Pacific Marine Fog and California Cool-Climate

ATF's 1999 final rule establishing the San Francisco Bay AVA (T.D. ATF-407) sets out the mechanism. The Pacific high pressure system is centered a thousand miles off the Pacific Coast. In winter it sits south of San Francisco, allowing rain-producing storms 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 transports inland. How long the fog stays varies by place. David Hirsch of Hirsch Vineyards wrote in 2003 that during the summer fog usually covers the Sonoma Coast during the morning and burns off about noon, as quoted in TTB's 2011 Fort Ross-Seaview rule, and Napa Valley Vintners notes that in the 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 (ATF, 1999)
• 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 transports inland
• Fog timing varies: burn-off about noon on the Sonoma Coast (David Hirsch, 2003), late morning in Napa's Oak Knoll District (Napa Valley Vintners)

Fog and marine air reach inland valleys through low points in the coastal hills. North of the Golden Gate, ATF's 1999 rule identifies the northwesterly windflow through the Estero Gap, near Petaluma in Sonoma County, into the Petaluma Valley as the major source of marine influence, and adds that 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. TTB's 2017 Petaluma Gap rule (T.D. TTB-149) describes low, rolling hills not exceeding 600 feet, except in a few places along the boundary ridgelines, that 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. In the Russian River Valley, Sonoma County Vintners describes the regular intrusion of cooling fog from the Pacific Ocean a few miles to the west. Within Napa Valley, Napa Valley Vintners says the most moderate temperatures are found in the Carneros region, and that farther up the valley the Pacific Ocean exerts less influence and summer temperatures are warmer. On the Central Coast, TTB's 2010 Santa Maria Valley expansion rule (T.D. TTB-88) records the petition's account that during the summer growing season marine air moves onshore through low-elevation passes in the Coast Range into the Santa Maria Valley, which T.D. ATF-89 describes as a natural funnel-shaped valley; 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. In Santa Barbara County, T.D. ATF-454 (2001) describes the Santa Rita Hills AVA as situated 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, and says the area's 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 (ATF, 1999)
• 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 (TTB)
• Santa Maria Valley: summer marine air moves onshore through low-elevation passes in the Coast Range into a funnel-shaped valley (TTB, 2010)
• 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 (ATF, 2001)

The marine-cooled zones lean toward Pinot Noir and Chardonnay. T.D. ATF-454 (2001) records that the Santa Rita Hills area has a cool climate and soils more conducive to growing Region One cool-climate grape varietals, while the surrounding Santa Ynez Valley AVA provides a warmer climate for Region Two varietals, and that Chardonnay and Pinot Noir are the predominant winegrapes in the Santa Rita Hills. The same rule records an average post-veraison ripening temperature 14.7°F hotter within the Santa Ynez Valley AVA than in the Santa Rita Hills AVA, and soils based on large percentages of dune sand, marine deposits, recent alluvium, river wash, and terrace deposits. ATF's 1999 rule notes that the degree-day concept was developed by UC Davis Professors Amerine and Winkler as a measure of climate support for vine growth and grape ripening, with large degree-day values indicating warmer climates. In Sonoma County, Sonoma County Vintners lists Chardonnay and Pinot Noir as key varieties of both the Russian River Valley and the Sonoma Coast, and Pinot Noir, Chardonnay and Syrah for the Petaluma Gap, where TTB's 2017 rule links prolonged high wind speeds to reduced photosynthesis and a longer hang time. For Carneros, Sonoma County Vintners describes a cool climate appellation known for Chardonnay, Pinot Noir and sparkling wines, and Napa Valley Vintners lists Chardonnay, Pinot Noir and Merlot as the principal varieties of Los Carneros.

• Santa Rita Hills: Region One cool climate; Chardonnay and Pinot Noir predominant (ATF, 2001)
• Santa Ynez Valley to the east: Region Two, warmer, known for varieties such as Cabernet Sauvignon, Cabernet Franc, Merlot, and Sauvignon Blanc (ATF, 2001)
• Russian River Valley and Sonoma Coast key varieties: Chardonnay and Pinot Noir (Sonoma County Vintners)
• Carneros: Chardonnay, Pinot Noir and sparkling wines (Sonoma County Vintners); Chardonnay, Pinot Noir and Merlot (Napa Valley Vintners)

The fog line sits at different heights in different places, and the federal record gives local figures rather than one statewide number. On the Sonoma Coast, David Hirsch wrote in 2003 that "This marine fog layer seldom rises above 900 feet," and Peay Vineyards grower and winemaker Vanessa Wong described 1,000 feet as the mean top of the inversion layer, with vineyards above it warmer, both as recorded in TTB's 2011 Fort Ross-Seaview rule (T.D. TTB-98). Sonoma County Vintners says the Sonoma Coast is warm enough to ripen wine grapes because most vineyards are above the fog line. In Mendocino County, ATF's 1997 rule (T.D. ATF-392) limits the Mendocino Ridge AVA, made up of non-contiguous ridge-top sites, to land at or above 1200 feet, approximately the fog line. It records that summer mornings are characterized by "lakes of fog" with the ridge-tops protruding like "small islands", and that while the valleys may be full of coastal fog, the vineyards are fully exposed to the sun while receiving the cooling influences of the fog. In Napa, Napa Valley Vintners places Howell Mountain above the fog line, warmer and drier than other AVAs with more hours of sunshine and little-to-no marine influence, and says most vineyards in Mount Veeder and in Spring Mountain District sit above the fog line, with typical mid-summer highs of about 85°F (29°C). By contrast, the same source says the Crystal Springs AVA, below 1,400 feet, gets shrouded in fog on many days.

• Sonoma Coast: the marine fog layer "seldom rises above 900 feet" (David Hirsch, 2003, in T.D. TTB-98)
• Mendocino Ridge AVA: ridge-tops at or above 1200 feet, approximately the fog line (ATF, 1997)
• Napa: Howell Mountain above the fog line; most Mount Veeder and Spring Mountain District vineyards above it (Napa Valley Vintners)
• Crystal Springs, below 1,400 feet, is shrouded in fog on many days (Napa Valley Vintners)

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 2026-09-28