One of the most underappreciated hazards in advanced operations is the effect of extreme cold temperatures on pressure altimetry. Your altimeter is calibrated to the International Standard Atmosphere (ISA), which assumes a sea-level temperature of 15°C (59°F) and a standard lapse rate of approximately 2°C per 1,000 feet. When actual air temperatures deviate significantly below ISA, the atmosphere is denser and more compressed than the instrument assumes. The result: your altimeter will read higher than your aircraft's true altitude. In extreme cold, this discrepancy can be hundreds of feet — enough to put an aircraft into terrain or obstacles during an approach.
The foundational principle comes directly from basic gas laws: cold air is denser, so a given column of air exerts the same pressure at a lower physical height than it would in a standard atmosphere. When the altimeter senses a certain pressure and converts it to an indicated altitude, it does not know the air is colder and denser than ISA. It displays a number that is optimistic — you appear higher than you actually are. The old aviator's memory phrase captures this perfectly: