It creeps up on you slowly. At first you don't really notice the rolling fog. Maybe you find it calming, maybe you find it cooling against your sweaty clothing since you've been climbing for hours and you're nearing treeline.
It's not until, perhaps, you look back to the trail you've just come from or you finally near the summit, that you realize how far you can see. Not very far at all that is. And now it has become dangerous as you begin to contemplate-- should you head back down before the fog gets worse or continue on?