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Napa County is the best-known county (in wine terms, at least) in the larger North Coast AVA of California, largely because it includes the world-famous Napa Valley AVA, which covers most of the region. Napa County lies between Yolo County to the east and Sonoma County to the west. Napa township, the county seat, is located in the southern quarter of the county, just 35 miles (55km) from downtown San Francisco.

All local wine producers have the right to use the Napa County AVA on their wine labels but few do, opting instead to use more location-specific appellations such as the prestigious Napa Valley, Carneros, Yountville, Mount Veeder or Howell Mountain. Naturally, the options open to them depend on the location of their vineyards.

Yellow mustard blooming in a Napa County vineyard

The first grape vines in Napa County were planted in the late 1830s by an early settler, George Calvert Yount (from whom the wine town of Yountville takes its name). The Napa Valley is home to Beringer Vineyards, the county's oldest continuously operated winery, which is listed on the National Register of Historic Places.

A Mediterranean climate prevails here, with hot, dry summers and cool, wet winters; the average temperature in August is 92F/27.8C, falling to 37F/2.8C in winter. As one of California's smaller counties, Napa County covers a total of 485,000 acres (196,275ha) with less than 10% of the land under vine. Perhaps surprisingly, despite its formidable reputation, the Napa Valley represents just 4% of California's total wine production – but it accounts for 30% of the state's wine economy.

Napa County's latitude of 38°N places it parallel with such southerly European vineyard areas as Portugal's Setubal Peninsula, Alicante in Spain, Italy's Calabria and the Greek Ionian Islands. While latitude is far from the defining factor of climatology, this does give some indication of how warm Napa Valley summer afternoons can be.

A key factor in Napa County's success as a viticultural region is the famous Bay Area fog and its interactions with the Napa Valley. As the sun warms the land on summer mornings, a blanket of cool, moist air rises from San Pablo Bay and is sucked inland by warm air currents moving up the valley sides. At its northern tip, as the Napa river approaches Calistoga, the valley floor measures just one mile (1.6km) across and sits at an altitude of roughly 500ft (150m). As it drops steadily towards San Pablo Bay it widens substantially, spanning almost nine miles (14km) east to west around Napa town. This funnel shape draws the refreshing, cooling fog far further inland than it would otherwise reach, influencing sites which would otherwise be too hot and dry to support quality viticulture.

For more specific information on each of the AVAs within Napa County, please see Napa Valley and its sub-regions.