‘Japan current’ plays big role in weather pattern
The Sportsman’s Report
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By Bill Hanson  April 18, 2014 12:00 am

At a recent winery tour, the guide said Sonoma County has a unique summer weather pattern and a moderate winter compared to other areas. When pressed for details, she could not come up with a suitable answer.

The big factor in our weather pattern is the huge, icy cold body of water to the west. Our part of the Pacific Ocean is part of the “Japan current,” a huge clockwise rotation with the middle of the clock midway between Japan and Washington state. The current passes the eastern shore of Japan, at the nine o’clock position.

The current runs north to the Bering Sea and the Alaskan Gulf and the freezing waters of the north pole. It’s now passing south, brushing Canada and the San Juan Islands in the three o’clock position on our imaginary clock. The icy waters support abundant sea life, including whales and near shore kelp forests. It also is home to huge numbers of small critters that litter the bottom end of the ocean food chain.

The current passes our coast and, just a few hundred miles south, heads back out to sea near Monterey, then due west a thousand miles or so above Hawaii and back to the Japanese coast completing the circuit.

For the Sonoma County coast, the salt water temperature varies between 48 to 54 degrees. That cold water passes the Golden Gate and is flushed in and out of San Francisco Bay, forming the chilly waters of San Pablo Bay.

In summer, the radiant heat plays over the cold salt water, which resists by forming an insulating blanket of fog. After a week or so of summer heat, the marine fog layer builds up enough that the cold air sneaks in at night from San Pablo Bay to the south and the Pacific to the east in a giant pincer movement.

The fog layer is so powerful it pushes back the hot summer air and covers much of Sonoma County with a deep, foggy blanket. This “natural air conditioning” is cold enough on some summer days to wear a light jacket. The battle is lost after a few days, and the heat of summer creeps back, pushing the fog back out to sea.

Only 20 or 30 miles inland, the summer heat dominates night and day, which means hotter summer days inland. This seesaw of hot and cold makes for excellent wine grape production. It also provides people with relief from the summer heat. 

My cousin, Mitchell Cari lives just east of Sacramento, where overnight lows can be above 90. He said, “Summer fog? What’s that?”


Bill Hanson is a Sonoma County native and a lifelong sportsman. He is the former president of the Sonoma County Mycological Association. Look for his column in The Community Voice each week.

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