Instrument Flight Rules (IFR) Practice Exam 2026 – The All-in-One Guide to Master Your IFR Certification!

When the altimeter indicates a lower altitude than the true altitude, it is primarily caused by changes in atmospheric pressure, particularly when the atmospheric pressure is lower than the standard atmosphere. The altimeter is calibrated to a standard atmospheric pressure of 29.92 inches of mercury (Hg). If the actual atmospheric pressure is lower than this standard, the altimeter will read an altitude that is higher than what the actual altitude is, because the altimeter senses the pressure around it, not the actual height above sea level.

While air temperature can affect altitude readings, it does so in a different context. If the air temperature is warmer than standard, the altimeter will typically indicate a higher altitude than what is truly present, as warmer air is less dense and causes the altimeter to react as though the aircraft is at a higher altitude than it actually is.

The concept of humidity essentially contributes to the density of air, but comparatively, it does not primarily affect the altimeter readings in this specific context as significantly as changes in atmospheric pressure do.

Thus, understanding that the altimeter functions based on standard atmospheric pressure is crucial in determining how deviations from that standard lead to discrepancies in altitude readings.