Which condition would cause the altimeter to indicate a lower altitude than true altitude?

• A. Air temperature warmer than standard
• B. Air temperature cooler than standard
• C. Low atmospheric pressure
• D. High relative humidity

Answer: (A)

Temperature deviations from standard change how pressure is mapped to altitude on the altimeter. The altimeter is essentially a sensitive barometer calibrated to the standard atmosphere, so it uses a fixed pressure-to-altitude relationship. When the air is warmer than standard, the atmosphere expands and the pressure surfaces are higher than the standard model predicts. At a given true altitude, the ambient pressure is higher than the standard-pressure prediction, and the altimeter interprets that higher pressure as being closer to the surface, so it shows a lower altitude than you actually have. If the air were cooler than standard, the opposite would occur and the altimeter would indicate a higher altitude than true. Low atmospheric pressure shifts the scale but doesn’t inherently produce a lower indication unless temperature effects are considered; humidity has only a small effect on the pressure-to-altitude relationship and isn’t a primary driver of this error.