San Diego’s coastal areas consistently register cooler average temperatures than Los Angeles, particularly during the late spring and summer months, often referred to as “May Gray” and “June Gloom.” This climatic variation results from a specific combination of oceanographic conditions, unique regional topography, and atmospheric dynamics. The three primary factors maintaining a cooler environment near the San Diego coast are the temperature of the Pacific Ocean water, the distinct physical landscape, and the behavior of the marine layer.
The Influence of Colder Ocean Water
The foundation of Southern California’s cool coastal climate is the California Current, a cold, southward-flowing mass of water originating in the North Pacific. This current acts as the primary cooling agent, chilling the air immediately above the ocean surface.
The temperature difference between the two cities is amplified by upwelling, where strong northwesterly winds push surface water away from the shore. This movement is compensated by deeper, colder water rising to the surface. The intensity of upwelling depends on the underwater topography, or bathymetry, near the coast.
The continental shelf is generally narrower and steeper off the San Diego coast compared to the wider shelf found in the Los Angeles Bight. This narrower structure allows deeper, colder water to be brought closer to the coastline more effectively. Consequently, sea surface temperatures adjacent to the San Diego coast are consistently cooler, establishing a lower baseline temperature for onshore air masses.
Differences in Coastal Topography and Airflow
The physical geography of the two regions dictates how cool Pacific air moves inland and interacts with warmer continental air. The Los Angeles Coastal Plain is a wide area, covering hundreds of square miles before meeting the steeper Transverse Ranges. This broad plain allows the cooling sea breeze to penetrate many miles inland, where it mixes quickly with warmer air generated by the heated land.
San Diego’s coastal zone is much narrower, immediately backed by the steeper hills and canyons of the Peninsular Ranges. This configuration constrains the cool, dense marine air to a tighter corridor along the coastline. The terrain acts like a funnel, channeling the cool air parallel to the shore or trapping it against the coastal slopes.
This physical barrier prevents the rapid inland penetration and mixing seen in the wider Los Angeles basin, keeping the cooling effect concentrated at the coast. The elevated terrain also prevents solar heating from inland areas from easily reaching and warming the coastal air mass. This topographic channeling creates a more pronounced microclimate difference in San Diego.
The Behavior of the Marine Layer
The final factor is the persistent atmospheric feature known as the marine layer, a low cloud or fog layer capped by a temperature inversion. This inversion layer is a boundary where warm, dry air aloft traps the cool, moist air near the surface, acting like a lid. Cooling is maximized when this lid is strong and keeps the cool air compressed near the ground.
While both cities experience the marine layer, its cooling effect is more prolonged in San Diego due to local topography. In Los Angeles, the wider coastal plain and greater inland heating often cause the marine layer to dissipate, or “burn off,” faster during the day. As inland air warms, it pushes the inversion base higher or breaks the layer, allowing sunlight to warm the coast sooner.
In San Diego, the combination of colder water and narrow, confining topography enhances the persistence of the marine layer, especially through the mornings and evenings. This physical constraint helps seal the cool air against the coast for longer periods, resulting in more frequent extended days of low clouds and fog. This atmospheric trapping mechanism ensures San Diego maintains a lower daily high temperature and smaller overall temperature fluctuation.