FAA Ground School Practice Test

What effect does warm air have on the altimeter readout in terms of altitude measurement?

1. It increases the indicated altitude

2. It decreases the indicated altitude

3. It provides an accurate readout

4. It has no effect

The correct answer is: It decreases the indicated altitude

Warm air affects the altimeter readout by causing it to indicate a lower altitude than the actual altitude. This occurs because altimeters are calibrated based on the assumption that the standard atmosphere is present, which involves a specific temperature gradient with altitude. When the air is warmer than standard, it becomes less dense and allows an aircraft to rise higher before sensing a pressure change. Since the altimeter is measuring pressure, which decreases with altitude, the instrument may incorrectly show a lower indicated altitude compared to the true altitude. In summary, when flying in warm air, the altimeter reads lower than it should, leading to the misunderstanding that the aircraft is at a higher altitude than its actual position. This phenomenon is particularly crucial for pilots to understand, as flying with an incorrect altitude readout can lead to safety issues, especially in relation to terrain and other aircraft.