Some kind of *radiation [fog][1]* (*ground fog* is a synonym):

> **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.
> 
> **Ground fog** is fog that obscures less than 60% of the sky and does not
> extend to the base of any overhead clouds. However, the term is
> usually a synonym for radiation fog.
> 
> [ Wikipedia ]


  

[1]: http://en.wikipedia.org/wiki/Fog