13.1: Physical Geography
The definition of Southern California is geographically complex and has been shaped by both natural and political forces. The region is generally considered to encompass the area south of the 35°45' latitude, but its northern boundary is not a straight line. Instead, it follows the northern boundaries of San Luis Obispo, Kern, and San Bernardino counties, or alternatively, Point Conception and the Tehachapi Mountains.
Following the acquisition of California by the United States, pro-slavery politicians attempted to divide the state at 36°30', the line of the Missouri Compromise. This would have created a separate slave state in the southern half of California. However, the Compromise of 1850 admitted California to the Union as a free state, preventing the division of the state. Despite the abolition of slavery, tensions between Northern and Southern California persisted. In the 1850s, Californians (dissatisfied with inequitable taxes and land laws) and pro-slavery Southerners in the lightly populated "cow counties" of southern California attempted three times to achieve a separate statehood or territorial status separate from Northern California. The last attempt, the Pico Act of 1859, was passed by the California State Legislature and approved by nearly 75 percent of voters. However, the secession crisis following the election of Abraham Lincoln in 1860 and the subsequent American Civil War led to the proposal never coming to a vote.
The complex history of Southern California's definition reflects the region's unique geographical and cultural identity. The region has been shaped by a diversity of influences, including its mountains, dry climate, its proximity to Mexico, and its role as the center of the American film industry.
Climate
Despite the popular image of California as a place of sunshine and perfect weather, the local climate can be very diverse, with some areas experiencing more extreme conditions. However, the weather in the region is usually mild, especially in the winter, and dry, with rainfall ranging from moderate in the coastal regions to almost none in the desert.
Around the coastal areas, the weather does not vary as dramatically as it does inland. Climate is affected by factors such as latitude, topography, and proximity to water masses - primarily the cold currents along the coast of the Pacific Ocean, and southern California's mountain ranges. The Transverse Ranges and the Peninsular Ranges are key players in the region's climate.
Essentially, the mountain ranges separate southern California into two distinct climatic regions: The heavy-populated coastal area west of these mountains is the one most associated with the term "southern California" and is characterized by pleasant weather all-year round, without frequent heat spells in the summer and without low temperatures in winter. By comparison, the area east of the ranges, between the mountains and the borders with Nevada and Arizona, is dominated by the Sonoran Desert and the Mojave Desert and tends to be more arid and register more extreme temperatures in both summer and winter.
Low Clouds, "May Gray" & "June Gloom"
Low clouds and fog are common weather phenomena in Southern California. This is due to the region's topography and proximity to the Pacific Ocean. Coastal fogs are often created by the interaction of seasonal inversion layers and the coastal marine layer. These fogs can reach as far inland as 20 miles, butting up against inland mountains or coastal mountain ranges. Coastal fogs can vary in density, from light "ground fogs" to dense "Tule fog." Tule fog is most common in the winter and spring when cold air from the mountains interacts with warmer ocean air masses.
In recent decades, the prevalence of low clouds and fog on the California coast has decreased. This is due to a combination of urban heat and climate change. Scientists have found that stratus cloud cover has decreased between 25% and 50% since the 1970s.
In contrast with the sunny summer, late spring in southern California is often overcast, due to the marine onshore air. This period, known to the locals as "May Gray" and "June Gloom", dims the coastal skies of sunny southern California. During this time, the coastal clouds may remain all day, but often give way to some hazy afternoon sunshine. The number of gloomy days during this period vary from year to year. Years with warmer ocean temperatures, influenced by the broader El Niño weather pattern, may result in fewer gray days in June, whereas the cooler ocean temperatures associated with a La Niña pattern, usually foretell more gray days in the season.
June Gloom is caused by the interaction of the California Current and the marine layer. The California Current is a cold ocean current that flows southward along the coast of California. The marine layer is a layer of cool, moist air that sits on top of the ocean. When the marine layer spreads over the California Current, it cools down and condenses, forming low-altitude stratus clouds. These clouds then move inland, bringing overcast skies and cool temperatures to the coastal regions of California.
Wildfires
Because of the hot, dry, and windy nature of southern California, wildfires are common. While most of them only affect small areas, wildfire can quickly get out of control and spread across large areas of woodland, particularly when fanned by the Santa Ana winds. Hot and extremely dry, they are also known as "Devil winds", and move towards the coastal region from the continental interior of the American West, where high pressure systems over the southwestern deserts increase in temperature.
Because natural wildfires are more likely to appear towards the end of long, dry summers, they have become a more serious problem in recent years because of the changing climate, overpopulation and decreased controlled fires, which makes wildfires increasingly larger and more frequent. Climate change has also widened the wildfire "season" from a few summer months to virtually the entire year.