Pacific Marine Fog and California Cool-Climate

The 1999 Treasury decision that established the San Francisco Bay AVA (T.D. ATF-407) sets out the weather of the San Francisco Bay area, including how marine fog forms. The Pacific high pressure system is centered a thousand miles off the Pacific Coast. In winter it sits south of San Francisco, letting rain-producing storms pass through; in summer it moves closer to the latitude of San Francisco, deflecting storms to the north and producing a dry summer climate. The winds from the high flow onshore from the northwest to the southeast and produce a cold, southward-flowing surface current off the California coast, the California Current, through upwelling, in which cold deep water is brought to the surface. When moist marine air from the Pacific High flows onshore over this cold water, it cools, producing fog or stratus clouds that the wind carries inland. During the summer, fog usually covers the Sonoma Coast in the morning and burns off about noon. In Napa's Oak Knoll District, marine air and fog can remain until late morning.

• Summer: the Pacific High moves closer to the latitude of San Francisco, deflecting storms north and producing a dry summer climate
• Its onshore northwest-to-southeast winds drive upwelling, bringing cold deep water to the surface as the California Current
• Moist marine air cooling over that cold water forms fog or stratus that the wind carries inland
• Fog timing: burn-off about noon on the Sonoma Coast, late morning in Napa's Oak Knoll District

North of the Golden Gate, the northwesterly windflow through the Estero Gap, near Petaluma in Sonoma County, into the Petaluma Valley is the major source of marine influence. Airflow inland from San Pablo Bay also affects the climate of southern Napa and Sonoma Counties, while the air that enters the Golden Gate does not flow north of San Francisco Bay. In the Petaluma Gap AVA, established by T.D. TTB-149 in 2017, low, rolling hills not exceeding 600 feet, except in a few places along the boundary ridgelines, create a corridor for marine winds from the Pacific Ocean to San Pablo Bay, so cool air and marine fog enter the vineyards when temperatures would normally be at their highest. Wind speeds there increase significantly in the afternoon, because rising inland temperatures cause hot air to rise and pull the cooler, heavier marine air in from the coast. The Russian River Valley climate is shaped by the regular intrusion of cooling fog from the Pacific Ocean a few miles to the west. Within Napa Valley, the most moderate temperatures are found in the Carneros region; farther up the valley the Pacific Ocean exerts less influence and summer temperatures are warmer. During the summer growing season, marine air moves onshore through low-elevation passes in the Coast Range into the Santa Maria Valley, a natural funnel-shaped valley whose AVA was expanded in 2010 (T.D. TTB-88). The cooling wind and fog meet little resistance until the Sierra Madre Mountains on the north side of the valley and the Solomon Hills on the south. The Santa Rita Hills AVA, established in 2001 by T.D. ATF-454, sits within an east/west transverse maritime throat that gives diurnal ocean breezes direct access to the coastal valleys between the Purisima Hills and the Santa Rosa Hills. Its proximity to the Pacific fills its hills and valleys with coastal fog in the late night and early morning hours. The AVA name was later abbreviated to Sta. Rita Hills (T.D. TTB-37, 2005).

• Estero Gap near Petaluma: the major source of marine influence north of the Golden Gate
• Petaluma Gap AVA (effective January 8, 2018): hills not exceeding 600 feet form a corridor from the Pacific Ocean to San Pablo Bay, with winds strongest in the afternoon
• Santa Maria Valley: summer marine air moves onshore through low-elevation passes in the Coast Range into a funnel-shaped valley
• Sta. Rita Hills: an east/west transverse maritime throat between the Purisima Hills and the Santa Rosa Hills, filled with coastal fog in the late night and early morning

