Freezing level
Freezing level is the height where the air cools to 0 °C. Below it the air is above freezing, so it marks where airframe icing becomes possible in cloud or precipitation.
Cool the surface temperature at the standard (ISA) lapse rate, 1.98 °C per 1000 ft (TC AIM AIR 1.5.1): freezing level (ft AMSL) = elevation + (surface temp ÷ 1.98) × 1000.
Use that standard rate, not the dry (3 °C) or saturated (1.5 °C) adiabatic rate (TC AIM RPA 3.2.22.8); those describe a rising parcel, not the air sitting in place.
Note: estimating the freezing level this way assumes the real air follows the standard lapse rate (a rule of thumb); the GFA icing/turbulence chart forecasts the actual freezing level directly (TC AIM MET 4.12).
Unit trap: freezing is 0 °C = 32 °F. A surface temperature in °F must be converted to °C first; the elevation may be in metres (×3.28 to feet).
Trap: the answer is height above sea level. Add the station elevation, do not stop at the height above the station.
References
- TC AIM AIR 1.5.1
- TC AIM RPA 3.2.22.8
- TC AIM MET 4.12