Why is there a discrepancy between freezing altitude and air temperature?

[bernk]

I'm looking at areas of the map where the temperature is -20°C or even lower, yet the freezing altitude iso line passing through same area shows 1400, which I assume is altitude in meters? Is freezing altitude different from freezing level? What am I missing?

[idefix37]

@bernk
Temperature shows the forecast temperature at 2m high above ground.
Freezing altitude is given by the temperature of the air mass without the effect of the ground on this temperature.
2m temperature varies daily due to radiative input with sun (heating) or loss at night (cooling).
So the air mass temperature does not show this daily variation.
In addition the 2m temperature in mountain is not very precise with low resolution models.