Inland Los Angeles areas frequently experience higher temperatures than coastal San Diego neighborhoods due to differences in geography and meteorology. While both cities are on the Pacific coastline, the specific shape of the land and the influence of air masses create vastly different microclimates. This temperature disparity is explained by how effectively each region accesses the ocean’s cooling power, the physical barriers that trap heat, and the influence of dry wind systems.
The Ocean’s Moderating Effect
The primary mechanism regulating Southern California’s temperatures is the cold California Current, which flows southward and keeps the ocean surface cool. This cold water chills the air above it, creating a persistent layer of cool, moist air known as the marine layer. A temperature inversion forms when this dense, cool air is trapped beneath a layer of warmer air higher up, often resulting in coastal fog or low clouds.
San Diego benefits from a strong and consistent maritime influence because its coastal plain is relatively narrow and open. This allows the cool marine air to penetrate several miles inland without significant obstruction, providing a reliable, year-round temperature-regulating effect. The cooling effect is pronounced, meaning the temperature difference between the immediate coast and inland San Diego is often less extreme than in the Los Angeles region.
Geographic Differences and Heat Trapping
The Los Angeles area’s unique topography significantly limits the inland reach of oceanic cooling, leading to greater temperature extremes. The massive Los Angeles Basin encompasses extensive inland valleys, such as the San Fernando and San Gabriel Valleys. These areas are effectively walled off from the Pacific by mountain ranges, most notably the Santa Monica and San Gabriel Mountains.
These physical barriers act as a dam, blocking the cool, moist marine layer air from penetrating the basin, especially during the summer. As a result, the inland air heats up substantially without the daily influx of ocean air, causing temperatures to soar higher than at the coast. Additionally, the dense infrastructure of the urban core, particularly Downtown Los Angeles, contributes to the Urban Heat Island effect. Materials like concrete and asphalt absorb and re-radiate heat, further exacerbating high temperatures.
Influence of Specific Weather Patterns
The most dramatic temperature spikes in Los Angeles are often caused by the Santa Ana winds, a meteorological phenomenon that intensifies the heat trapping effect. These winds originate from high-pressure systems over the Great Basin, typically during the fall and winter. The air begins cool and dry over the high-elevation deserts, but as it flows toward the coast and descends thousands of feet into the Los Angeles basin, it is compressed.
This compression causes the air temperature to rise significantly, a process called adiabatic heating, at a rate of approximately 5.5°F for every 1,000 feet of descent. The winds are then funneled through mountain passes and canyons, arriving on the coast as hot, dry blasts that override the normal cooling effect of the ocean. While San Diego also experiences Santa Ana winds, their trajectory and broader coastal plain often mitigate the most intense heating. This makes the heat spikes in the Los Angeles inland valleys more pronounced and frequent.