Fog

As Central Valley fog disappears, fruit, nut crops decline

Peter Fimrite

The soupy thick tule fog that regularly blanketed the Central Valley and terrorized unsuspecting motorists during the winter has been slowly disappearing over the past three decades, a UC Berkeley study has found.
 
The blinding mists may not be missed by those who remember white-knuckle drives in zero visibility and regular multiple-car pileups, but the fog dearth is bad news for farmers, according to a study published this month in the journal Geophysical Research Letters.

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

Baldocchi and co-author Eric Waller, a UC Berkeley doctoral student, used weather station data and compared it with satellite records and photographs kept by NASA and the National Oceanic and Atmospheric Administration during the past 32 winters.

The satellite evidence was clear. During fog days, he said "it's just all white, like a big bathtub full of cream."
The number of days when the valley was socked-in varied widely from year to year, he said, but the average amount of tule fog between November and the end of February declined 46 percent during the study period.
 
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.

Tule fog, named after the tule grass wetlands that covered much of the Central Valley, is a dense ground fog that usually forms during calm winds and cold temperatures after the first significant rainfall of the season.
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).

The milky blanket is often so dense that people can barely see 5 feet in front of them. This is why tule fog has been the leading cause of weather-related casualties in California. Chain-reaction crashes caused by a lack of visibility have occurred on many roads and freeways, including a 25-car pileup that killed five people on Interstate 5, near Elk Grove (Sacramento County) in 1997. Two people were killed and there were multiple injuries in 2007 when 108 cars, including 18 big-rig trucks, crashed into each other on Highway 99 between Fowler and Fresno.

Baldocchi said a combination of factors may be contributing to the tule retreat, including global warming and, possibly, a decrease in crop burning. He said records indicate the amount of fog increased in the Central Valley from the 1930s through the 1970s, and then decreased starting about the same time farmers cut down on winter burning. Smoke in the sky can help produce fog as the air cools, he said.

Whatever it is, he said, "it is happening before our eyes. The trends are very distinct."

The decline is obvious to anyone who remembers how it once was.

"I grew up in the valley, and when I was a kid we had these terrible foggy winters, and I go out there now and there is hardly any fog at all," said Baldocchi, whose father grew almonds and walnuts.

The problem is that 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," said Baldocchi, a professor of environmental science, policy and management. "An insufficient rest period impairs the ability of farmers to achieve high-quality fruit yields."

That's trouble for the state's multibillion-dollar agricultural industry and could have ramifications across the country. California produces 95 percent of U.S. fruit and nut crops, which are suffering already from three years of drought.

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."