California is globally recognized for its sunshine, yet its most famous city, San Francisco, is notorious for its cool, often chilly summer weather. The city’s summer climate presents a geographical paradox, where the nearby inland valleys experience scorching heat while the coast remains shrouded in a cool, damp marine layer. This unique phenomenon, which keeps summer temperatures consistently low, is the result of a precise interaction between a cold ocean current, a specific form of fog, and a large-scale atmospheric pressure system.
The Influence of the Cold Pacific Current
The primary source of San Francisco’s persistent cool air is the cold water of the Pacific Ocean, driven by the California Current. This current transports frigid water from the North Pacific and Alaska southward along the western coast of North America. The temperature of this water is significantly cooler than at comparable latitudes on the East Coast of the United States, which is influenced by currents from the tropical Atlantic.
This naturally cool current is intensified near the coast by a process called coastal upwelling. Prevailing northwesterly winds push the surface water away from the coastline due to the Coriolis effect. As the surface water moves offshore, deep, cold water rises to replace it. This upwelling causes sea surface temperatures immediately off the coast to remain consistently low, typically in the range of 52–58 °F (11–14 °C) year-round, chilling the air that comes into contact with it. This forms a dense, cold air mass that is the foundation of the local weather pattern.
The Fog’s Cooling Effect
The cold air mass created by upwelling provides the perfect conditions for the formation of the characteristic San Francisco fog, also known as the marine layer. As warmer, moist air from the open Pacific Ocean blows over the chilled coastal waters, it cools rapidly. This cooling lowers the air temperature to its dew point, causing the water vapor to condense into tiny liquid droplets that form thick fog. This type of condensation is known as advection fog, forming when warm, moist air moves horizontally over a cooler surface.
The resulting marine layer acts as a natural shield, preventing the city from heating up under the summer sun. By reflecting incoming solar radiation back into the atmosphere, the fog effectively blocks daytime heating. Even when the fog is not thick enough to be visible, the cool, dense marine air flowing onshore keeps temperatures subdued. The persistent onshore flow of this chilled air is why San Francisco often feels “naturally air-conditioned.”
How Inland Pressure Draws the Cold Air In
The final mechanism that pulls this cold air and fog directly over San Francisco involves the intense summer heat of the California interior. During summer, the vast Central Valley heats up dramatically, creating a persistent, large-scale area of low atmospheric pressure known as the Central Valley heat low. This intense heat causes the air above the valley to expand and rise.
The low pressure acts like a vacuum, drawing the cold, high-pressure, dense marine air from the Pacific toward the interior. The most direct pathway for this air to move inland is through the Golden Gate, the narrow opening into San Francisco Bay. This funnels the fog and cold air directly into the city, creating strong winds. The greater the temperature contrast between the hot Central Valley and the cold Pacific, the stronger the pressure gradient, resulting in more persistent fog and lower temperatures for San Francisco.