For radiation fog to form you need a light wind of between 2 to 8 kts to stir the air sufficiently. Less than 2 kts and no fog, more than 8 kts low stratus (usually).
Commonest time for fog to form - what's called "after dawn thickening" - the sun comes up and gives the air a bit of a stir with some mild thermal activity and, hey presto, fog.
It has also been known for aircraft taking off to be the catalyst for the formation of radiation fog.
Hope this helps.