Those pistachios you like? Chances are, they came from California’s Central Valley – a region that provides more than half the nation’s non-citrus fruit and nuts.

The area’s climate is ideal for growing cherries, apricots, and walnuts, in part because each winter, a thick ground fog called tule fog blankets the valley.


Baldocchi: “This Tule fog can be very, very, very dense. As children, we’d be living in this, and you couldn’t see from the windshield to the edge of your car, it was so dense.”

That’s Dennis Baldocchi, a biometeorologist at UC-Berkeley. He says the tule fog helps keep temperatures low in winter, so fruit and nut trees stay dormant.

Baldocchi: “When you have fog, you’ve got this blanket of clouds essentially hugging the ground. It reflects the sunlight, so it doesn’t penetrate to the ground and warm the ground and the air.”

But Baldocchi has found that tule fog has decreased more than forty percent in the last thirty years – putting many crops at risk. Climate change and drought are both likely causes.

To reflect the sun, some farmers are now covering their crops with a white powder. Others are using chemicals to keep the trees dormant. But both methods are expensive, and likely to lead to higher consumer prices.

Reporting credit: ChavoBart Digital Media.
Photo: Fog rolling into the central valley, California. Copyright protected.

More Resources
Winter fog is decreasing in the fruit-growing region of the Central Valley of California
Central Valley sees big drop in wintertime fog needed by fruit and nut crops (News release)
NASA/Earth Observatory Image (on Tule fog in Central CA)
Climatic Change Study on declining winter chill
Loathed by motorists, loved by fruit trees, California’s tule fog fades away
California Department of Food and Agriculture Statistics

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Bruce Lieberman

Bruce Lieberman, a long-time journalist, has covered climate change science, policy, and politics for nearly two decades. A newspaper reporter for 20 years, Bruce worked for The San Diego Union-Tribune...