When flying into an area of higher pressure without resetting the altimeter, is your true altitude higher or lower than your indicated altitude?

• A. Lower
• B. Not determinable
• C. Higher
• D. Same

Answer: (C)

Pushing the discussion is the fact that an altimeter shows altitude based on the pressure setting you have selected. The air pressure at a given height changes with weather systems, so moving into an area with higher pressure means the ambient pressure at your true height is higher than what your current setting expects. If you don’t reset to the local pressure (local QNH), the altimeter reading will come out lower than your actual height above mean sea level. In other words, your true altitude is higher than what the altimeter indicates. Resetting to the local setting would bring the indicated altitude back in line with your true altitude. If you were moving from high to low pressure without resetting, the opposite would happen and the altimeter would read higher than your actual height.