High pressure creates an inversion, trapping cooler air near the Earth's surface. Moisture on the ground condenses into a low-lying layer of fog. Because the coolest air settles into the lowest elevations, tule fog is most likely in valleys, along rivers and streams.
The phenomenon is common in California's Santa Clara, Livermore and Central Valleys. Tule fog usually forms during the summer when the difference in temperature between the San Francisco Bay area and the valleys is small during the day and much greater at night.
The dense fog can reduce visibility to near zero and causes more auto accidents in California than any other weather condition. Tule fog was responsible for an accident in December 1997, in which five people were killed and 28 injured on Interstate 5 near Sacramento. The pile-up involved 25 cars and 12 tractor-trailer trucks.
Tule fog gets its name from a Native American word for the bulrush grass that grows in the freshwater marshes of California's central valleys. According to the Bay Nature Institute, wetlands once covered the Sacramento and San Joaquin River floodplains in such abundance that fog covered the valley floor during the rainy season.
According to Invisible 5, an organization that tries to raise awareness of pollution-producing activities along the I-5 corridor in California, air pollutants intensify the formation of tule fog by readily absorbing moisture and acting as condensation nuclei that lead to smaller droplets and, therefore, thicker fog. The group says high concentrations of particulate matter from diesel exhaust, incineration and other pollution sources help fog form more easily and may make tule fog more frequent.
