** LESSON: What's radiation fog? **
Radiation fog is formed by the cooling of land after sunset by thermal radiation in calm conditions with clear sky. The cool ground produces condensation in the nearby air by heat conduction.
It is similar to misting up of a mirror when taking a shower.
Calm winds prevent the fog from being mixed with drier layers and dissipating.
In this case, the snow covered fields in the valley cool the lower air layers and create an inversion (stable).