Fog

Fog forms either by air cooling its dew point—resulting in radiation fog, advection fog, or upslope fog—or by evaporation and mixing, when moisture is added to the air by evaporation and then mixes with drier air to form evaporation fog or frontal fog.

Other types of fog include ice fog (a fog of suspended ice crystals, frequently forming in Arctic locations), acid fog (fog forming in polluted air, and turning acidic due to oxides of sulfur or nitrogen), or smog (fog consisting of water and smoke particles). While any type of fog can be hazardous because of the potential dangers of reduced atmospheric visibility—especially for ground and air transportation—acid fog and smog can pose additional risk to human health, causing eye irritations or respiratory problems.