On Thursday, 11 December, the Grand Canyon experienced a “total cloud inversion.” If this sounds like something backwards, that’s because it is. Normally, temperatures are much higher in the canyon than above it (and it is not uncommon to have summer-ish temps at the very bottom while snow sits on the rim above), but once every few years it gets reversed and a layer of cold air is trapped in the canyon by a layer of warmer air above. If there’s enough moisture in the system, the colder air condenses, forming fog.
You can download wallpaper-sized versions of this photo on the National Parks Flickr page. The NPS also posted a time-lapse video, condensing (ha) 15 minutes of time into one.
(Hat tip to Karyn and Mary.)