STOCKTON - Valley fog - scourge of white-knuckled drivers, savior of the cherry crop - has declined by about one-third over the past three decades, according to new research.


There have been hints of this for years, mostly in the stories people tell. University of California, Berkeley, Professor Dennis Baldocchi grew up near Brentwood and worked the Delta's orchards.

"My experience is there's less fog," he said. "My dad feels that way. All my cousins who work in the Delta feel that way."

So Baldocchi and researcher Eric Waller decided to find out what was really going on.

Fog records can be fickle, so they analyzed 30 years of satellite data instead - more than 10,000 images, altogether - and concluded that about 20 percent of winter days are foggy today, compared with 30 percent before.

You might consider this good news, if you're a morning commuter or a Valley pilot. Less fog might also bring down your home heating bill.

But it's decidedly bad news for farmers who need a certain number of chill hours each winter. Less fog means more direct sunlight on the buds of sensitive fruit trees, a threat to the agricultural industry's $10 billion annual fruit and nut crop.

"It is getting tougher and tougher in California," said University of California, Davis, plant scientist Kitren Glozer. "This is not just anecdotal, although the anecdotes are well worth listening to."

The decrease in fog days appears to be most obvious in November and December, with very little change in January. That's a problem because chilling is especially important early in the season, Glozer said.

"You cannot easily play catch-up," she said.

Valley fog materializes when the air is colder than the soil. That differential causes moisture on the ground to rise and condense in the air, forming that familiar, thick fog that blocks out the sun.

Warmer temperatures as a result of global climate change make it more difficult to create that difference in temperature between the ground and the air, Glozer said.

But there may be more to it than global warming.

First of all, fog these days appears to be less thick over cities, which are warmer than surrounding undeveloped areas. So the outward growth of Valley cities could hinder the development of fog.

Second, air pollution in the Valley has declined in recent decades. Why does that matter? Fog forms when water vapor latches onto particles such as dust.

Less pollution, less fog, said Bill Patzert, an oceanographer with the NASA Jet Propulsion Laboratory in Pasadena.

"I'm a big global warming person, but global warming has not been that big yet," Patzert said. "In the Central Valley and in the great urban areas like Los Angeles, the bigger effect is what's happening locally."

And of course, the amount of fog any given year varies because of the weather. A wet winter with many storms is likely to clear out much of the fog. (Although, strangely, a dry 2011-12 included below-average fog accumulation in the Valley.)

The long-term trend means farmers must adapt, but it's not such bad news on the roadways. After repeated deadly pileups on Highway 99 near Fresno, Caltrans has instituted a fog alert system using message boards and weather stations.

A spokesman could not say if the agency had noted any long-term changes in fog hazard.

Less fog also means less hassle at Stockton Metropolitan Airport. The airport doesn't have the same sophisticated technology as, say, San Francisco International Airport, so foggy days can hinder incoming flights.

"It cuts down on business, since people can't get in," airport manager Patrick Carreno said. "We hate foggy days. If we have fewer of them, that works for us."

Longtime flight instructor Rick Tutt shared one of those anecdotes that the Berkeley researchers had heard so much about before breaking out the satellite images and confirming what everyone had said.

"I grew up in Lodi in the late '50s and early '60s, and sometimes we'd have six weeks of fog where we never saw the sun until late in the afternoon just before it went down," Tutt said. "We just haven't had those kind of fog seasons anymore."
