Petaluma Gap Archives

Fog is to central and Northern California’s wine country what garrigue is to Southern France’s—it’s an intrinsic part of California wine’s character. Several prized American Viticultural Areas (AVAs)—including Alexander Valley, Fort Ross-Seaview, Los Carneros, the Russian River Valley, Petaluma Gap, Rutherford and Yountville—specifically called out fog as a distinguishing characteristic in their petitions for official AVA status to the Alcohol and Tobacco Tax and Trade Bureau (TTB). “I’ve been living in Fort Ross-Seaview for 35 years,” says Fort Ross Vineyard’s co-owner and cofounder, Lester Schwartz. “My wife, Linda, and I shepherded it through the appellation application process, and one of the main features that distinguished it was the fog inversion layer. We’re above that layer here, and because of the way the sun radiates off of that layer, we get a lot of sun and produce unique grapes. Now, I’m not standing outside recording my observations of fog on a daily basis, but I can say that over the years, we’re definitely seeing a lot less of it.” Fog Ghosts California Official and unofficial wildfire, extreme heat and drought alert systems have been set up by governmental and academic organizations around the state, and Governor Gavin Newsom has set one of the most aggressive climate change agendas in the country. It aims to achieve net-zero carbon pollution by 2045, with an 85% cut in greenhouse gas emissions through clean energy investments and wildland and water conservation projects. But while billions of dollars are being invested in the study and potential resolution of these three pervasive problems, few people are truly considering the implications of the sudden, drastic reduction in fog in key growing regions around the state. You May Also Like: Northern California Winemakers Shift Practices Amid Drought Emergency Chief Meteorologist at WBRZ in Baton Rouge, Louisiana, Dr. Josh Eachus explains that fog’s importance is in its role of creating high-humidity conditions. “It forms when the air temperature cools to the dew point temperature, which is the temperature at which air becomes saturated. Fog also happens when the dew point temperature climbs to the air temperature. In both cases, it results in 100% relative humidity and saturated air, which appears as fog.” Its slow disappearance in California, Eachus continues, is primarily because “with climate change, the winds forming over the Pacific Ocean, which produces fog that blows inland, are growing weaker. This has a twofold effect: reducing fog production and reducing the winds that blow the fog that is produced inland.” Using weather data at airports from central to Northern California that measure fog hours by day researchers inferred a reduction in the frequency of fog by 33% from the early 20th century, according to an analysis coauthored in 2010 by Todd E. Dawson, PhD, a professor of integrative biology at University of California, Berkeley, and then-post-doctoral fellow, James A. Johnston. More recently, a 2022 report issued by the California Environmental Protection Agency cited an average 46% drop in winter fog events by analyzing weather data and satellite imagery of the Central Valley from 1981 through 2014. Effects on Grape-growing Fog acts like a cool sip of water on a hot day for parched vines—not as great as a dip in the pool (rain), but better than the alternative (nothing). “Fog acts like a stress reliever during the summer,” says Fulldraw Vineyard’s cofounder and winemaker, Connor McMahon. “In drought years especially, I love seeing fog, because it holds moisture in soil, tucks it in like a blanket. Fog can also protect grapes from the sun. In Paso Robles, it gets so hot—in August and September we had six days of 115 degrees. A little bit of fog makes a huge difference.” But McMahon says that since 2011 when he began working in Paso, he’s seen a significant decrease in fog events. In a bid to create relief for struggling grapes, McMahon says he started using shade cloth in 2013. “It’s essential to protect the fruit from direct sunlight when the rain and fog aren’t there,” McMahon says. “And we don’t want to overstress our water table and water the grapes all day, which is the alternative.” Enrico Bertoz, winemaker at Napa’s Flora Springs, has also been alarmed by the drop in fog in recent years. “In the past few years, we have experienced a decline in the duration of the fog during the summer months,” Bertoz notes. “Ten years ago, the fog lasted through much of the day, and now it burns up more quickly in the morning. Where most of our vineyards are located in Rutherford and Oakville, we receive fog from both the San Pablo Bay and the Chalk Hill Gap in the Mayacamas, which is incredibly important.” You May Also Like: Climate Change Forces California Winemakers to Reconsider What Grapes Grow Where Bertoz explains that the fog helps “retain natural acidity in the grapes and prevent sunburn and heat damage.” And this year, Bertoz is happy to reporty, the growing season has delivered plenty of fog, something he also links to a longer growing season, which in turn ensures even ripening and full phenolic maturity. At this point, a clear, traceable line from fog—or the lack thereof—to definitive effects on the development of grapes has not been established. Observations like Bertoz’s are backed up by one of the only comprehensive academic studies of fog’s impact on vegetation in general, published in Geophysical Research Letters. Tracking the impact of fog through satellite imagery and remote sensing data, researchers say that fog has a “continuous positive impact” on vegetation in drier regions. Fog, the authors write, helps plants maintain “photosynthetic function and sustain biogeochemical dynamics,” and can reduce drought stress by up to 36% off the coast of southern California. At Flora Springs, when the fog doesn’t deliver that much-needed hit of moisture, they cope by “adding plenty of water and pruning as few leaves as possible [to encourage shading] in the fruit zone to ensure as much water as possible was retained within the vines,” Bertoz explains, adding that the periodic watering helps counteract what would otherwise be an “expedited sugar ripening process.” Take Nothing for Granted Coming off the wettest winter in decades, winemakers are clear on one thing: Nothing is a given anymore. Joe Nielsen, general manager and winemaker at Ram’s Gate Winery in Sonoma, concurs that fog is another shifting factor that vintners should begin to monitor. “I come from the Midwest, and when I came to Northern California, the fog struck me as such an anomaly,” Nielsen says. “Fog is so important, because it acts as a natural air condition, and is essential for mitigating both the highs and lows of temperature during the growing season.” Anecdotally, Nielsen says that he’s noticed differences in the fog, but says he couldn’t begin to estimate how much it has changed. “There’s no way I could say we get 20% more or less at this point,” Nielsen admits. “It’s just clearly more erratic, along with everything else. But winemaking is just getting more refined every day, and because fog is so important to grape-growing, we definitely plan to monitor it more closely. We measure and track everything else in the vineyard—why not fog?” “The weather has been so weird for the past years, it’s clear that we can’t take anything for granted,” says Dave Low, winemaker at Papapietro Perry in Healdsburg. “We’ve definitely noticed less fog in the past few years, but I haven’t noticed that it has led to a chemical difference in the grapes. They look the same, and they taste the same. But we are picking them a month earlier than we used to, and some of that is probably due to less fog.” Fog, Low says, is something he plans to keep an eye on in the coming years, to establish if there is any link between a reduction of fog and changes in the glass. This article originally appeared in the October 2023 issue of Wine Enthusiast magazine. Click here to subscribe today! Bring the World of Wine to Your Doorstep Subscribe to Wine Enthusiast Magazine now and get 1 year for $70 $29.99. Subscribe

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