All wine grown within Napa County falls within the Napa Valley AVA. Established in 1981, this is California’s most famous region, although it accounts for only 4% of the state's vineyard plantings. There are now 16 sub-AVAs scattered throughout the valley and the mountain slopes. The valley itself is 50 kilometres long and eight kilometres wide and is surrounded on both sides by mountain ranges: Mayacamas to the west separates Napa from Sonoma and provides protection from the coast, and Vaca to the east separates Napa from the desert-like Central Valley. The climate is moderate, but heat spikes are common around harvest, and most rainfall occurs during winter–a starkly Mediterranean climate. Rainfall levels are similar to Bordeaux, and the mountain ranges to the west protect the eastern side of the valley from rainfall. Napa Valley enjoys cool nights and a wide diurnal range. The warm conditions are moderated by San Pablo Bay, which brings in cool breezes and fogs in the morning–this effect dissipates as you move up the valley. Soils on the valley floor are alluvial and fertile, while more rocky and volcanic types can be found on the mountain slopes, and elevations can range from zero to 600 metres. It is easiest to categorise Napa Valley into mountain AVAs and valley floor AVAs as you dive into the complexity of a compact but complex wine region. Spring Mountain District One of Napa Valley’s famed mountain AVAs, Spring Mountain District makes just 2% of Napa’s wine but has a reputation that belies its size. Located west of St. Helena on the eastern slope of the Mayacamas Range, it’s Napa Valley’s coolest and wettest region, in part due to its proximity to the Pacific Ocean (50 kilometres) and the San Pablo Bay (40 kilometres). Conditions are further moderated by aspect and altitude; most of the region faces east, and the steep terrain ranges from 120-800 metres. Vineyards in the AVA tend to be located above the fog line, are small and enjoy shelter from the surrounding forest. The region is planted to Cabernet Sauvignon, Merlot, Cabernet Franc, Chardonnay and Zinfandel, among others. The soils are primarily weathered sandstone and shale, giving low fertility, small yields and good drainage. The relatively cool conditions generally result in structured, ageworthy, elegant Cabernet Sauvignon wines. Mount Veeder Mount Veeder AVA, located west of Napa and Yountville, was established in 1990 and is home to just under 450 hectares of vines. Napa County does not allow plantings on slopes steeper than 30 degrees, rendering much of Mt Veeder unplantable and, therefore, one of Napa’s least-developed AVAs. The region is on the southern end of the Mayacamas mountains with an easterly aspect, so moisture and cool air funnelled from San Pablo Bay easily reach the vineyards. Shale, sandstone, clay and sandy loams are common soil types, along with a sub-surface layer of compressed ash rock, which is very porous and creates a natural aquifer (which facilitates dry farming). Most vineyards are located above the fog line and enjoy a significant diurnal shift. Oak Knoll District Located at the southern end of central Napa Valley, Oak Knoll was officially designated in 2004. It’s known for its long growing season, cooler conditions and high-quality wines. It’s a relatively flat appellation and, together with Yountville, is considered among Napa’s coolest areas. Soils are deep loam and gravel, and elevations range from zero to 244 metres. The area is exposed to ocean influence from San Pablo Bay, so fogs and cooling winds moderate the climate and provide a long growing season with a wide diurnal range. Daytime temperatures can reach 33 degrees Celsius, plunging to 10 degrees at night. Yountville Yountville AVA, recognised in 1999, takes its name from George Yountville, the first person to plant vines in the area in 1836. Soils vary from alluvial, silt, gravel and volcanic, and elevations top out at 60 metres. The area is exposed to ocean influences from San Pablo Bay; fogs and cooling winds moderate the climate and provide a long growing season with a diurnal range just slightly less exaggerated than in Oak Knoll District. Oakville In the heart of the Napa Valley, Oakville is named for the dense thickets of oak once prevalent in the area. Today, the AVA is famed as the source of some of Napa’s most premium Cabernet Sauvignon and famous producers of the 100pt ilk: Harlan, Opus One and Screaming Eagle, to name but a few. The region is three kilometres wide and reaches 300 metres up the Vaca mountains to the east and 150 metres up the Mayacamas range to the west. Soils to the west are sandstone and shale, with some gravel and alluvial loam in the lower parts. Soils to the east are heavier and more volcanic. Oakville is far enough south to receive morning fog from San Francisco Bay, and this, combined with the warmer inland airflow from the San Joaquin Valley, provides a temperate Mediterranean climate for winegrowers. Rutherford, St. Helena and Calistoga Continuing north from Oakville are Rutherford and St. Helena, which sprawl across a number of alluvial fans in the foothills of the Mayacamas Mountains until the valley floor comes to a point and ends in Calistoga―the warmest of the Napa Valley AVAs. Other Mountain AVAs Diamond Mountain District is north of Mount Veeder along the east-facing slopes of the Mayacamas Mountains on the west side of the Napa Valley. Howell Mountain and Atlas Peak are nestled in the Vaca Mountains at the north and south ends, respectively. A wide diurnal shift dramatically influences the climate here. Valley Floor AVAs Several famed Napa AVAs are flanked by mountains on gently sloped foothills that reach the valley floor. Some produce the most famous and sought-after wines from Napa Valley, and lie close to the vigorous valley floor (often within the same AVA). Other Napa AVAs The Stags Leap District is a small western valley enclave surrounded by unplanted land with a strong reputation thanks largely to Stags Leap Cellars. The Vaca Mountains are far less extensively planted than the Mayacamas Mountains. Chiles Valley lies east of the famed Howell Mountain, while Wild Horse Valley and Coombesville are at the southeast end of Napa Valley. Along with Chiles Valley, the latter two have not achieved the fame and reverence the rest of Napa enjoys. Lake County Eight AVAs comprise an area that has long supported the Napa industry, providing quality Cabernet Sauvignon for inclusion in many blends.