What exactly is freezing fog, and what's the difference between freezing fog and ice fog?
When temperatures at ground level drop to or below freezing, the water droplets making up fog often freeze on contact. That's freezing fog. The result can be black ice, which makes driving or walking very slick and dangerous. Ice fog, on the other hand, forms at temperatures well below zero...generally twenty below or colder. It's made up of ice crystals suspended in the air, and is most common near water sources. It may seem odd that water wouldn't be frozen at those temperatures, but fast-moving stretches of rivers can often remain unfrozen at frigid temperatures. In fact, ice fog tends to become thickest at temperatures of forty to fifty below zero.