Fog is considered a low cloud that is either close to ground level or in contact with it. As such, it is made up of water droplets that are in the air like a cloud. Unlike a cloud however, the water vapor in fog comes from sources close to the fog like a large water body or a moist ground. For example, fog usually forms over the city of San Francisco, California during the summer months and the moisture for that fog is produced by the cool ocean waters that are nearby. By contrast, moisture in a cloud is gathered from large distances that are not necessarily near where the cloud forms.
Formation of Fog
Like a cloud, fog forms when water evaporates from a surface or is added to the air. This evaporation can be from the ocean or another body of water or moist ground like a marsh or a farm field, depending on the type and location of the fog. According to Wikipedia, water vapor is also added to the air via winds, precipitation, daytime heating and the evaporation of water from a surface, plant transpiration or air rising over mountains (orographic uplift).
As water begins to evaporate from these sources and turn into water vapor it rises into the air. As the water vapor rises, it bonds with aerosols called condensation nuclei (i.e. - small dust particles in the air) to form water droplets. These droplets then condense to form fog when the process occurs close to the ground.
There are however, several conditions that need to first occur before the process of fog formation can be complete. Fog usually develops when relative humidity is near 100% and when the air temperature and dew point temperature are close to one another or less than 4˚F (2.5˚C). When air reaches 100% relative humidity and its dew point it is said to be saturated and can thus hold no more water vapor. As a result, the water vapor condenses to form water droplets and fog.
Types of Fog
There are various types of fog that are categorized based on how they form. The two main types though are radiation fog and advection fog. According to the National Weather Service, radiation fog forms at night in areas with clear skies and calm winds. It is caused by the rapid loss of heat from the Earth's surface at night after it was gathered during the day. As the Earth's surface cools, a layer of moist air develops near the ground. Over time the relative humidity near the ground will reach 100% and fog, sometimes very dense forms. Radiation fog is common in valleys and often when the fog forms it remains for long periods when winds are calm. This is a common pattern seen in California's Central Valley.
Another major type of fog is advection fog. This type of fog is caused the movement of moist warm over a cool surface like the ocean. Advection fog is common in San Francisco and it forms in the summer when warm air from the Central Valley moves out of the valley at night and over the cooler air over the San Francisco Bay. As this process occurs, the water vapor in the warm air condenses and forms fog.
Other types of fog identified by the National Weather Service include upslope fog, ice fog, freezing fog and evaporation fog. Upslope fog occurs when warm moist air is pushed up a mountain to a place where the air is cooler, causing it to reach saturation and the water vapor to condense to form fog. Ice fog develops in Arctic or Polar air masses where the air temperature is below freezing and is composed of ice crystals suspended in the air. Freezing fog forms when the water droplets in the air mass become supercooled. These drops remain liquid in the fog and immediately freeze if they come into contact with a surface. Finally, evaporation fog forms when large amounts of water vapor are added to the air through evaporation and mixes with cool, dry air to form fog.
Because certain conditions must be met for fog to form, it does not occur everywhere, however there are some locations where fog is very common. The San Francisco Bay Area and Central Valley in California are two such places, but the foggiest place in the world is near Newfoundland. Near Grand Banks, Newfoundland a cold ocean current, the Labrador Current, meets the warm Gulf Stream and fog develops as the cold air causes the water vapor in the moist air to condense and form fog.
In addition, southern Europe and places like Ireland are foggy as is Argentina, the Pacific Northwest and coastal Chile.
Bodine, Alicia. (n.d.). "How Does Fog Form." Ehow.com. Retrieved from: http://www.ehow.com/how-does_4564176_fog-form.html
National Weather Service. (18 April 2007). Types of Fog. Retrieved from: http://www.crh.noaa.gov/jkl/?n=fog_types
Wikipedia.org. (20 January 2011). Fog- Wikipedia, the Free Encyclopedia. Retrieved from: http://en.wikipedia.org/wiki/Fog