<P> California is a very geologically diverse region and varies greatly in the range of climates and terroirs that can be found . Most of the state's wine regions are found between the Pacific coast and the Central Valley . The Pacific Ocean and large bays, like San Francisco Bay, serve as tempering influences to the wine regions nearby providing cool winds and fog that balance the heat and sunshine . While drought can be a vinicultural hazard, most areas of California receive sufficient amounts of rainfall with the annual rainfall of wine regions north of San Francisco between 24 - 45 inches (615--1150 mm) and the more southern regions receiving 13 - 20 inches . Winters are mild with little threat of frost damage though springtime . To curb the threat of frost, vineyard owners will often employ the use of wind machines, sprinklers and smudge pots to protect the vines . </P> <P> While California's wine regions can be generally classified as a Mediterranean climate, there are also regions with more continental dry climates . Proximity to the Pacific or bays as well as unobstructed access to the cool currents that come off them dictate the relative coolness of the wine region . Areas surrounded by mountain barriers, like some parts of Sonoma and Napa counties will be warmer due to the lack of this cooling influence . The soil types and landforms of California vary greatly, having been influenced by the plate tectonics of the North American and Pacific Plates . In some areas the soils can be so diverse that vineyards will establish blocks of the same vine variety planted on different soils for purpose of identifying different blending components . This diversity is one of the reasons why California has so many different and distinct American Viticultural Areas . </P> <P> California has over 427,000 acres (1,730 km) planted under vines mostly located in a stretch of land covering over 700 miles (1,100 km) from Mendocino County to the southwestern tip of Riverside County . There are over 107 American Viticultural Areas (AVAs), including the well - known Napa, Russian River Valley, Rutherford and Sonoma Valley AVAs . The Central Valley is California's largest wine region stretching for 300 miles (480 km) from the Sacramento Valley south to the San Joaquin Valley . This one region produces nearly 75% of all California wine grapes and includes many of California's bulk, box and jug wine producers like Gallo, Franzia and Bronco Wine Company . </P> <P> The wine regions of California are often divided into 4 main regions - </P>

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