Radiation fog, a common wintertime feature of the Central Valley, has distinct formation processes compared to California’s coastal fog
By MADELEINE PAYNE — firstname.lastname@example.org
Winter in Davis often means chilly bike rides to class, sunsets that start as early as 5 p.m. and the occasional presence of a dense, motionless fog known as tule fog. In the Central Valley, this weather phenomenon is known for causing limited visibility and for posing dangerous conditions for travelers. According to the U.S. Department of Transportation, tule fog is the leading cause of weather-related traffic accidents in California.
Yet, according to a study published in 2019, the annual number of winter tule fog days is declining. From 1980 to 2016, tule fog in the Central Valley decreased by 76% — a stark contrast from the trends present in 1930 to 1970, when fog days increased by 85%.
A UC Berkeley research team, led by doctoral student Ellyn Gray, conducted the study and analyzed several factors that could explain the decrease in fog, including temperature, wind speed, dew point depression, precipitation and air pollutant levels. They found that emission trends and the NOx concentration, or the abundance of oxides of nitrogen in the air, were longer-term drivers in the changing number of days of tule fog.
Allen Goldstein, the senior author of the study and distinguished professor in the Department of Environmental Science, Policy, & Management and Civil and Environmental Engineering at UC Berkeley, explained how the formation of fog, which is essentially a cloud at ground level, is affected by the number of available particles in the air.
“Fog droplets, just like cloud droplets, typically start by water condensing onto a small particle we often refer to as a cloud condensation nuclei,” Goldstein said. “When you get more of these small particles in the air, particularly if they’re hygroscopic, or if they want to absorb water, then you’re more likely to condense the water that’s in the air on these particles rather than, say, dew forming on the ground.”
The study discussed how tule fog in the Central Valley exhibits a north-south trend, with southern regions averaging more days of tule fog than northern regions. This is similar to trends in emission pollution, with higher rates of NOx concentration occupying the southernmost part of the valley. Researchers found that climate fluctuations drove the short-term annual changes in fog frequency and that dew point depression, the difference in degrees between the air temperature and the dew point, played a crucial role. Emission trends were responsible for the longer-term temporal and spatial changes across the Central Valley.
Tule fog, also known as radiation fog, has some distinct features when compared to fog found in coastal areas of California. Ian Faloona, a professor in the Department of Land, Air and Water Resources at UC Davis, elaborated on the physical processes that make tule fog unique.
“At night, the ground radiates in the infrared out into space,” Faloona said. “Some of the atmosphere absorbs some of that radiation and also radiates it back to the ground, but [it’s] not nearly as much as the ground radiates up. It’s constantly cooling overnight, and, when there’s enough water vapor in the air, it cools down to the dew point, and then you create the fog.”
In reference to the large-scale geographic factors that affect tule fog formation, the Central Valley is a perfect mix of low elevation with protection in the east from the Sierra Nevadas, according to Faloona. This helps create the right conditions for relatively unmoving air, a key component to the still, lifeless quality of radiation fog.
“If you have enough wind coming in, that will mix in enough dry air to eliminate any kind of fog formation,” Faloona said. “One of the key ingredients [to radiation fog] is having stagnant winds.”
Fog by the shoreline of California often takes on a more mobile form, rolling in and out of coastal areas accompanied by a breeze. While the coastal fog formation is spurred by wind, the physical processes of formation for coastal fog and tule fog are fairly similar, according to Qi Zhang, a professor in the Department of Environmental Toxicology at UC Davis.
“If you go to the fundamentals, it’s the same process, which is the cooling of humid air,” Zhang said. “In the summertime in the coastal areas, you have this body of water that has lower temperatures, and you have this warm air that goes above the cooler surface. Then, it depends on the humidity of that air and the temperature difference between the warm air and the cool surface. This can lead to the temperature inside the air mass to approach the dew point, leading to the formation of fog.”
Yet one of the more distinctive features of coastal fog is the large-scale processes which create the proper conditions for fog formation, including oceanic movements and the Earth’s rotation, according to Faloona.
“The ocean water is particularly cold because of a process called upwelling, where the wind blows on the ocean which moves the ocean southward and that’s what’s called the California Current,” Faloona said. “As that current is moving towards Baja and equatorward, because the earth is rotating, it gets pulled to its right and therefore offshore.”
This shift of water away from the coast opens a void by the coast that the deeper, colder waters move upward to fill, according to Faloona. Thus, the colder water can interact with the warmer air to produce fog. Besides creating cool, humid conditions on the coast, this upwelling is important for stimulating phytoplankton growth and for supporting the food web, while the fog from the upwelling can also have crucial climate implications, Faloona explained.
“It’s usually associated with these giant shields of low cloud over the ocean which are called stratocumulus,” Faloona said. “It turns out these clouds are very important and have been studied tremendously because they reflect so much sunlight. […] The difference between that cloud being there and not being there actually has very strong fulcrum on our climate. These are very important centers of climate cooling.”
Besides the role it plays in food webs and global climate change, fog has also become a cultural symbol within California communities. In San Francisco, the coastal fog has even been given a name, “Karl the Fog,” to describe the low clouds that thread through the hilly city streets. After a two-year hiatus on Twitter, Karl the Fog posted on Jan. 20 to share a recent update since the start of the pandemic — marking the triumphant return of a beloved Bay Area persona to the public sphere.
“Oh, hey down there,” the fog wrote. “I know it’s been awhile, so hope you still recognize me [Crossed fingers emoji] (I put on a few metric tons over the last two years). Moved in with my parents in Point Reyes at the start of the pandemic. The free rent was great, but nothing beats hanging 6ft away from you.”
Written by: Madeleine Payne — email@example.com