Santa Maria Valley is an American Viticultural Area (AVA) which straddles the boundary of Santa Barbara and San Luis Obispo counties in the south Central Coast of California. It was established as the nation's third, the state's second and both counties' initial appellation on August 5, 1981, by the Bureau of Alcohol, Tobacco and Firearms (ATF), Treasury after reviewing the petition submitted by area vintners and wine grape growers proposing the viticultural area named "Santa Maria Valley." A portion of the AVA crosses the Cuyama River into the southernmost corner of San Luis Obispo County. The east–west orientation of the 97,483-acre (152 sq mi) open valley and rolling hills allows cool winds and fog to freely flow in from the Pacific Ocean, settling most noticeably in lower-lying areas. The result is a mild Mediterranean climate that lengthens the growing season and contributes to the eventual sugar/acid balance in the grapes from Santa Maria Valley's 7,500 acres (3,000 ha) cultivated vineyards. According to the petition, the area has recently cultivated wine grapes for a decade. On December 29, 2010, the AVA was designated a 29.4-square-mile (18,790-acre) expansion to its southern boundary.