Cold air moved in over the weekend, and a high pressure cell pushed cold, wet air up against the mountains from the east. Such conditions produce an upslope condition in which that wet air rises and cools still more and cloaks our foothills in the Rockies with dense fog. With the temperature depressed just below freezing, the super-cooled water droplets in the fog accumulate to form a coating of Rime Frost on the upwind side of nearly everything.