“Like clouds, fog is made up of condensed water droplets which is the result of the air being cooled to the point (actually, the dewpoint) where it can no longer hold all of the water vapor it contains. For clouds, that cooling is from the rising of air parcels, which cools from expansion. For fog, which occurs next to the ground, there are usually other reasons for this cooling. For instance, rain can cool and moisten the air near the surface until fog forms. Also, infrared cooling of a cloud-free, humid air mass at night can lead to fog formation – this is called “radiation fog”. Radiation fog is most common in the fall, when nights get longer, and land and water surfaces that have warmed up during the summer are still evaporating alot of water into the atmosphere. Finally, a warm moist air mass blowing over a cold surface (usually snow or ice) can also cause fog to form-this is called “advection fog”.”
Fog can form in a number of ways, depending on how the cooling that caused the condensation occurred:
Radiation fog is formed by the cooling of land after sunset by thermal radiation in calm conditions with clear sky. The cool ground produces condensation in the nearby air by heat conduction.
Ground fog is fog that obscures less than 60% of the sky and does not extend to the base of any overhead clouds.
Advection fog occurs when moist air passes over a cool surface by advection (wind) and is cooled. It is common as a warm front passes over an area with significant snowpack.
Steam fog, also called evaporation fog, is the most localized form and is created by cold air passing over much warmer water or moist land. It often causes freezing fog.
Precipitation fog (or frontal fog) forms as precipitation falls into drier air below the cloud, the liquid droplets evaporate into water vapor. The water vapor cools and at the dewpoint it condenses and fog forms.
Upslope fog or hill fog forms when winds blow air up a slope (called orographic lift), adiabatically cooling it as it rises, and causing the moisture in it to condense. This often causes freezing fog on mountaintops, where the cloud ceiling would not otherwise be low enough.
Valley fog forms in mountain valleys, often during winter. It is the result of a temperature inversion caused by heavier cold air settling into a valley, with warmer air passing over the mountains above. It is essentially radiation fog confined by local topography, and can last for several days in calm conditions.
Freezing fog occurs when liquid fog droplets freeze to surfaces, forming white soft or hard rime. This is very common on mountain tops which are exposed to low clouds. It is equivalent to freezing rain, and essentially the same as the ice that forms inside a freezer.
Ice fog is any kind of fog where the droplets have frozen into extremely tiny crystals of ice in midair. Generally this requires temperatures at or below −35 °C (−30 °F), making it common only in and near the Arctic and Antarctic regions.
Artificial fog is artificially generated fog that is usually created by vaporizing a water and glycol-based or glycerine-based fluid. The fluid is injected into a heated block, and evaporates quickly. The resulting pressure forces the vapor out of the exit. Upon coming into contact with cool outside air the vapor condenses and appears as fog.
Garua fog is a type of fog which happens to occur by the coast of Chile and Peru. The normal fog produced by the sea travels inland, but suddenly meets an area of hot air. This causes the water particles of fog to shrink by evaporation, producing a transparent mist. Garua fog is nearly invisible, yet it still forces drivers to use windshield wipers.
Hail fog sometimes occurs in the vicinity of significant hail accumulations due to decreased temperature and increased moisture leading to saturation in a very shallow layer near the surface. It most often occurs when there is a warm, humid layer atop the hail and when wind is light. This ground fog tends to be localized but can be extremely dense and abrupt. It may form shortly after the hail falls; when the hail has had time to cool the air and as it absorbs heat when melting and evaporating.
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