A pressure altimeter does not measure height. It measures air pressure and converts it to an altitude using a model of the atmosphere, and when the real atmosphere is much colder than that model, the number on the dial flatters you. On a freezing day near high ground, that gap between indicated and true altitude is worth understanding.
This is general educational information, not operational, legal, or regulatory advice. Rules differ by authority and change over time. Always verify against current official sources and follow your operator's approved procedures.
Why cold air fools the altimeter
A pressure altimeter is calibrated to the standard atmosphere: a fixed assumption about how pressure falls with height. Cold air is denser than standard, so pressure falls off more quickly with height, and a given pressure level sits lower than the instrument assumes.
The practical result, as the FAA Pilot's Handbook puts it, is that in colder-than-standard air the altimeter over-reads: it shows a higher altitude than you are actually at, so your true height above the ground is less than indicated. The classic reminder is "from high to low, look out below", covering both moving toward lower pressure and toward lower temperature. This is distinct from the altimeter setting itself, which corrects for pressure, not temperature.
How large the error is
The error grows with two things: how far you are above the altimeter setting source, and how far the temperature sits below standard. As a rule of thumb it is about 4 per cent of the height above the source for every 10 degrees Celsius below standard.
So at 2000 feet above the source on a day 30 degrees colder than standard, the error is roughly 3 times 4 per cent, or about 12 per cent of 2000 feet, which is about 240 feet lower than indicated. That is small at low level on a mild day and large when you combine a cold day with a high minimum altitude over terrain. The rule of thumb is for awareness; the precise figures come from the cold-temperature correction table in ICAO Doc 8168 (PANS-OPS), keyed to the reported aerodrome temperature and the height above the source.
When it matters, and what you correct
The danger case is an instrument procedure in cold conditions, particularly in mountainous terrain, where the minimum altitudes are there to keep you clear of high ground. If the altimeter over-reads, your true clearance is eroded just when you have least to spare.
The fix is to add the correction to the published minimum altitudes so your true height meets the minimum. As the FAA AIM and SKYbrary describe, some States and operators require this only at or below a published temperature, and some airports are specifically designated for cold-temperature correction. When you adjust an altitude that ATC has assigned, tell the controller, so the change is understood.
Crucially, separation from other aircraft is not the concern. Every altimeter on the same setting shares the same temperature error, so aircraft stay correctly separated from one another. What the cold takes away is true height above terrain and obstacles.
A worked example
Imagine an approach with a step-down minimum of 3000 feet above the setting source, flown on a day 20 degrees colder than standard:
- The rule of thumb is 4 per cent per 10 degrees below standard, so 20 degrees gives about 8 per cent.
- Eight per cent of 3000 feet is about 240 feet.
- Left uncorrected, your true height at the step-down could be roughly 2760 feet, not 3000.
- Adding the 240-foot correction and flying 3240 indicated restores about 3000 feet of true clearance.
The exact correction from the State's table may differ a little, but the lesson holds: the colder it is and the higher you are above the source, the more you must add.
Common pitfalls
- It is the height above the source, not your altitude, that counts. A high field elevation with a low procedure altitude can mean a small height above the source and a small correction.
- Standard altimetry does not catch it. Setting the correct QNH is right and necessary, but it corrects for pressure, not temperature; the cold error remains until you apply a temperature correction.
- Do not correct away separation. Apply corrections to terrain-driven minimum altitudes per procedure, not to assigned levels at random, and coordinate any change with ATC.
In Pilot EFB
Pilot EFB includes a cold-temperature altitude correction among its built-in calculators, so you can see the correction for a given height above the source and temperature with the working shown. The figures are for your own awareness and planning; Pilot EFB is offline-first and is not a certified Electronic Flight Bag, so apply your operator's and the State's published procedures and the official correction tables before you act on them.