Sonoma County

California State Parks dates the Russian-American Company's settlement at Fort Ross on the Sonoma coast to 1812–1841, as a farming base for Alaska. Sonoma County Tourism says the Russians grew vines there as early as 1821, and that Padre José Altimira planted several thousand vines at Mission San Francisco Solano in Sonoma, the order's northernmost mission. General Mariano Vallejo, sent north in 1833, founded the town of Sonoma and planted profitable vineyards of his own, and the county's wine industry was, in the tourism office's words, "just beginning" when the Mexican-American War ended in 1848.

Sonoma Valley was among the first AVAs, established on 4 December 1981. Russian River Valley followed on 21 October 1983, Northern Sonoma on 17 May 1985 and Sonoma Coast on 11 June 1987. Sonoma County Winegrowers says that Carneros, recognised in 1983, was the first California wine region defined by its climate rather than by political boundaries.

Sonoma County Winegrowers describes vineyards from sea level to about 790 m, on valley floors, benchlands and mountains, with 11 major geological formation types and 31 soil series. The Pacific cools the county through three openings: the Petaluma Gap, the channel of the Russian River and San Pablo Bay. TTB's 2011 rule calls fog the single most unifying feature of the Russian River Valley; fog entering through the Petaluma Gap can lower temperatures by as much as 28 °C, and the area has a "coastal cool" climate of about 1,110–1,560 degree days (°C).

Rain almost stops from May to September. Summer highs range from about 21 to 33 °C, and even in warm inland Alexander and Knights valleys nights fall to about 4–12 °C. The Sonoma Coast AVA, close to the ocean, receives more than twice the rain of inland areas, but many of its vineyards lie above the fog line. Sonoma Valley lies between the Mayacamas and Sonoma mountains, which shelter it from Pacific weather.

Sonoma County Winegrowers ties wine character to temperature. In the Russian River Valley, evening fog drops the temperature by 19–22 °C from its daytime high, so the grapes develop full flavour over a growing season 15–20% longer than in neighbouring areas while keeping their acidity. In Alexander Valley, heat spikes ripen big varieties such as Cabernet Sauvignon and cool mornings and evenings preserve acidity. The rocky, well-drained soils of Dry Creek Valley stress the vines late in the season. In the Petaluma Gap, wind and fog give smaller yields and later ripening.

"Sonoma County" is a county appellation under 27 CFR 4.25, not an AVA: at least 75% of the wine must come from the county, and the word "county" must appear. Its 19 AVAs need 85%. They include Sonoma Valley, Russian River Valley with Chalk Hill and Green Valley of Russian River Valley, Dry Creek Valley, Alexander Valley, Knights Valley, Sonoma Mountain, Bennett Valley, Moon Mountain District, Fountaingrove District, Rockpile, Fort Ross-Seaview, Petaluma Gap, Pine Mountain-Cloverdale Peak, Sonoma Coast, West Sonoma Coast and Northern Sonoma; Carneros is shared with Napa County.

The Winegrowers associate Pinot Noir and Chardonnay with the Russian River Valley and Sonoma Coast, Zinfandel and Sauvignon Blanc with Dry Creek Valley, and Cabernet Sauvignon with Alexander, Knights, Sonoma Valley and Sonoma Mountain.