Suppose it is foggy outside and the temperature is 30 F. Fog tends to not produce measurable precipitation by itself but it can still wet and moisten objects. In the case of freezing fog, the fog cloud droplets are supercooled. When a droplet contacts an object below freezing it will turn to ice. When only freezing fog occurs, there will be just about as much freezing of the fog droplets onto surfaces as there will be sublimation from the surface, thus there is not much accumulation of ice. Often freezing fog will be accompanied with freezing drizzle. In that case, a film of ice will coat surfaces.

Ice fog is a fog composed of tiny ice crystals. In the ice fog situation the temperature is becoming too cold for only supercooled water to occur. Ice fog will only be witnessed in cold Arctic / Polar air. Generally the temperature will be 14 F or colder in order for ice fog to occur.