Fog is defined as a cloud with its base at or very near the ground. Physically, there are no differences between fog and a cloud; their appearances and structures are the same. The essential difference is the method and place of formation. While clouds mainly form when air rises and cools adiabatically, fog occurs because air either is cooled below its dew-point temperature or gains water through evaporation until it becomes saturated (evaporation fog).

Fog is considered an atmospheric hazard because it reduces visibility (▼). Official weather stations report fog only when it is thick enough to reduce visibility to  kilometer ( mile) or less. Dense fog can cut visibility to a few dozen meters or less, making travel by any mode difficult and dangerous.

Fogs Caused by Cooling

When the temperature of a layer of air in contact with the ground falls below its dew point, condensation produces fog. Depending on the prevailing conditions, fogs formed by cooling are called either radiation fog, advection fog, or upslope fog.

Radiation Fog

As the name implies, radiation fog results when ground cooled by radiation cools the overlying layer of air. It is a nighttime phenomenon that requires clear skies and a high relative humidity. As the night progresses, a thin layer of air near the ground is cooled below its dew point, resulting in the formation of fog.

Because the air containing the fog is relatively cold and dense, it flows downslope in hilly terrain. As a result, radiation fog is thickest in valleys, whereas the surrounding hills may remain clear (▲A). Normally, radiation fog dissipates within 1 to 3 hours after sunrise. The common phrase that fog “lifts” is misleading; actually, as the Sun warms the ground, the fog evaporates from the bottom up.

Advection Fog

When warm, moist air blows over a cold surface, it becomes chilled by contact with the cold surface below. If cooling is sufficient, the result is a blanket of fog called advection fog. (The term advection refers to air moving horizontally.) A classic example is the frequent advection fog around San Francisco’s Golden Gate Bridge (▼). The fog experienced in San Francisco, California, as well as many other west coast locations, is produced when warm, moist air from the Pacific Ocean moves over the cold California Current.

Advection fog is also a common wintertime phenomenon in the Southeast and Midwest when relatively warm, moist air from the Gulf of America (Gulf of Mexico) and Atlantic moves over cold and occasionally snow-covered surfaces to produce widespread foggy conditions. This type of advection fog tends to be thick and produce hazardous driving conditions.

Upslope Fog

When relatively humid air moves up a gradually sloping landform or, in some cases, up the steep slopes of a mountain, upslope fog can form. Because of the upward movement, air expands and cools adiabatically. If the dew point is reached, an extensive layer of fog will form.

It is easy to visualize how upslope fog might form in mountainous terrain. However, in the United States, upslope fog also occurs in the Great Plains, when humid air moves from the Gulf of America (Gulf of Mexico) toward the Rocky Mountains. (Recall that Denver, Colorado, is called the “mile-high city,” and the Gulf of America (Gulf of Mexico) is at sea level.) Air flowing “up” the Great Plains expands and cools adiabatically by as much as 12°C (22°F), which can result in extensive upslope fog in the western plains.

Evaporation Fogs

When air forms fog mainly because of the addition of water vapor, the fog is called an evaporation fog. Two types of evaporation fogs are recognized: steam fog and frontal (precipitation) fog.

Steam Fog

When cool, unsaturated air moves over a warm water body, enough moisture may evaporate to saturate the air directly above, generating a layer of fog. The added moisture and energy often makes the saturated air buoyant enough to rise. Because the foggy air looks like the “steam” that forms above a hot cup of coffee, the phenomenon is called steam fog (▼). Steam fog is a fairly common occurrence over lakes and rivers on clear, crisp autumn mornings when the water is still relatively warm but the air is comparatively cold.

Frontal (Precipitation) Fog

Frontal boundaries where a warm, moist air mass is forced to rise over cooler, dryer air below generate frontal fog, also referred to as precipitation fog. Fog develops because the raindrops falling from relatively warm air above the frontal boundary evaporate in the cooler air below, causing it to become saturated. Frontal fog, which can be quite thick, is most common on cool days during extended periods of light rainfall.

Where Is Fog Most Common?

The frequency of dense fog varies considerably from place to place (▼). As might be expected, fog incidence is highest in coastal areas, especially where cold currents prevail, as along the Pacific and New England coasts. Relatively high frequencies are also found in the Great Lakes region and in the humid Appalachian Mountains of the eastern United States.

• Fog is a cloud with its base at or very near the ground. Fogs form when air is cooled below its dew point or when enough water vapor is added to the air to cause saturation.

• Fogs formed by cooling include radiation fog, advection fog, and upslope fog. Fogs formed by the addition of water vapor are steam fog and frontal fog.

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