On my birthday, I got out for a brief stroll in the foggy frost (or frosty fog?) that was still lingering about mid day. Here's a photo from that excursion.

I wanted to capture the dull blanket of the fog. The relative clarity (a term used loosely in this case) of the foreground tree with those in the background captures that effect. Here, positioning is important. Ideally, I would liked to have been closer to the foreground tree and made it a bit clearer, but that would not have allowed a similar composition given the surroundings (the foreground tree's lowest branches are quite high, and there is a tall chainlink fence just below frame of the background trees).
Believe it or not, this is a colour photo. The fog is thick, and the trees are backlit, so the scene looks pretty much as it was when I stood there. I did fuss with the tone curve a bit to get a satisfactory amount of detail in the foreground tree's branches.
A few more photos from this outing can be found on my Flickr photostream, tagged with 'fog'.