FAA Ground School Practice Test

What environmental condition causes the altimeter to read a lower altitude than true altitude?

1. Standard air temperature

2. Air temperature colder than standard

3. Air temperature warmer than standard

4. Humidity levels above standard

The correct answer is: Air temperature warmer than standard

When the air temperature is warmer than standard, the altimeter will read a lower altitude than the true altitude. This phenomenon occurs because the altimeter is designed to measure air pressure, which decreases with altitude. In warmer air, the air density is lower, resulting in a pressure reading that does not correspond accurately to the altitude. The altimeter assumes a standard atmospheric pressure model based on a specific temperature. If the temperature is warmer than the standard atmospheric model, then the actual air pressure at a given altitude is higher than what the altimeter is calibrated to expect. Consequently, the altimeter will indicate a lower altitude than the true altitude, leading to potential navigation errors. This understanding is crucial for pilots to maintain accurate altitude awareness, especially when flying in varying temperature conditions.