At 2,000 feet with an ambient temperature of 0°C, how does the true altitude compare to the indicated altitude?

• A. Higher
• B. The same
• C. Lower
• D. Cannot be determined

Answer: (C)

The main idea is that the altimeter assumes a standard (ISA) temperature profile. When the air is colder than ISA, the vertical spacing of pressure surfaces is reduced, so for a given pressure the actual geometric height above the ground is less than what the standard atmosphere would predict. The altimeter, using that standard relationship, will indicate a higher altitude than you actually have. So your true altitude is lower than the indicated altitude.

In this case, 0°C is well below ISA at 2,000 feet, so the true altitude would be noticeably lower than 2,000 feet. A practical estimate often used is that the correction is roughly 4 feet per 1,000 feet per degree Celsius of deviation, so with about an 11°C deviation and 2,000 feet, the true altitude would be on the order of ~90 feet lower than indicated, keeping the key takeaway: true altitude is lower than what the altimeter shows.