Thick low hanging clouds can fill a valley so that from above you can’t see much of anything. When it starts to clear, that is when fog can have an even more magical effect on a place.
Taking a hike on a rainy day can be risky when your main goal is finding something to photograph. Get out there anyway! The worst that will happen is you will get some exercise.
Sometimes just getting out there can be a reward in itself. Other times, with some planning, and a little bit of luck, you will be rewarded in ways you cannot imagine.