I have seen a clear morning with the air saturated in which one aircraft took off and within 2 minutes or so the field was fogged in. In southern Ontario there is a region in which fog forms almost instantly. I think in these cases the dewpoint is actually higher than the temperature but fog doesn't form until there is a disturbance or some nucleii to form centres of condensation. I am also sure that the lost precision between F and C is important as other contributors have said.
After an excellent landing you can use the aircraft again!