How do variations in temperature affect the altimeter?

• A. Pressure levels are raised on warm days and the indicated altitude is lower than true altitude
• B. Pressure levels are lowered on warm days and the indicated altitude is higher than true altitude
• C. Indicated altitude remains unchanged by temperature
• D. Altimeter readings are not affected by temperature changes

Answer: (A)

When temperature deviates from standard atmosphere, the vertical distribution of air pressure shifts, and the altimeter’s reading is biased because it assumes standard pressure–temperature relations. On warm days, the air expands and pressure surfaces rise higher than in the standard model. At a fixed true altitude, the ambient pressure remains what the warmer column produces, which the altimeter interprets as being at a lower altitude. So the pressure levels are raised, and the indicated altitude is lower than the true altitude. Temperature effects do not leave readings unchanged, nor do they make pressure surfaces lower on warm days, so the other options don’t fit.