What happens to pressure altimeters calibrated under ISA conditions in non-standard temperatures?

When pressure altimeters are calibrated under International Standard Atmosphere (ISA) conditions and then operate in non-standard temperatures, the readings can be affected. Specifically, in cooler than standard temperatures, the altimeter tends to indicate a lower altitude than the true altitude. This is because pressure decreases with altitude, and a colder atmosphere is denser, causing the altimeter to interpret the higher pressure as being lower in altitude than it actually is.

In non-standard temperatures, particularly colder than ISA, the atmosphere does not behave as the altimeter is calibrated to expect. Therefore, instead of accurately reflecting the true altitude, the altimeter underestimates the altitude due to the denser air. This phenomenon is critical for pilots to understand as it can impact their altitude readings significantly, leading potentially to a risk of flying at lower altitudes than required for safety, particularly in mountainous terrain or congested areas.