The marine-cooled zones lean toward Pinot Noir and Chardonnay. The Santa Rita Hills area has a cool climate and soils more conducive to growing Region One cool-climate grape varieties, while the surrounding Santa Ynez Valley AVA provides a warmer climate for Region Two varieties, and Chardonnay and Pinot Noir are the predominant winegrapes in the Santa Rita Hills. The average post-veraison ripening temperature is 14.7°F hotter within the Santa Ynez Valley AVA than in the Santa Rita Hills AVA, and the Santa Rita Hills soils are based on large percentages of dune sand, marine deposits, recent alluvium, river wash, and terrace deposits. The degree-day concept, developed by UC Davis Professors Amerine and Winkler, measures climate support for vine growth and grape ripening, and large degree-day values indicate warmer climates. In Sonoma County, Chardonnay and Pinot Noir are the key varieties of both the Russian River Valley and the Sonoma Coast, and Pinot Noir, Chardonnay and Syrah are the key varieties of the Petaluma Gap. There, prolonged high wind speeds are linked with reduced photosynthesis, so the grapes need a longer hang time to reach a given sugar level. Sonoma's vintner association counts Carneros as a cool climate appellation known for Chardonnay, Pinot Noir and sparkling wines, and the principal varieties of Los Carneros on the Napa side are Chardonnay, Pinot Noir and Merlot.

• Santa Rita Hills: Region One cool climate; Chardonnay and Pinot Noir predominant
• Santa Ynez Valley to the east: Region Two, warmer, known for varieties such as Cabernet Sauvignon, Cabernet Franc, Merlot, and Sauvignon Blanc
• Russian River Valley and Sonoma Coast key varieties: Chardonnay and Pinot Noir
• Carneros: known for Chardonnay, Pinot Noir and sparkling wines; Napa lists Chardonnay, Pinot Noir and Merlot as its principal varieties

On the Sonoma Coast, David Hirsch of Hirsch Vineyards, on whose behalf a 2003 petition for what became the Fort Ross-Seaview AVA was submitted, wrote in a 2003 letter that "This marine fog layer seldom rises above 900 feet." The 2011 rule establishing that AVA (T.D. TTB-98) also records Peay Vineyards grower and winemaker Vanessa Wong placing the mean top of the inversion layer at 1,000 feet, with vineyards above it warmer. The Sonoma Coast is warm enough to ripen wine grapes because most vineyards there sit above the fog line. In Mendocino County, the Mendocino Ridge AVA (T.D. ATF-392, 1997), made up of non-contiguous ridge-top sites, covers only land at or above 1200 feet, approximately the fog line. Summer mornings there bring "lakes of fog" with the ridge-tops protruding like "small islands", and while the valleys may be full of coastal fog, the vineyards are fully exposed to the sun while still receiving the cooling influence of the fog. In Napa, Howell Mountain sits above the fog line and is warmer and drier than other AVAs, with more hours of sunshine and little-to-no marine influence. Most vineyards in Mount Veeder and in the Spring Mountain District also sit above the fog line, with typical mid-summer highs of about 85°F (29°C). By contrast, the Crystal Springs AVA, below 1,400 feet, is shrouded in fog on many days.

• Sonoma Coast: in David Hirsch's words, the marine fog layer "seldom rises above 900 feet"
• Mendocino Ridge AVA: ridge-tops at or above 1200 feet, approximately the fog line
• Napa: Howell Mountain sits above the fog line, as do most Mount Veeder and Spring Mountain District vineyards
• Crystal Springs, below 1,400 feet, is shrouded in fog on many days

Primary sources (Federal Register final rules on govinfo.gov, Napa Valley Vintners at napavalley.wine and napavintners.com, and Sonoma County Vintners at sonomawine.com): https://www.govinfo.gov/content/pkg/FR-1999-01-20/html/99-1209.htm, https://www.govinfo.gov/content/pkg/FR-2017-12-07/html/2017-26410.htm, https://www.govinfo.gov/content/pkg/FR-2011-12-14/html/2011-32016.htm, https://www.govinfo.gov/content/pkg/FR-2001-05-31/html/01-13645.htm, https://www.govinfo.gov/content/pkg/FR-2016-08-22/html/2016-19998.htm, https://www.govinfo.gov/content/pkg/FR-2010-12-29/html/2010-32873.htm, https://www.govinfo.gov/content/pkg/FR-1997-10-27/html/97-28280.htm, https://www.govinfo.gov/content/pkg/FR-2018-12-14/html/2018-27017.htm, https://napavalley.wine/region, https://napavintners.com/napa-valley/science/climate/, https://sonomawine.com/appellations/. Last verified 2026-10-04