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In cold air your altimeter reads higher than your true altitude. At –30 °C, an aircraft 3,400 ft above the airfield can be about 600 ft lower than the altimeter shows. On an instrument approach, that error eats directly into obstacle clearance.
Why cold air fools the altimeter
A barometric altimeter measures pressure, then converts it to height using the International Standard Atmosphere. ISA assumes 15 °C at sea level, cooling about 2 °C per 1,000 ft.
Cold air is denser, so the pressure levels sit closer together. With the correct QNH set, the altimeter still assumes standard spacing between levels. The result: you are lower than indicated, and the error grows with height above the station that supplied the altimeter setting.
The old memory aid still works: from high to low (pressure or temperature), look out below.
The quick rule of thumb
The altimeter error is about 4% of your height above the airport for every 10 °C the airport temperature is below 15 °C.
Example: airport at –15 °C, you are 2,000 ft above it. That is 30 °C below 15 °C, so 3 × 4% = 12%. 12% of 2,000 ft = 240 ft too high on the altimeter.
Use the rule for awareness and cross-checks. Use the official ICAO table, or your aircraft’s temperature compensation, for the actual correction.
The ICAO formula
The correction tables come from this formula, where H is height above the airport (ft) and t is the airport temperature (°C):
ΔH = H × (15 − t) ÷ (273 + t − 0.5 × 0.00198 × H)
Approximate corrections (ft to add)
Values below come from the formula, rounded to the nearest 10 ft. They match the published ICAO table closely, but fly with the official table in the AIM or your operations manual.
| Airport temp | 200 ft | 500 ft | 1,000 ft | 2,000 ft | 3,000 ft | 5,000 ft |
|---|---|---|---|---|---|---|
| 0 °C | 10 | 30 | 60 | 110 | 170 | 280 |
| –10 °C | 20 | 50 | 100 | 190 | 290 | 480 |
| –20 °C | 30 | 70 | 140 | 280 | 420 | 710 |
| –30 °C | 40 | 90 | 190 | 370 | 560 | 950 |
| –40 °C | 50 | 120 | 240 | 480 | 720 | 1,210 |
Above 5,000 ft height above the airport, the FAA lets you use the 5,000 ft column.
FAA Cold Temperature Restricted Airports
In the US, some airports carry a snowflake symbol and a temperature (for example ❄ –30 °C) on the approach chart. When the reported airport temperature is at or below that value, you must correct the published altitudes on the affected approach segments.
Key rules from the FAA notice:
- Correct the FAF/PFAF, step-down fixes, the MDA or DA, and the final missed approach holding altitude, as required for the segment.
- Tell ATC the corrected altitude for any segment except the final segment.
- Do not correct altitudes on SIDs, obstacle departure procedures or STARs.
- ATC does not correct its minimum vectoring altitudes, so query any vectored altitude that is below your corrected altitude.
- Never change the altimeter setting to make a correction. Keep the current setting and add the correction to the altitudes.
Worked example: Yampa Valley (KHDN), –30 °C
This is the FAA’s own example for the RNAV (GPS) RWY 28 approach.
- Airport elevation 6,606 ft. FAF altitude 10,000 ft. Height above airport = 3,394 ft.
- From the table at –30 °C: about 600 ft. FAF becomes 10,600 ft; the IAFs at 13,000 ft become 13,600 ft.
- LP MDA 7,080 ft is 474 ft above the airport. Correction about 90 ft, so fly a corrected MDA of 7,170 ft.
A 600 ft error at the FAF is far larger than most pilots expect. That is the point of the rule.
What cold temperatures do not affect
An ILS glideslope and a WAAS (LPV) glidepath are geometric, so cold air does not bend them. A baro-VNAV glidepath is barometric, so it is affected. That is why RNAV approaches publish separate temperature limits for the LNAV/VNAV line of minima.
Outside the United States
Under ICAO PANS-OPS, cold temperature corrections are the pilot’s responsibility, and most operators apply them whenever the airport temperature is below 0 °C. Transport Canada and many European operators publish their own tables and procedures. Follow your operator’s manual first.
Check it yourself
- Pull the airport temperature with the METAR decoder.
- Compare ISA with the actual temperature using the ISA guide.
- Convert the setting with the QNH hPa/inHg converter.
FAQ
Does cold weather make my altimeter read high or low?
High. You are lower than the altimeter shows.
At what temperature should I correct?
In the US, at or below the temperature next to the snowflake on the chart. Elsewhere, usually at or below 0 °C, per your operator.
Do I correct the decision altitude on an ILS?
Yes. The glideslope is unaffected, but the DA is a barometric altitude, so add the correction.
Can I just set a different altimeter setting?
No. Keep the current QNH and add the correction to the altitudes.
Reference only. Always use official charts, the AIM or your operator’s procedures for real flight decisions.
Sources and further reading
- AIM 7-3, Cold Temperature Barometric Altimeter Errors and Cold Temperature Airports
- ICAO Doc 8168, PANS-OPS, Volume I (temperature correction)
- FAA Instrument Procedures Handbook (FAA-H-8083-16)
Sources checked November 2026. Rules differ between countries and change over time: for real flights, use the current publications of the authority that applies to you, your aircraft flight manual and your operator’s procedures.