Identify the basic types of fog and describe how each forms.

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. Whereas clouds result when air rises and cools adiabatically, fog results from cooling or when air becomes saturated through the addition of water vapor (evaporation fog) rather than the changes in pressure that cool rising air.

Although not inherently dangerous, fog is generally considered an atmospheric hazard (Figure 5.9). During daylight hours, fog reduces visibility to 2 or 3 kilometers (1 or 2 miles). When the fog is particularly dense, visibility may be cut to a few dozen meters or less, making travel by any mode difficult and dangerous. Official weather stations report fog only when it is thick enough to reduce visibility to 1 kilometer (0.6 mile) or less. Table 5.2 summarizes the basic fog types.

Fogs Formed 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 from radiation cooling of the ground and adjacent air. It is a nighttime phenomenon that requires clear skies, high relative humidity, and relatively light wind. Under clear skies, the ground and the air immediately above cool rapidly. Because of the high relative humidity, a small amount of cooling lowers the temperature to the dew point. If the air is calm, the fog is usually patchy and less than 1 meter (3 feet) deep. For radiation fog to be more extensive vertically, a light breeze of 3 to 5 kilometers (2 to 3 miles) per hour is necessary, to create enough turbulence to carry the fog upward 10 to 30 meters (30 to 100 feet) without dispersing it. High winds, in contrast, mix the air with drier air above and disperse the 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 (Figure 5.9A). Normally, radiation fog dissipates within 1 to 3 hours after sunrise—and is often said to “lift.” However, the fog does not actually “lift.” Instead, as the Sun warms the ground, the lowest layer of air is heated first, and the fog evaporates from the bottom up. The last vestiges of radiation fog may appear as a low layer of stratus clouds.

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 will be 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 (Figure 5.10). 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.

A certain amount of turbulence is needed for proper development of advection fog; typically winds between 10 and 30 kilometers (6 and 18 miles) per hour are required. Not only does the turbulence facilitate cooling through a thicker layer of air, but it also carries the fog to greater heights. Thus, advection fogs often extend 300 to 600 meters (1000 to 2000 feet) above the surface and persist longer than radiation fogs. An example of such fog can be found at Cape Disappointment, Washington—the foggiest location in the United States. The name is indeed appropriate because the station averages about 2552 hours of fog each year—equivalent to 106 days.

Advection fog is also a common wintertime phenomenon in the Southeast and Midwest when relatively warm, moist air from the 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 can produce hazardous driving conditions.

Upslope Fog

As its name implies, upslope fog forms when relatively humid air moves up a gradually sloping landform or, in some cases, up the steep slopes of a mountain (Figure 5.11). 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 Mexico toward the Rocky Mountains. (Recall that Denver, Colorado, is called the “mile-high city” and that the 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 saturation occurs primarily because of the addition of water vapor to air, the resulting fogs are called evaporation fogs. 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 cause it to rise. Because the foggy air looks like the “steam” that forms above a hot cup of coffee, the phenomenon is called steam fog (Figure 5.12). Steam fog is a fairly common occurrence over lakes and rivers on clear, crisp mornings in the autumn when the water is still relatively warm but the air is comparatively cold. Steam fog usually forms a shallow foggy layer because as it rises, the fog droplets mix with the cool unsaturated air above and dissipate (evaporate).

In a few settings, steam fogs can be dense—especially during the winter, as cold arctic air pours off the continents and ice shelves over the comparatively warm open ocean. The temperature contrast between the warm ocean surface and overlying cold air mass has been known to exceed 30°C (54°F). The result is thick steam fog produced as the rising water vapor saturates a large volume of air. Because of its source and appearance, this type of dense steam fog is given the name arctic sea smoke.

Frontal (Precipitation) Fog

Frontal boundaries where a warm, moist air mass is forced to rise over cooler, dryer air below generates frontal (precipitation) fog. The foggy conditions result because the raindrops falling from relatively warm air above the frontal surface evaporate in the cooler air below, causing the cooler air to become saturated. Frontal fog, which can be quite thick, is most common on cool days during extended periods of light rainfall. Although less common, it is possible for frontal fog to form behind a cold front by the same process.

Figure 5.13 shows the predominant fog type and frequency of dense fog for various locations. 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. In contrast, fogs are rare in the interior of the continent, especially in the arid and semiarid areas of the West (the yellow areas in Figure 5.13).

• advection fog: Fog formed when warm moist air is blown over a cool surface and chilled below the dew point.

• fog: A cloud with its base at or very near Earth’s surface.

• frontal (precipitation) fog: Fog formed when rain evaporates as it falls through a layer of cool air.

• radiation fog: Fog resulting from radiation cooling of the ground and adjacent air; primarily a nighttime and early morning phenomenon.

• steam fog: Fog that has the appearance of steam and that is produced by evaporation from a warm water surface into the cool air above.

• upslope fog: Fog created when air moves up a slope and cools adiabatically.

• Fog, generally considered an atmospheric hazard, is a cloud with its base at or very near the ground.

• 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.