Fimrite, P. May 22, 2014. As Central Valley fog disappears, fruit, nut crops decline. San Francisco Chronicle.

California produces 95% of U.S. fruit and nut crops that depend on disappearing Tule fog.

The soupy thick tule fog that regularly blanketed the Central Valley has been slowly disappearing over the past three decades, declining by 46%, a University of California, Berkeley study has found. Tule fog is a dense ground fog that usually forms during calm winds and cold temperatures after the first significant rainfall of the season and can be so dense there is only 5 feet of visibility.

“It is jeopardizing fruit growing in California, we’re getting much lower yields” said Dennis Baldocchi, a biometeorologist at UC Berkeley and lead author of the study.

Almonds, pistachios, cherries, apricots and peaches rely on the thick ground fog to hold down temperatures and bring on a dormant period, a necessary physiological process that helps them produce buds, flowers and fruit during the growing season.

“If we don’t get enough chill, the flowers and fruit doesn’t form, an insufficient rest period impairs the ability of farmers to achieve high-quality fruit yields,” said Baldocchi, a professor of environmental science, policy and management.

In 1980, for instance, there was an average of 37 foggy days in Fresno compared with 22 now. Long-term averages were used in an attempt to correct for times of drought. Only two foggy days were recorded this past winter.

Held down by warmer air from the surrounding mountains, the fog can linger for days or even weeks and cover as much as 400 miles from Bakersfield to Red Bluff (Tehama County).

And things aren’t expected to get better. Climate forecasters predict steadily warming winters in the Central Valley. Baldocchi said temperatures have increased 3 to 4 degrees Fahrenheit in places like Chico, Davis and the foothills since the 1940s. Various other studies have shown dramatic declines since 1950 in the number of hours temperatures in the Central Valley have been below 40 degrees, according to the report.

“Farmers may also need to consider adjusting the location of orchards to follow the fog, so to speak,” Baldocchi said. “Some regions along the foothills of the Sierra are candidates, for instance. That type of change is a slow and difficult process, so we need to start thinking about this now.”