Can there be fog on the ground and clouds in the sky?

Question

When you see a dense fog outside is it just a single cloud or all the clouds in the area?

Answer 1

The fog is simply a cloud that is in contact with the ground.  Most of the time there are few if any other clouds above the fog layer, but in some cases there can be.

Answer 2

Fog is a cloud in contact with the ground. Fog differs from other clouds only in that fog touches the surface of the Earth. The same cloud that is not fog on lower ground may be fog where it contacts higher ground such as hilltops or mountain ridges. Fog is distinct from mist only in its density. Fog is defined as cloud which reduces visibility to less than 1 km, whereas mist is that which reduces visibility to less than 2 km.