Ice fog is a special type of fog which forms when the humidity is close to 100% and the temperature is below freezing. The suspended water which makes up the fog can be in either supercooled liquid or ice form. When that moisture-laden air touches any solid surface, the cold moisture immediately precipitates out as a layer of ice.
At humidities close to 100% and temperatures close to the triple point of water, large amounts of ice crystals, water droplets, and water vapour exist simultaneously. This causes an extremely dense ice fog! Sometimes this kind of ice fog is called freezing fog, to differentiate it from fog which is made up entirely of ice crystals.
Freezing fog is rare in most of the world. However, the extended close-to-freezing winter temperatures and large amount of natural running water between the Great Lakes means that much of southern Ontario east of the Great Lakes is prone to freezing fog, especially in valleys and along rivers. It's not unusual here to see a farmer's field covered in ice fog, while the higher gravel roadbeds around it are fog-free. In eastern parts of Ontario up to the Ottawa Valley, freezing fog is often accompanied by freezing drizzle.
Freezing fog can deposit just a little ice or a great deal of ice, depending on the difference between the air temperature and the ground temperature. If it's a rough farm field or there's already snow on the ground, the moisture just forms rime crystals on top of it. That can turn into a snow crust over time, or sometimes just crunchy snow.
However, if ice fog meets cold bare pavement, the moisture precipitates onto that pavement as black ice. If the temperature's close to freezing, it might look wet. If it's well below freezing and no other chemicals are involved, it will look dry. This is particularly common where bridges span open water, even if it's just a small river.
The special type of ice fog known in some regions as pogonip is always caused by suspended ice crystals, which coat everything they touch in a sheet of ice. When it forms over water, it is referred to as arctic sea smoke.
Pogonip can happen only when the temperature gets very, very cold while the relative humidity stays high. Northern British Columbia and Alaska see a great deal of this kind of ice fog, usually when arctic outflow warnings are issued. There's other places that get much colder in winter, but few that are wetter!
There's an old belief that pogonip can kill. That's partly false and partly true. The ice crystals which make up pogonip can't kill you, even if they do get into your lungs. However, whenever you're breathing air that cold, the air itself is definitely cold enough to freeze your lungs. Wear a scarf!
What causes ice fog
Ice fog requires three conditions to form: extremely high humidity, freezing cold temperatures, and still air during temperature inversions. That's why valleys and rivers are prone to ice fog in the winter. It's extremely unusual to see ice fog if there's any real wind at all.
That's why there's rarely ice fog during a flash freeze. Flash freeze warnings are usually issued when an Arctic cold front sweeps through the area while there's still a lot moisture in the air and on the ground. Even though it's cold enough and and humid enough, Arctic fronts are usually accompanied by strong northwest winds which make it impossible for ice fog to form. Of course, those winds also often bring snow squalls downwind of the Great Lakes or ocean, but that's completely a different issue.
Alaskan ice fog also requires the freezing cold temperatures and still air temperature inversions. However, natural sources of humidity aren't sufficient to produce ice fog by themselves. Most of the high humidity in the Fairbanks region is caused by local power plants, followed by automobile combustion and even human and dog breath. Together, these sources pump about 4000 tons of water vapor into the air each day. When the temperature hits around -40 or so, all that water vapor snap-freezes into tiny ice crystals, causing ice fog.