If the atmospheric visibility near the earth's surface is reduced to 0.62 mi (1 km) or less due to floating water droplets in the air, it is called fog. Fog can form in two ways: either by cooling the air to its dew point (e.g., radiation fog, advection fog, upslope fog), or by evaporation and mixing, when moisture is added to the air by evaporation, and then it is mixed with drier air (e.g., evaporation fog, 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 its effects on atmospheric visibility for ground and air transportation, acid fog and smog can pose...