Fog

Factors involved: Geography, temperature, weather conditions

Regions affected: Coasts, especially those where a cool ocean current is present; mountains; cities; towns

Definition

Fog is a cloud of atmospheric water droplets near the level of the earth's surface that reduces visibility to one kilometer (0.62 miles) or less. The loss of visibility can make fog a serious transportation danger.

Science

Fog most commonly occurs when the temperature of any surface falls below the dew point of the air directly above it, causing water vapor in the air to condense around microscopic solid particles in the air, such as dust, ice, or air pollution. Massed, the droplets make visibility difficult. By definition, fog exists when visibility is less than 0.62 mile (l kilometer), while higher visibility under similar conditions is considered mist. There are a number of different kinds of fog, depending on the circumstances that lead to its generation.

Radiation fog (also known as ground fog) occurs in the night and early morning hours, when the cooling of the ground has created a temperature differential between the ground and the moist air directly over it. The lower ground temperature (at lower temperatures the air is less able to hold moisture) causes the moisture in the air immediately above it to condense into tiny droplets. Radiation fog typically dissipates as the morning sun warms the ground, though this may not be observed at higher altitudes.

Advection fog occurs when moist air moves over colder water. This is the kind of fog common along coasts, especially those where a colder ocean current tends to parallel the coast. If wind speeds increase, the density of this kind of fog also tends to increase, unless the wind speed is such as to blow away the moist air mass constituting the fog. For that to occur, wind speeds greater than 15 knots are needed.

Upslope fog occurs in areas in which the prevailing winds blow over a large surface area from a moist region toward a region of increasing altitudes. As the wind blows upslope, it creates the temperature gradient between the ground and the moister air that can induce fog.

Occasionally, precipitation in the form of drizzle can turn into fog. This can occur if the drizzle is falling through cool air that becomes saturated as a result of the drizzle. Such fogs can become very dense and are most apt to occur in places where relatively high rainfall is the norm.

In high latitudes, what are known as ice fogs are rather common. In these cases, below-freezing temperatures cause moisture in the air to become suspended ice crystals that dominate the atmosphere, creating the effect of fog. For such fogs to form, very low temperatures are needed—at least minus 25 degrees Fahrenheit.

The fog that comes off the surface of a body of water in early winter is often warmer than the air above. Even though the vapor pressure of the water is higher (the reverse of the normal condition creating fog) droplets will sometimes move upward from the water creating the effect of fog. Such fog is called steam fog or evaporation fog.

Geography

Radiation fog may occur anywhere if the proper temperature differential exists, but it is most common in areas where there are different elevations. This type of fog tends to concentrate in depressions or river valleys. It tends to "burn away" during the early morning hours if the day is sunny—the heat of the sun dries up the condensed water vapor.

Advection fog is most common along coasts and most frequent along coasts where a cold ocean current flows and creates the necessary temperature differential. The cold ocean current is the defining factor, and for this reason fog is very common on seacoasts where such currents exist. The West Coast of the United States, from San Francisco northward, is subject to such conditions, with the prevailing wind blowing the moisture off the ocean onto the land. The Pacific coast of North America averages at least sixty days of dense fog each year.

The east coast of Canada, especially Newfoundland and Labrador, is notorious for its dense fogs. These fogs result from the cold Labrador current that runs up that coast. Even farther south, on Cape Cod, Massachusetts, fogs are fairly common, although they lack the intensity of those along the east coast of Canada.

Iceland and the British Isles are notorious for their fogs, again resulting from the temperature differential between the land and the surrounding ocean. On the other hand, fogs are rare in the lower latitudes farther south in Europe, although radiation fog may exist in, for example, the Alpine valleys of Switzerland.

Although true fog (rather than mist) is rare in tropical areas, there are some regions that do experience it. One is the Peruvian coastline, where, although there is little actual precipitation, vegetation can survive in an essentially desert climate from the condensation of the moisture contained in the fogs. In some communities in Peru, inhabitants rely on fog for the majority of their water supply, using fog-catcher devices in the form of large screens that collect the water droplets from the air. Another tropical area that experiences fog is the coast of Somalia, in eastern Africa, where some unusual coastal currents create the necessary temperature differential.

