San Diego County has more than 70 miles (113km) 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.[12] These lushly forested mountains receive more rainfall than the 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.
San Diego County became part of the United States as a result of the Treaty of Guadalupe Hidalgo in 1848, ending the Mexican–American War. This treaty designated the new border as terminating at a point on the Pacific Ocean coast which would result in the border passing one Spanish league south of the southernmost portion of San Diego Bay, thus ensuring that the United States received all of this natural harbor.
San Diego County was one of the original counties of California, created at the time of California statehood in 1850.[18]:221
At the time of its establishment in 1850, San Diego County was relatively large, and included all of southernmost California south and east of Los Angeles County. It included areas of what are now Inyo and San Bernardino Counties, as well as all of what are now Riverside and Imperial Counties.[18]:221
During the later part of the 19th century, there were numerous changes in the boundaries of San Diego County, when various areas were separated to make up the counties mentioned above. The most recent changes were the establishments of Riverside County in 1893[18]:207 and Imperial County in 1907.[18]:113 Imperial County was also the last county to be established in California, and after this division, San Diego no longer extended from the Pacific Ocean to the Colorado River, and it no longer covered the entire border between California and Mexico.
According to the U.S. Census Bureau, the county has an area of 4,526 square miles (11,720km2), of which 4,207 square miles (10,900km2) is land and 319 square miles (830km2) (7.0%) is water.[19] The county is larger in area than the combined states of Rhode Island and Delaware.[20]
San Diego County has a varied topography. On its western side is more than 70 miles (113km) of coastline.[21] Most of San Diego between the coast and the Laguna Mountains consists of hills, mesas, and small canyons. Snow-capped (in winter) mountains rise to the east, with the Sonoran Desert farther to the east. Cleveland National Forest is spread across the central portion of the county, while the Anza-Borrego Desert State Park occupies most of the northeast.
Although the county's western third is primarily urban, the mountains and deserts in the eastern two-thirds are primarily undeveloped backcountry. Most of these backcountry areas are home to a native plant community known as chaparral. San Diego County contains more than 1,000,000 acres (4,000km2) of chaparral, twice as much as any other California county.[22]
Periodically the area has been subject to wildfires that force thousands to evacuate. The most recent are the December 2017 Lilac Fire and the May 2014 San Diego County wildfires; before them was the Witch Creek Fire in 2007 and the Cedar Fire in 2003. California defines a fire season in which fires are most likely to occur, usually between late July and late October (which are the driest months of the area). Signs posted in numerous spots of the county provide information on the level of threats from fires based on weather conditions.[citation needed]
Regions
Northern San Diego County is known as North County; the eastern suburbs are collectively known as East County; the rural areas located further east and extending to the Imperial County line are known as the Mountain Empire; and the southern suburbs, extending to the Mexican border, are collectively known as the South Bay or South County, including South San Diego, an exclave of the city of San Diego which has no land connection to the rest of the city.
Under the Köppen climate classification system, the urban and suburban San Diego area straddles areas of Mediterranean climate (Csa) to the north and hot semi-arid climate (BSh) to the south and east.[23] As a result, it is often described as "arid Mediterranean" and "semi-arid steppe." Farther east, arid desert conditions prevail. Western San Diego's climate is characterized by warm, dry summers and mild winters with most of the annual precipitation falling between November and March. The city has mild, mostly dry weather, with an average of 201 days above 70°F (21°C) and low rainfall (9–13 inches (23–33cm) annually). Summer temperatures are generally warm, with average highs of 70–78°F (21–26°C) and lows of 55–66°F (13–19°C). Temperatures exceed 90°F (32°C) only four days a year. Most rainfall occurs from November to April. Winter temperatures are mild, with average high temperatures of 66–70°F (19–21°C) and lows of 50–56°F (10–13°C).
The climate in the San Diego area, like much of California, often varies significantly over short geographical distances resulting in microclimates. In San Diego's case this is mainly due to the city's topography (the Bay, and the numerous hills, mountains, and canyons). Frequently, particularly during the "May gray/June gloom" period, a thick marine layer will keep the air cool and damp within a few miles of the coast, but will yield to bright cloudless sunshine approximately 5–10 miles (8.0–16.1km) inland. This happens every year in May and June.[24] Even in the absence of June gloom, inland areas tend to experience much more significant temperature variations than coastal areas, where the ocean serves as a moderating influence. Thus, for example, downtown San Diego averages January lows of 48°F (9°C) and August highs of 77°F (25°C).[25] The city of El Cajon, just 10 miles (16km) northeast of downtown San Diego, averages January lows of 42°F (6°C) and August highs of 89°F (32°C).[26] Julian, in the mountains, has an average January low of 29°F (−2°C) and August high of 85°F (29°C).[27] Borrego Springs, in the Colorado Desert, has an average January low of 43°F (6°C) and August high of 106°F (41°C).[28]
Rainfall along the coast averages about 10 inches (25cm) of precipitation annually, which occurs mainly during the cooler months of December through April. Though there are few wet days per month during the rainy period, rainfall can be heavy when it does occur. However, the rainfall is greater in the higher elevations of San Diego. Some of the higher areas of San Diego, such as Palomar Mountain and the Laguna Mountains, receive 20–40 inches (51–102cm) of rain per year, supporting lush forests similar to the Sierra Nevada and California Coast Range. The Colorado Desert portion of the county lies to the east of the mountains, which receives the least amount of precipitation; Borrego Springs, the largest population center in the desert, averages only 5 inches (13cm), with a high evaporation rate.
