San Diego County, officially the County of San Diego, is a county in the southwestern corner of the state of California, in the United States. As of the 2010 census, the population was 3,095,313, making it California’s second-most populous county and the fifth-most populous in the United States. Its county seat is San Diego, the eighth-most populous city in the United States. It is the southwesternmost county in the 48 contiguous United States, and is a border county.
San Diego County has more than 70 miles of coastline. This forms the most densely populated region of the county, which has a mild Mediterranean to semiarid climate and extensive chaparral vegetation, similar to the rest of the western portion of southern California. Precipitation and temperature extremes increase to the east, with mountains that receive frost and snow in the winter. These lushly forested mountains receive more rainfall than average in southern California, while the desert region of the county lies in a rain shadow to the east, which extends into the Desert Southwest region of North America.