Definition of fog patches: a discontinuous layer of fog deeper than 2 meter over land surface or 10 meter over sea surface. When the layer is not deeper than 2 meter (or 10 meter over sea) it is referred as shallow fog. It is formed on calm clear radiation nights especially over open fields or above water surfaces. When entering, visibility is suddenly reduced to less than 1 km, which may cause some dangerous effects on the road traffic.

The picture was taken at sunrise with clear skies and no wind.