Some kind of 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. In perfect calm the fog layer can be less than a meter deep but turbulence can promote a thicker layer. Radiation fogs occur at night, and usually do not last long after sunrise, but they can persist all day in the winter months especially in areas bounded by high ground such as the Vale of York in England. Radiation fog is most common in autumn and early winter. Examples of this phenomenon include the Tule fog.
[ Wikipedia ]