i) By cooling the atmospheric air to its dew point (like advection fog, radiation fog, upslope fog)
ii) By evaporation and mixing, when the moisture is added to the particular volume of air by evaporation, and then it’s mixed with the drier air (like frontal fog, evaporation fog).
Other forms of fog include acid fog (fog that forms in polluted air, and the turning acidic because of oxides of nitrogen or sulfur), ice fog (a fog of suspended ice crystals, regularly developing in Arctic locations), or smog (fog comprising of smoke and water particles). Though any fog can be very dangerous due to its impacts on atmospheric visibility for air and ground transportation, smog and acid fog can pose an extra danger to human health, resulting in respiratory problems or eye irritations.
Radiation fog (or ground fog) happens at night when radiational cooling of the surface of Earth cools the thin, moist air layer close to the ground to its dew point or below; therefore, the moisture in the air condenses to form fog droplets. It happens under calm weather conditions, in the presence of light wind, or no wind, since a powerful wind would mix the higher-level dry air with the lower-level cold air, hence preventing the air at the lowest level from becoming saturated adequately to form fog. The availability of clouds at night can also assist in preventing the formation of the fog of this kind, since they trap the heat of Earth, not enabling the cooling of the air for condensation. Radiation fog usually forms in late winter and fall nights, specifically in lower areas, since dense and cold air moves downhill, and eventually gathers in the valleys. Consequently, radiation fog is also known as valley fog. In the morning, it often dissipates or ‘burns off’ when the heat of the Sun warms the air and ground.
Advection fog usually forms when warm, and moist air horizontally moves (that’s referred to as advection) over a cold surface that cools the air to its dew point. This type of fog can form anytime, and can be so persistent. It’s common along the coastlines where moist air primarily moves from over the water to over the land, or when the particular mass of air moves over a cold surface (like snow), and the moisture in this air condenses to form fog as the surface cools it. Advection-radiation fog forms when warm, a humid column of air moves over a cold surface that’s cold due to radiation cooling. When warm, and humid air moves over cold water, it’s referred to as sea fog.
Upslope fog forms in higher regions, where a humid air mass is forced to move up along the mountain. Though the air mass is moving up the slope, it’s cooled beyond its dew point and forms fog. It needs a fast wind, as well as warm and humid conditions at the surface. Unlike radiation fog, this kind of fog dissipates when there’s no wind, and it can also develop under cloudy skies. Upslope fog is often dense and spreads to high altitudes.
Evaporation fog develops by the mixing of any two unsaturated air masses. Steam fog is a kind of evaporation fog that forms when cold, dry air moves over warm, moist land or warm water. When some amount of the water evaporates into the low layers of air, and the warm water warms the air, forces it to rise, mixes with the colder air, cools, and then condenses some of its water vapor. Over oceans, it is known as sea smoke. Instances of cold air over warm water happen over hot tubs or swimming pools, where steam fog can form easily. It’s common, specifically in the fall season, when the winds are getting colder but the water is only gradually turning colder.
Precipitation fog is a form of evaporation fog which occurs when relatively snow or warm rain falls through cool, nearly saturated air, and the evaporation from the precipitation eventually saturates the cool air. It can turn thick, persist for a long period, and may extend over huge areas. Even though it is commonly associated with the warm fronts, it can happen with stationary fronts or slow cold fronts as well; thus, the name frontal fog is used as well.
Below are some of the fascinating facts you might not know about fog. Enjoy!
1. The foggiest region in the world goes to part of the Atlantic Ocean is known as “Grand Banks,” lying off Newfoundland coast.
The area creates the meeting site of the much warmer Gulf Stream from the south and the cold Labrador Currents from the north. This mixture of currents frequently results in the formation of fog with more than 200 days of fog each year.
2. United States city of Seattle is popular for having several foggy days annually.
3. In order for the fog to be formed the difference between dew point and temperature must be smaller than 2.5 °C (4 °F).
4. Argentina is the foggiest nation in the world, having over 200 foggy days annually.
5. The main difference between mist and fog is in their density. Mist lowers visibility to no less than 1 km while fog lowers visibility to less than 1 km.