Definition
Altitude indication errors caused by air that is colder than the standard atmosphere assumed by the altimeter. Because cold air is denser and a column of it is physically shorter than a standard column at the same pressure, the altimeter reads higher than the aircraft is actually flying. The colder the temperature and the higher the indicated altitude above the altimeter setting source, the larger the error becomes, meaning true altitude can be significantly lower than indicated altitude.
Plain English
When the air outside is much colder than normal, your altimeter lies to you. It shows you higher than you really are. The colder it is, and the higher you are above the airport reporting the altimeter setting, the bigger the lie.
Context Anchor
Seen in instrument flying when planning or flying approaches, checking minimum altitudes, and applying cold-temperature altitude corrections.
Derivation
Altimeter comes from Latin roots meaning “height” and “measure.” That helps here because the problem is not the airplane’s actual height changing; it is the height-measuring instrument giving a reading that needs correction in cold air.
Why Pilots Care
Failure to apply the correction can place the aircraft below published minimum altitudes and increase the risk of controlled flight into terrain.
Analogy
Think of a ruler whose marks have been squeezed closer together. If you count the marks normally, the number can look right, but the real distance is less than you think.
Grounding Statement
If your altimeter reads 5,000 feet on a very cold day, you may actually be at 4,700 feet -- the colder it is, the bigger that gap.
Intuition Check
Do not assume a correct altimeter setting removes all altimeter error. In cold weather, the altimeter can still read too high because the air column itself is colder and compressed.
Example Sentence 1
Because the surface temperature was -20 °C, the pilot applied a cold weather altimeter error correction to the minimum altitudes on the approach.
Example Sentence 2
During the approach briefing the crew discussed how cold weather altimeter errors would affect the 1,200-foot minimums.