Arctic fogs have created problems for weather-gathering stations in Greenland for a number of years. They are particularly intense on the east coast of Greenland.

Although the reduction in the use of coal-fired steam engines has reduced the amount of steam vented into the atmosphere around cities, auto exhausts and emissions from power plants can, if added to natural fog, produce what is often called smog. This mixture of natural fog and emissions can be a health hazard as well as a navigational challenge. Some cities, located where prevailing winds cannot disperse such atmospheric collections because of adjacent mountains, have severe problems with smog—Denver and Los Angeles are examples.

Prevention and Preparations

Because of the hazard to transportation, especially air transport, at various times efforts have been made to try to disperse fog, especially at transportation hubs. Seeding a fog with salt has been found effective but has some obvious environmental drawbacks. Another method, creating a blast of hot air along the runways of airports, has been used in some critical situations but demands an extremely large fuel input. However, when temperatures are below freezing (below 25 degrees Fahrenheit), success has followed seeding of fog with solid carbon dioxide crystals. Another method occasionally used has been spraying with propane gas. However, when temperatures are above freezing (and most fogs form under such conditions), no satisfactory method has been found to disperse fogs at airports.

Foghorns have been the traditional method of warning vessels both at sea and on large bodies of inland waters, such as the Great Lakes. The most effective antidote to fog has been the development of radar, which sees through fog. This methodology has become increasingly successful in handling air traffic, although the radar devices have had to become more exact as the volume of traffic has grown. Even so, and even though all commercial pilots must be able to land a plane solely with the use of instrument indicators, fog can shut down air operations. Most pilots prefer being able to see a runway before they land. Even localized fogs can disrupt the schedules of virtually all airlines because they interrupt interconnecting flights.

Although airplane crashes in the United States resulting from fogs are now relatively rare because the flights are regularly shut down when fog closes in at an airport, there is always an intensive investigation by the Federal Aviation Administration (FAA) if there is a crash. Because the federal government controls all the airline flights through its air traffic control system, flights are routinely canceled when a serious fog situation exists.

Fog continues to be a problem in automobile travel, although the development of the interstate highway system, with its dual road structure, has helped reduce the dangers. However, the majority of roads remain two-lane roads, and it is up to the individual motorist to drive with exceptional care in foggy conditions.

As shipping has become more a system for moving freight than for moving people, the risk of marine accidents is no longer what it once was. Still, in certain areas fog continues to be a problem for oceangoing traffic despite the assistance of radar.

Historical Overview of Fog Disasters

Although fogs can occur anywhere in the world under the appropriate conditions, they are more common in the northerly latitudes, especially along seacoasts. Consequently, they began to pose a major navigation problem as exploration and trade took people on longer journeys. As the development of new vessels made people more venturesome on the sea, fog became even more of a risk factor. Vessels tended to hug the shoreline, where the lack of visibility in a fog posed the risk of running aground on a difficult-to-see coast.

While fog has affected navigators throughout the world, the history of European exploration illustrates the challenges faced by those traveling through regions known for significant fogs. As European fishermen braved the Atlantic to fish in the rich waters of the Grand Banks off Newfoundland, the fogs that often enshroud that peninsula often became deadly. Breton fishermen risked their lives in search of cod as early as the fifteenth century, and thousands of fishermen have lost their lives in shipwrecks brought about by the inability of the crew to see. In the lobby of one of the principal hotels on the island of Miquelon near Newfoundland, for example, there is a chart listing more than 300 wrecks that have occurred along the Newfoundland coastline, most as a result of fog. The locals maintain that the list significantly undercounts the number of wrecks that have cost fishermen their lives.

Throughout the nineteenth century, as residents of coastal areas of New England went to sea to make a living, the risk of shipwreck along the rocky New England coast remained great. In the middle of the nineteenth century the whaling vessels of New England numbered more than seven hundred. Because the ships possessed only relatively rudimentary navigational instruments and navigated by sight, fogs posed a real danger. Even the adoption of foghorns at many risky coastal points did not relieve the danger for sailing vessels.

New technology, especially the invention of wireless radio in the late nineteenth century enabling ship-to-shore communication, reduced some of the risks posed by fog. Radio communication from shore stations to vessels during fog was only introduced gradually, however; the first such signal on the Great Lakes was sent in 1925. The invention of radar in World War II vastly reduced the risks of fog at sea, as it enabled vessels to "see" even under conditions of heavy fog.

