Question

Why can we see the fog when we are far away from it, but not when we come close to it?

Answer 1

We see fog when we are at some distance because there is a larger space between (consisting of more layers of fog) us and that point compared to when we are nearer to it. All that fog in the intervening space accumulates, and to us it looks like a thick blanket of fog, making it hard to see further. If we were nearer, less fog would be accumulated and thus making it a lot easier for us to see.

We also observe that fog is more in open spaces because of the same reason.