Colder-than-standard temperatures at altitude cause the true altitude to be lower than the indicated altitude. This statement is:

• A. False
• B. Not enough information
• C. True
• D. Depends on altitude

Answer: (C)

Nonstandard temperature affects how far you are from the surface for a given pressure reading. When the air is colder than standard, the pressure surfaces are lower than in the standard atmosphere. The altimeter reads altitude by converting the measured ambient pressure to a height using the standard (ISA) relationship. Since colder air pushes those pressure surfaces downward, the pressure you sense at a given true height corresponds to a higher altitude in the ISA model. In other words, the altimeter tends to show a higher altitude than your actual height above the ground. Therefore, the true altitude is lower than the indicated altitude. This effect is present across flight levels and is why temperature corrections are considered in planning and when flying high or near obstacles.