As winter moisture rolls in from the Pacific Ocean and makes snow on the Sierra Nevada range, it also can create blankets of white across the Central Valley below. Known as Tule (too-lee), this opaque fog spread across California in time for the 2020 winter solstice. Though it can look pretty from above, the dense fog can be hazardous for people on the ground.
With more frequent rain and lighter winds during winter, moist coastal air masses moving in through the lowlands, and cooler air slipping down from the mountains, the San Joaquin and Sacramento valleys are fertile territory for fog formation. Daytime sunlight warms the soil and causes evaporation, while nighttime cooling can cause that moisture to saturate the air. If winds are calm, the water droplets accumulate until fog clouds form near the ground.
Tule fog tends to form after sunset and becomes thickest just before dawn. It slowly thins as the air warms in daytime sunlight. The name is derived from its formation over reedy, wetland vegetation—Los Tules or tullin.
The National Weather Service (NWS) in Hanford, California, issued dense fog advisories for several consecutive days in mid-December 2020. Drivers were cautioned to reduce speed and to use low-beam headlights as visibility dropped to near-zero in places. “It is normal for fog to come in this time to year, but it was unique how quick and thick it came in,” NWS meteorologist Jim Anderson told The Fresno Bee. The Hanford team even tweeted a photo of conditions right outside their office. The pattern is expected to continue sporadically into next week.
The natural-color images above were acquired between December 19–21, 2020, by the Moderate Resolution Imaging Spectroradiometer (MODIS) instruments on NASA’s Terra and Aqua satellites. Tule fog spread out across hundreds of miles of central California around midday, the timing of each day’s NASA satellite overpass.
Despite the hazards, the fog is important to California’s crops because fruit and nut trees need sufficient rest between growing seasons to be most productive. They get that rest in winter when cooler temperatures bring on a dormant period. Fog typically indicates such cooler weather; it also shields trees from direct sunlight that would otherwise warm the plant buds.
According to scientists, tule fog has been forming less often in California in recent decades. University of California–Berkeley researchers using satellite images found that the number of foggy days between November and February decreased by 46 percent from 1981 to 2014. Fewer fog days correspond with fewer cold days, and other studies have confirmed that trees in the valley are now exposed to hundreds fewer cold hours per year.
NASA Earth Observatory images by Joshua Stevens, using MODIS data from NASA EOSDIS/LANCE and GIBS/Worldview. Story by Michael Carlowicz with Holli Riebeek.