Fog is a cloud that is in contact with the ground. Stratus clouds are usually the only clouds that touch the ground. Fog differs from other clouds only in that fog touches the surface of the Earth. The same cloud that is not fog on lower ground may be fog where it contacts higher ground such as hilltops or mountain ridges. Fog is distinct from mist only in its density. Fog is defined as cloud which reduces visibility to less than 1 km, whereas mist is that which reduces visibility to less than 2 km.
The foggiest place in the world is the Grand Banks off the island of Newfoundland, Canada. Fog is frequent there as the Grand Banks is the meeting place of the cold Labrador Current from the north and the much warmer Gulf Stream from the south. The foggiest land areas in the world are Point Reyes, California, and Argentia, Newfoundland and Labrador, both with over 200 foggy days a year. Even in generally warmer southern Europe, thick fog and localized fog is often found in lowlands and valleys, like the lower part of Po Valley and the Tiber Valley, especially in late autumn and winter.
Fog begins to form when water vapor (a colorless gas) condenses into tiny liquid water droplets in the air. Conversely, water vapor is formed by the evaporation of liquid water or by the sublimation of ice. Since water vapor is colorless, it is actually the small liquid water droplets that are condensed from it that make water suspended in the atmosphere visible in the form of fog or any other type of cloud.
Fog normally occurs at a relative humidity near 100%. This can be achieved by either adding moisture to the air or dropping the ambient air temperature. Fog can form at lower humidities, and fog can sometimes not form with relative humidity at 100%. A reading of 100% relative humidity means that the air can hold no additional moisture and the air will then become supersaturated if additional moisture is added.
Fog formation does require all of the elements that normal cloud formation requires with the most important being condensation nuclei. When the air is saturated, additional moisture tends to condense rather than staying in the air as vapor. Condensation nuclei must be present in the form of dust, aeresols, pollutants, etc. for the water to condense upon. When there are exceptional amounts of condensation nuclei present, especially hydroscopic (water seeking such as salt, see below) then the water vapor may condense below 100% relative humidity.
Fog can form suddenly, and can dissipate just as rapidly, depending what side of the dewpoint the temperature is on. This phenomenon is known as flash fog.
Another type of formation also common is associated with sea fog (also known as haar or fret). This is due to the peculiar effect of salt. Clouds of all types require minute hygroscopic particles upon which water vapor can condense. Over the ocean surface, the most common particles are salt from salt spray produced by breaking waves. Except in areas of storminess, the most common areas of breaking waves are located near coastlines, hence the greatest densities of airborne salt particles are there. Condensation on salt particles has been observed to occur at humidities as low as 70%, thus fog can occur even in relatively dry air in suitable locations such as the California coast. Typically, such lower humidity fog is preceded by a transparent mistiness along the coastline as condensation competes with evaporation, a phenomenon that is typically noticeable by beachgoers in the afternoon. Another recently-discovered source of condensation nuclei for coastal fog is kelp. Researchers have found that under stress (intense sunlight, strong evaporation, etc.), kelp release particles of iodine which in turn become nuclei for condensation of water vapor.
Fog occasionally produces precipitation in the form of drizzle or very light snow. Drizzle occurs when the humidity of fog attains 100% and the minute cloud droplets begin to coalesce into larger droplets. This can occur when the fog layer is lifted and cooled sufficiently, or when it is forcibly compressed from above. Drizzle becomes freezing drizzle when the temperature at the surface drops below the freezing point.
The thickness of fog is largely determined by the altitude of the inversion boundary, which in coastal or oceanic locales is also the top of the marine layer, above which the airmass is warmer and drier. The inversion boundary varies its altitude primarily in response to the weight of the air above it which is measured in terms of atmospheric pressure. The marine layer and any fogbank it may contain will be "squashed" when the pressure is high, and conversely, may expand upwards when the pressure above it is lowering.
Fog reduces visibility and thus contributes to accidents, particularly with modes of transportation. Ships, trains, cars and planes cannot see each other and collide. Notable examples of accidents due to fog include the July 28, 1945 crash of a B-25 Mitchell into the Empire State Building, and the July 25, 1956 collision of the ocean liners the and . Although most sea vessels can penetrate fog using radar, road vehicles have to travel slowly and use low-beam headlights. Localised fog is especially dangerous, as drivers can be caught by surprise. On busy highways, multiple-vehicle collisions have resulted -- such as Interstate 4's 70-vehicle pileup that occurred on January 9th, 2008.
In aviation, fog normally prevents aircraft from taking off or landing at airports other than those equipped with an Instrument Landing System. Fog can be especially deceptive for light aircraft because often they will overfly the airfield to assess the surface conditions before landing. When the pilot is overhead the field looking straight down, he will usually only be looking through about 100 or 200 meters of fog and if it is thin enough, the pilot will be able to see the runway with a minimal amount of interference. However, when the pilot lines up for the final approach phase, instead of looking vertically through the fog, he is now looking through it at an angle, which increases the amount of haze he has to see through, often completely obscuring the runway. This presents a hazard because the pilot may have developed a mindset whereby the visibility is (or soon will be) suitable to land when it is not. The worst accident in aviation history occurred in the fog when 2 Boeing 747s collided in 1977 in Tenerife. One 747 had clearance to taxi down a foggy runway and the other could not see any distance down the runway when the captain decided to take off without proper clearance.
At airports, some attempts have been made to develop methods (such as using heating or spraying salt particles) to aid fog dispersal. These methods enjoy some success at temperatures below freezing.
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. In perfect calm the fog layer can be less than a meter deep but turbulence can promote a thicker layer. Radiation fogs occur at night, and usually do not last long after sunrise. Radiation fog is common in autumn, and early winter. Examples of this phenomenon include the Tule fog.
Ground fog is fog that obscures less than 60% of the sky and does not extend to the base of any overhead clouds. However, the term is sometimes used to refer to radiation fog.
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. It's most common at sea when tropical air encounters cooler waters, including areas of cold water upwelling, such as along the California coast. The advection of fog along the California coastline is propelled onto land by one of several processes. A cold front can push the marine layer coastward, an occurrence most typical in the spring or late fall. During the summer months, a low pressure trough produced by intense heating inland creates a strong pressure gradient, drawing in the dense marine layer. Also during the summer, strong high pressure aloft over the desert southwest, usually in connection with the summer monsoon, produces a south to southeasterly flow which can drive the offshore marine layer up the coastline; a phenomenon known as a "southerly surge", typically following a coastal heat spell. However, if the monsoonal flow is sufficiently turbulent, it might instead break up the marine layer and any fog it may contain. Moderate turbulence will typically transform a fog bank, lifting it and breaking it up into shallow convective clouds called stratocumulus.
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, or sometimes hoar frost.
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 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. In California's Central Valley, Valley fog is often referred to as Tule fog.
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. It is most often seen in urban areas where it is created by the freezing of water vapor present in automobile exhaust and combustion -products from heating and power generation. Urban ice fog can become extremely dense and will persist day and night until the temperature rises. Extremely small amounts of ice fog falling from the sky form a type of precipitation called ice crystals, often reported in Barrow, Alaska. Ice fog often leads to the visual phenomenon of light pillars.
Freezing fog occurs when liquid fog droplets freeze to surfaces, forming white rime ice. 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 which is not of the "frostless" or "frost-free" type. In some areas such as in the State of Oregon, the term "freezing fog" refers to fog where water vapor is super-cooled filling the air with small ice crystals similar to very light snow. It seems to make the fog "tangible", as if one could "grab a handful".
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 disappears.
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.
Fog is known to disappear with the the air slowly moving it away from the water front and eventually vanishing back into the air.