What is the opposite word of fog?

There isn't one single perfect opposite for "fog". It depends on what aspect of fog you want to contrast:

If you're thinking about visibility:

* Clear

* Bright

* Sunny

If you're thinking about the atmospheric condition:

* Dry (fog is often associated with moisture)

* Clear skies

If you're thinking about the metaphorical meaning of fog, like confusion or obscurity:

* Clarity

* Transparency

* Understanding

So, the best opposite for "fog" depends on the specific context you're using it in.