BERKELEY (KCBS) — Results of a new study reveal that tule fog–the dense ground fog that blankets the Central Valley during the winter months—has been steadily decreasing for the last 30 years.

Dennis Baldocchi, a biometeorologist and lead author of the UC Berkeley study, said that while drivers may be happy about this development, many farmers are not.

“Many fruit and nut crops need 500 to 1000 hours of temperatures between about 32 and 42 degrees Fahrenheit,” he said.

The blanket of fog helps to keep winter temperatures in the 40s throughout the day. When the weather is clear, overnight temperatures may be sharply colder but then warm up to the 50s and 60s when the sun comes out.

“It just doesn’t allow them to achieve the levels of dormancy they need to have to get the physiological rest,” Baldocchi said.

Baldocchi said a combination of factors may be responsible—including global warming and, possibly, a decrease in crop burning. But climate forecasts suggest this pattern of warmer winters and less tule fog will continue into the foreseeable future.

In the meantime, plant breeders are working to develop varieties that need less of a winter chill.

Tule fog is named after the tule grass wetlands that covered much of the Central Valley. It usually forms during calm winds and cold temperatures after the first significant rainfall of the season.Warmer air from the surrounding mountains can hold the fog down causing it to linger for days or even weeks.

It can get so thick that can drivers along the freeways can of have about 5 feet of visibility and it’s the the leading cause of weather-related casualties in the state.


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