Freezing Level & Icing Layer Calculator (0 °C Altitude, −10 °C, −20 °C, Cloud Base & Icing Band)
Estimate the freezing level from surface temperature and lapse rate, the altitudes of −10 °C and −20 °C, the cloud base from the temperature–dew point spread, and the band of cloud where airframe icing is most likely, with the altitudes that keep you out of it.
How it works
altitude of temperature X = elevation + (surface T − X) ÷ lapse × 1,000 ft · cloud base AGL ≈ (T − Td) ÷ 2.5 °C × 1,000 ft · icing band = visible moisture between the freezing level (or cloud base) and the −20 °C level (or tops); highest risk 0 to −10 °C
Airframe icing needs two things: visible moisture and a temperature between about 0 and −20 °C, where cloud droplets stay liquid below freezing and freeze on impact. The freezing level and the −10 °C and −20 °C levels follow from the surface temperature and the lapse rate, and the cloud base from how far the temperature is above the dew point. Overlay the cloud on the temperature profile and the icing band appears — and with it the obvious choices: stay below the freezing level, get above the tops, stay clear of cloud, or do not go.
Worked example
At an 800 ft airport with 8 °C and a 4 °C dew point and a standard 2 °C/1,000 ft lapse rate, the freezing level is about 4,800 ft, −10 °C about 9,800 ft and −20 °C about 14,800 ft. The cloud base is about 2,400 ft MSL; with tops at 9,000 ft, the icing band is 4,800–9,000 ft, and a 7,000 ft cruise at about 4 °C below freezing sits in the worst part of it. Flying at 4,000 ft, below the freezing level, is the way out if terrain allows.
Frequently asked questions
How do I estimate the freezing level?
Take the surface temperature, divide by the lapse rate and add to the elevation: 10 °C at sea level with the standard 2 °C per 1,000 ft puts 0 °C at about 5,000 ft. Moist, cloudy air cools slower (1.5 °C/1,000 ft) and pushes it higher; inversions make the rule useless — check the forecast freezing level.
At what temperatures is icing worst?
Between 0 and about −10 °C, where large supercooled drops freeze slowly and spread back as clear or mixed ice. From −10 to −20 °C rime is more likely; below −20 °C most cloud is ice crystals and the risk falls, though not to zero in convective cloud.
What about freezing rain?
Freezing rain happens when rain falls from a warm layer aloft into sub-freezing air below — the opposite of the normal profile. It can build ice faster than any anti-ice system handles; the escape is usually to climb into the warm layer, but the answer is to not be there. A simple lapse-rate estimate will not show it.
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For training, planning and education only. The aircraft's approved flight manual (POH/AFM), current charts, NOTAMs, weather and regulations govern every real flight. Verify with your instructor and official sources.