if the actual air is colder, the pressure at a given altitude is higher than what the altimeter assumes under standard conditions. As a result, the aircraft's true altitude – the actual height above mean sea level – is less than what the altimeter indicates.

Understanding this concept is crucial for pilots, as it can affect flight planning, especially in colder regions or at higher altitudes where temperature deviations can be significant. This insight emphasizes the importance of compensating for temperature differences when interpreting altitude readings during flight.