4.5: Fog

Radiation Fog

Figure \(\PageIndex{1}\): Early morning radiation fog in Bangladesh. (CC BY-SA 4.0; Abdul Momin via Wikimedia Commons)

Fog is often described as a cloud resting near the ground. Fog forms when the air's temperature and dew point are equal. This can occur either by cooling the air or by adding sufficient moisture to raise the dew point. When composed of ice crystals, it is called ice fog.

Radiation fog is a very common type of fog throughout the United States and worldwide. Figure \(\PageIndex{1}\) shows radiation fog forming in the early morning hours in Bangladesh. It is most prevalent during the fall and winter, forming overnight as the air near the ground cools and stabilizes. When this cooling causes the air to reach saturation, fog will form. Fog will first form at or near the surface, thickening as the air continues to cool. The fog layer will deepen overnight as the air above it cools. As this air cools, the fog will extend upward. The most favored areas for fog development are sheltered valleys with little to no wind and locations near bodies of water. Wind would disrupt the formation of radiation fog. Radiation fog is usually patchy, tends to stay in one place, and goes away the next day under the sun’s rays. Thicker instances of radiation fog tend to form in valleys or over calm bodies of water.

Figure \(\PageIndex{2}\): Tule fog captured by NASA's Terra Satellite in January 2005. The Sacramento Valley and San Joaquin Valley are almost completely covered in tule fog. (Public Domain; Jeff Schmaltz via NASA)

Valley fog is a type of radiation fog. When air along ridgetops and the upper slopes of mountains cools after sunset, it becomes denser and heavier, draining into the valley floors below. As the air on the valley floor continues to cool through radiational cooling, it becomes saturated, and fog forms. Valley fog can be very dense at times. It is also called “tule” (TOO-lee) fog and occurs each winter in the Central Valley of California. This type of fog tends to dissipate quickly once the sun rises and begins to evaporate the fog layer. But under favorable conditions, it can persist for days or even weeks, as during the 2025 Tule Fog event in the California Central Valley. The combination of a cool, moist layer of air from the Pacific near the surface, clear skies above, and light winds results in exceptionally thick fog on many nights from late October to February. This phenomenon is often visible from satellite images like Figure \(\PageIndex{2}\).

Advection Fog

Figure \(\PageIndex{4}\): Advection fog moving across the Golden Gate Bridge in San Francisco. (CC BY 2.0; Runner310 via Flickr)

Advection fog often resembles radiation fog and results from condensation. However, condensation in this case is caused not by a reduction in surface temperature but by the horizontal movement of warm, moist air over a cold surface, such as warm, humid air flowing over snow. Advection fog can sometimes be distinguished from radiation fog by its horizontal motion along the ground. It is most common at sea when moist air encounters cooler waters, including areas of cold water upwelling, such as along the California coast. A sufficiently strong temperature difference between the Pacific Ocean and the coastal waters or bare ground results in persistent morning advection fog near San Francisco, as observed in Figure \(\PageIndex{4}\).

Evaporation/Mixing Fog

This type of fog forms when sufficient water vapor is added to the air by evaporation and the moist air mixes with cooler, relatively drier air. A common example is steam fog, which forms when cold air moves over warm water, as depicted in Figure \(\PageIndex{5}\). When cool air mixes with warm, moist air over water, the moist air cools until its humidity reaches 100%, and fog forms. This type of fog appears as wisps of smoke rising from the water's surface. This type of fog commonly occurs over the Great Lakes but can also occur on any lake or body of water, as well as on wet surfaces. Steam fog does not penetrate very deeply, but it is sufficient to block some sunlight.

Figure \(\PageIndex{5}\): When warm, moist air rising over a lake comes in contact with the cold air, steam fog forms. (CC BY 2.0; Michael Scialdone via Flickr)

Another common example is breath fog. When we exhale, our breath is warm and contains a lot of water vapor. On a cold day, the cold air rapidly lowers the temperature of our breath, causing it to reach the dew point. It creates a misty cloud, which we have experienced during cold weather.

Upslope Fog

This type occurs when sloping terrain lifts air, cooling it adiabatically to its dew point and saturation. Upslope fog may be viewed as either a stratus cloud or fog, depending on the point of reference of the observer. Upslope fog generally forms at higher elevations and moves downslope into valleys. This fog can persist at higher wind speeds due to increased lift and adiabatic cooling. Upslope winds of more than 10 to 12 knots usually result in stratus rather than fog. The east slope of the Rocky Mountains is a prime location for this type of fog.

Freezing Fog

Figure \(\PageIndex{6}\): When the temperature falls to 32°F (0°C) or below, the tiny droplets in the fog freeze when they come into contact with an object. (Public Domain; Freezing Fog via NWS)

Tiny, supercooled liquid water droplets in fog can freeze instantly on exposed surfaces when surface temperatures are at or below freezing. Some surfaces that these droplets may freeze on include stairs and rails, sidewalks, roads, vehicles, and tree branches, as depicted in Figure \(\PageIndex{6}\). Extreme caution should be taken if travel is necessary. Freezing fog can cause black ice to form on roadways. Black ice is difficult to see, making it particularly dangerous. Drive more slowly when you suspect icy conditions. For those flying, a thin layer of ice can form on aircraft, making flights very dangerous unless the aircraft is treated or equipped with effective de-icing equipment.

Ice Fog

Figure \(\PageIndex{7}\): A tree in a field during extreme cold with frozen fog. (CC BY-SA 3.0; Ian Furst via Wikimedia Commons)

Frozen fog (also known as ice fog) is any fog in which droplets have frozen into extremely tiny crystals of ice in midair. Generally, this requires temperatures at or below −35°C (−31°F), making it common only in and near the Arctic and Antarctic regions, as depicted in Figure \(\PageIndex{7}\). It is sometimes observed in urban areas, where it forms from the freezing of water vapor present in automobile exhaust and in combustion products from heating and power generation. Urban ice fog can become extremely dense and will persist day and night until the temperature rises. It can be associated with the diamond dust form of precipitation, in which very small ice crystals form and slowly fall to the ground. This often occurs under blue-sky conditions, which can produce a variety of halos and other effects of sunlight refraction by airborne crystals, such as light pillars.