Starting around the beginning of November and lasting through March, a dense ground fog settles in California’s Central Valley each year.
What is Tule Fog?
Named after the tule grass wetlands (tulles) found in the area, the Tule Fog is a radiation fog caused by the combination of a high relative humidity (typically after a heavy rain), calm winds, and rapid cooling during the night.
The fog develops on cold winter nights when the ground is moist from recent rain. The evaporating water from the soil condenses into a fog when the temperatures drop low enough.
Poor Visibility When the Tule Fog is Present
Visibility in a Tule Fog is poor, varying from about 600 feet or 200 meters to sometimes nearly zero.
Occurring in the San Joaquin Valley and Sacramento Valley areas, Tule Fog can extend over 400 miles (650 kilometers) from Bakersfield to Red Bluff.
The surrounding mountain ranges typically constrain the extent of the Tule Fog.
Decline of the Tule Fog
A group of researchers analyzed satellite imagery over a 33 year span to establish an understanding of the fog climatology in the Central Valley.
Using satellite imagery from MODIS and from NOAA’s advanced very high resolution radiometer (AVHRR), researchers counted the number of days each year when the Tule fog occurred.
In a paper published in Geophysical Research Letters, Baldocchi and Waller found that since 1981, “the number of winter fog events, integrated spatially, decreased 46%, on average, over 32 winters, with much year to year variability”.
Impact on Local Agriculture
Since the occurrence of the Tule fog traps cold air in the Central Valley area, years when the fog is not as prevalent are warmer. The decline in Tule fog has a negative impact on the local agriculture, particularly in its fruit and nut tree production.
The Central Valley is responsible for 95% of the fruit and nut tree production in the United States. These trees require a certain threshold of winter chill in order to rest for the next season’s production of buds and flowers.
The decline in the occurrence of Tule fog correlates to less winter chill which in turn affects productivity of these trees. High productivity in this trees requires a period of dormancy brought on by winter temperatures between 0° and 7° Celsius (32° and 45° F).
Hourly temperatures between 0° and 7° Celsius are known as “chill hours”.
Another study done in 2008 analyzed climatological records and predicted that chill hours are dropping by about 40 hours per decade.
Fruit and nut trees in Central California need from 200 to 1,500 hours at temperatures between 32 and 45°F during the winter. By the mid- to late-twentieth century, it is anticipated that climatic conditions would no longer support the growth of some of the varieties of California’s major tree crops.
In addition, the absence of Tule fog exposes the tree buds to sunlight which also brings up the temperature of those buds even when the surrounding air is cold.
Baldocchi, D., & Wong, S. (2008). Accumulated winter chill is decreasing in the fruit growing regions of California. Climatic Change, 87(1), 153-166. https://doi.org/10.1007/s10584-007-9367-8