Is this a special effect?

No, and images showing this strange fog appeared on the BBC today!
It's a combination of a natural Fog, and a great deal of smoke from the many fires set on the moor to burn off the old heather! you can actually tase as well as smell the burning.
Fryup Dale, lunchtime today.
You only get this effect with the sunbehind you, from the North it simply looks like fog, but from the South it was Hoizon to horizon!
And above it a lovely blue sky over a pale lower level