Marine disasters under foggy conditions did not disappear with the introduction of wireless radio, however. On May 29, 1914, the Empress of Ireland was struck by a Norwegian steam freighter on the St. Lawrence River; when the Norwegian vessel backed off, the Empress of Ireland quickly filled with water and went down within fifteen minutes. Although 444 people were saved, more than 1,000 died. All passengers were rescued when the George M. Cox struck Isle Royale in Lake Superior in 1933, despite foghorn warnings. Even the presence of radar did not prevent a collision between two vessels on Lake Michigan in October, 1973, although no one was injured.

The most spectacular shipping disaster attributed to fog was, however, the sinking of the Italian liner Andrea Doria in July of 1956. The Swedish liner Stockholm struck the AndreaDoria just after 11 p.m. in heavy fog. The Swedish ship had a reinforced prow, and, despite being equipped with numerous watertight bulkheads, the Andrea Doria could not be saved; it sank in the Atlantic eleven hours later. All passengers who survived the impact were saved, however, by other ships that came to the rescue.

Fog, when mixed with suspended particles in the air, can be a killer on its own. The famous London fogs, mixed with suspended particulate matter and called "smogs," proved to be particularly intense between December 5 and 9, 1952. That so-called Great London Smog is thought to be responsible for the deaths of more than 4,000 individuals. Other areas have also experienced severe smog problems, as in the case of Los Angeles's Black Wednesday in 1943.

Fog also poses a danger to surface transportation. The greatest problems have arisen in situations where numerous trains use the same track and are dependent on signals that may not be readily visible in heavy fog. London has a history of train disasters due to fog and smog. In 1947, in South Croydon, an overcrowded suburban train was rammed from behind by a faster-moving train. The signaling equipment, only partly automated, failed to alert the faster train to the presence of the suburban train on the same track. In 1957, on a day when fog reduced the visibility to as little as 66 feet (20 meters), an express train struck an electrified suburban train at St. John’s, outside of London, killing 92 people. The introduction of fully automated signaling equipment has helped prevent such disasters, although as late as 1966 a passenger train crashed into the rear of a freight train in Villafranca, Italy, causing the death of 27 people.

Fog is a major hazard to airplane traffic. Although most airplane accidents in the United States are not attributable to fog, in part because the stringent rules of the Federal Aviation Administration require that airports with severely reduced visibility be shut down, the danger is great. The crash of a Delta jet attempting to land at Logan International Airport in Boston on July 31, 1973, brought home the dangers posed by fog. Eighty-nine people lost their lives. Since then, airports have been shut down entirely when they are enveloped in fog, and incoming flights are diverted to other airports. The flight control system maintained by the federal government is entirely based on radar, which is unaffected by fog.

Bibliography

Barry, Roger G., and Richard J. Chorley. Atmosphere, Weather, and Climate. 8th ed. New York: Routledge, 2003. A strongly scientific presentation that treats fog as condensation. Provides numerous maps showing water vapor content at various locations.

"Fog." National Geographic, 2017, www.nationalgeographic.org/encyclopedia/fog/. Accessed 23 Oct. 2017.

Gedzelman, Stanley David. The Science and Wonders of the Atmosphere. New York: John Wiley & Sons, 1980. Contains numerous diagrams and maps. Provides descriptions of the climate in various geographical areas, with the resulting vegetation. Numerous photographs.

Hidore, John J., and John E. Oliver. Climatology: An Atmospheric Science. 2d ed. Upper Saddle River, N.J.: Prentice Hall, 2002. Contains excellent diagrams of the process of fog formation. A solid, scientific-based presentation for the general reader.

Lockhart, Gary. The Weather Companion. New York: John Wiley & Sons, 1988. Contains some information on foghorns. Otherwise, a compendium of popular weather lore.

Lydolph, Paul E. The Climate of the Earth.Totowa, N.J.: Rowman & Littlefield, 1985. Although a generalized text on climatology, this text contains good material on the different kinds of fogs.

"What Is Fog?" Met Office, 12 July 2017, www.metoffice.gov.uk/learning/fog. Accessed 23 Oct. 2017.