Under which atmospheric conditions does an altimeter tend to read higher than your true altitude?

• A. Going from high to low pressure or hot to cold temperature
• B. Going from low to high pressure or cold to hot temperature
• C. Altimeter readings are unaffected by pressure changes
• D. Only when the aircraft is on the ground

Answer: (A)

The main idea is that the altimeter converts ambient pressure into a height using the standard atmosphere. When actual conditions deviate from that standard, the pressure-zenith relationship changes and the altimeter’s reading shifts away from the true altitude.

If you move from a region of high pressure to low pressure, the ambient pressure at your true altitude becomes lower than the altimeter expects for that height, so the instrument indicates a higher altitude than you actually are. Similarly, if the air temperature cools (hot to cold), the pressure distribution changes so that the same ambient pressure corresponds to a higher indicated height on the altimeter’s standard scale. In both cases, the altimeter tends to read higher than your true altitude.

By contrast, rising pressure (low to high) or warming (cold to hot) would push the reading in the opposite direction. The other options either ignore the pressure/temperature influence or state effects that don’t occur in flight.