Valley fog covers a road in southern Wisconsin.

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The Weather Book
(Jack Williams)

WHAT is fog?

Fog is a cloud near the ground. The same processes that produce clouds thousands of meters above the ground can produce clouds near the surface. Both fog and clouds form when the atmosphere can no longer hold all of the moisture it contains. This happens when 1) the temperature of air drops to its dew point, which is the temperature at which air is holding as much moisture as it can or 2) the amount of moisture in the air increases until the air reaches its dew point. Once air has reached its dew point, the water in it condenses, forming tiny water droplets that we see as fog.

Fog is a hazard mostly for one very important reason: reduced visibility. Airport delays, automobile accidents, ship wrecks, plane crashes, and many other transportation problems are frequently caused by fog. On March 27, 1977, the worst air disaster in history occurred when two Boeing 747 passenger jets collided on a runway in the Canary Islands in dense fog. When air pollution (such as smoke) combines with fog, visibility decreases even more. Acid fog, resulting from the combination of air pollutants (such as nitrogen and sulfur oxides) with water droplets can create health problems, especially for people who have respiratory conditions. However, like several other natural hazards, fog can also be beneficial. Several species of plants, including some crops, depend on fog for moisture and cool temperatures from decreased sunlight.

Fog is responsible for an average of over $1 million in property damage, dozens of injuries, and several deaths every year in the United States. The financial cost of transportation delays caused by fog has not been calculated, but is substantial.

WHERE does fog occur?

Fog can occur almost anywhere in the world. Fog is classified based on how it forms, which is often related to where it forms. The following are the most common types of fog:

• Coastal Fog: When warm, moist air blows over a cold surface, the surface can lower the temperature of the air to its dew point. This type of fog is common in harbors and bays in the summer, such as the San Francisco Bay in California. In the winter, warm and humid air from the subtropical oceans blows over the cooler surface of the land, causing extensive coastal fog.
  
  
• Evaporation Fog: Water is always evaporating from the surface of streams, lakes, and oceans. When cold air blows over warm water, the moisture that is evaporating from the surface will increase the amount of moisture in the air, possibly to the point that the air can no longer hold all of the water it contains. In this case, the temperature of the air was not decreased to dew point. Instead, the moisture content of the air increased until the air reached its dew point. Steam rising from lakes is a common type of evaporation fog and is often called "steam fog".
  
  
• Radiation (or Ground) Fog: Common on clear nights with little or no wind, this type of fog is formed from the rapid cooling of the Earth's surface, usually on cloudless nights. Since the temperature of air near the surface is mostly influenced by the surface itself, the air temperature can easily drop to its dew point on calm, clear nights.
  
  
• Valley Fog: Cool air is denser than warm air, which causes cool air to sink and warm air to rise. As cool air drains down the slopes of hills or mountains in the evening, it accumulates in valleys below where it may reach its dew point and cause fog to form. Often, valley fog will "burn off" shortly after sunrise the next morning because the sun warms the air above its dew point temperature. However, valley fog can persist during the daytime hours, if temperatures do not increase above the dew point.
  
  
• Upslope Fog: As air rises over obstacles, it cools. If air is blown over hills or mountains, it may cool enough to reach its dew point. This can result in upslope fog in areas such as the eastern slope of the Rocky Mountains in Colorado. Here, moist air from the Gulf of Mexico or the southern Pacific Ocean is occasionally drifts eastward where it meets the Rocky Mountains.

WHEN does fog occur?

Fog can occur during any season. Certain seasons are more likely to have foggy days and nights in certain locations based on a variety of factors including the weather patterns that control the amount of moisture in the air and the temperature, the topography (the hills and valleys of the landscape), and how close a location is to a moisture source, such as an ocean or large lake. In the mid-latitudes, fog is common after frontal thunderstorms, which leave behind cooler air and ample moisture to form fog.

HOW do we cope with fog?

In the U.S., the National Weather Service forecasts fog and issues dense fog advisories when visibility is decreased to less than one quarter of a mile (0.4 km). These advisories alert travelers to potentially dangerous conditions. Traveling in fog requires reduced speed and careful navigation. At night, traveling in fog is especially dangerous because darkness combines with the fog to reduce visibility even more. Also, light from automobile headlights and other navigation lights is scattered off the water droplets of the fog limiting sight to only a short distance. In response, automobiles are often equipped with specially designed lights that illuminate a usually dry (and therefore clear) area just above surface.

WHO has more information on fog?

Because it is often considered only a nuisance, relatively little attention has been paid to fog as a natural hazard. Considering the losses that have resulted from fog, it may be appropriate to consider this atmospheric hazard more carefully.