Advection fog

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Advection fog

Advection fog is a type of fog that occurs when moist air passes over a cool surface by advection (wind) and is cooled. It differs from other types of fog, such as radiation fog, which is formed by the cooling of land after sunset by thermal radiation in calm conditions with clear skies. Advection fog is most common in coastal areas where sea breezes can blow the air over cooler landmasses. However, it can also occur in any area where air passes from a warmer to a cooler surface, such as over lakes, rivers, and snow-covered ground.

Formation

The formation of advection fog requires three primary conditions: a significant difference between the temperature of the air and the surface over which it is moving, moist air, and a wind that is strong enough to move the air mass but not too strong to disperse the fog particles. As the warm, moist air moves over a cooler surface, the air cools to its dew point, causing the moisture in the air to condense into tiny water droplets, which are suspended in the air and form fog.

Characteristics

Advection fog tends to be thicker and more persistent than radiation fog. It can last for several days, especially if the wind continues to supply moist air over the cool surface. This type of fog can also cover extensive areas and, depending on the temperature difference and humidity levels, can become quite dense, significantly reducing visibility. This reduction in visibility can have a considerable impact on transportation, particularly maritime and aviation operations.

Geographical Distribution

While advection fog can occur in many parts of the world, it is especially prevalent in coastal regions. One of the most famous examples is the fog that frequently envelops the San Francisco Bay Area, which is caused by moist air from the Pacific Ocean moving over the cooler landmass of California. Other notable areas that experience advection fog include the coasts of the United Kingdom, the Grand Banks off the coast of Newfoundland, and the coasts of the North and Baltic Seas.

Impact

The primary impact of advection fog is on transportation. Reduced visibility can lead to delays and accidents in shipping, aviation, and road transportation. In maritime environments, advection fog is a significant hazard, particularly in busy shipping lanes. Airports located in areas prone to advection fog often have to implement special procedures and technologies to operate safely during foggy conditions.

Prevention and Management

Managing the impact of advection fog primarily involves the use of technology to improve visibility and navigation. For maritime operations, radar and GPS are essential tools for navigating through fog. In aviation, Instrument Landing Systems (ILS) and other forms of navigational aids allow aircraft to land in reduced visibility. On the roads, fog lights and reflective road markings can help improve visibility, though the best advice is often to reduce speed and increase following distances.