The Pacific Ocean off San Francisco is cold, about 52°F to 58°F all year. The California Current carries cold water south from the Gulf of Alaska, and summer winds pull even colder water up from below in a process called upwelling. That cold water chills the air above it into the marine layer, which is why the Bay Area coast is foggy all summer. You can see the effect in NOAA monthly Bay and ocean water-temperature averages, with recent readings for Bay waterfronts and Pacific beaches.
Why Is the Pacific Ocean So Cold in California?
The Pacific Ocean is cold in California because of two forces. A cold current flows down the coast from Alaska, and upwelling brings cold deep water to the surface. On the East Coast, by contrast, the warm Gulf Stream flows north from the tropics. That is why the Atlantic at the same latitude is far warmer in summer.
The cold water also stays cold through the summer, because upwelling keeps replacing sun-warmed surface water with cold water from below. As a result, the ocean off San Francisco warms only about 5 degrees between May and August. Offshore water is warmest in September, averaging about 58°F.
What Is the California Current?
The California Current is a cold ocean current that flows south along the West Coast of North America, from the Gulf of Alaska to Baja California. It is the eastern edge of the North Pacific Gyre, a huge clockwise loop of ocean water driven by global winds and Earth's rotation. The California Current is a cold current, carrying subarctic water into the mid-latitude Pacific.
That water runs about 50°F to 58°F off the Northern California coast in summer, colder than the air moving over it. When warm, moist air crosses the cold water, the air cools from below. Once it cools to its dew point, the moisture condenses and the marine layer forms.
What Is Upwelling?
Upwelling is the rise of cold, deep ocean water to the surface when wind pushes surface water away from the coast. Along California, northwest winds driven by the North Pacific High blow along the coast from spring through summer. Because of Earth's rotation, the moving surface water turns about 90 degrees to the right of the wind, which sends it offshore, a process oceanographers call Ekman transport.
Cold water from about 200 to 600 feet down rises to replace the water pushed offshore. In active upwelling zones along the Bay Area coast, the sea surface can drop to 48°F to 52°F. Air over that water chills below 60°F even when inland valleys reach 80°F or more, and it often cools enough to form persistent fog.
The coast north of San Francisco, around Point Reyes and Point Arena, is one of the strongest upwelling zones in California. The coastline there runs nearly north-south into the prevailing northwest wind, which makes upwelling efficient and keeps the Bay Area coast foggier than the coast farther south.
Why Is San Francisco Bay Warmer Than the Ocean?
San Francisco Bay is warmer than the ocean in summer because it is shallow, mixes with river water and heats in the sun. NOAA monthly averages peak at 68.6°F in Alameda in August, compared with 58.3°F offshore San Francisco in September. The cold upwelled ocean water stays mostly outside the Golden Gate.
That is why fog forms over the open ocean, not over the bay itself. The fog is then pushed inland, and the Golden Gate is the main gap in the coastal hills. The strait runs roughly east-west, lined up with the prevailing ocean wind, so it funnels the marine layer into the bay and the cities around it.
How Does Cold Ocean Water Drive the Afternoon Fog?
Cold ocean water drives the afternoon fog by creating a sharp pressure difference between the cool coast and the hot interior. The Central Valley regularly reaches 95°F to 105°F on summer afternoons. Hot air rises and lowers the pressure inland, while the cool, heavy air over the ocean stays at higher pressure. Air flows from high to low pressure, so marine air rushes inland through the Golden Gate and other coastal gaps.
The colder the ocean and the hotter the valley, the stronger the push. That is why the foggiest days in San Francisco often line up with the hottest days in Livermore and the Central Valley. The inland heat is the pump, and the cold Pacific is the reservoir.
Changes in the North Pacific can change Bay Area fog. El Niño years, when the coastal ocean runs warmer than normal, tend to bring less summer fog, while La Niña years, with stronger upwelling, tend to be foggier. See whether Bay Area fog is declining for the long-term picture.
Deep Dive: How Does Wind Along the Coast Lift Cold Water?
Upwelling depends on Earth's rotation turning the ocean's surface water sideways. The northwest wind drags the top layer of the sea along with it, and the Coriolis effect bends that moving water to the right. Along California, the right of a wind blowing south is west, out to sea. Cold water rises from below to fill the gap left near the shore.
How Much Water Does the Wind Push Offshore?
In 1905, the Swedish oceanographer Vagn Walfrid Ekman showed that the net flow in this wind-driven surface layer runs at right angles to the wind. The amount depends on how hard the wind pulls on the water and how strongly Earth's spin turns it. Wind pull rises with the square of wind speed, so doubling the wind roughly quadruples the offshore flow.
By our calculation, a steady 22-mph wind off San Francisco moves about 1.7 cubic meters of water offshore each second for every meter of coastline. If that water is replaced from below across a coastal strip about 12 miles wide, the cold water rises about 25 feet a day. At that rate, water from 200 feet down can reach the surface in about a week. That is why the sea surface can drop several degrees after a few days of strong northwest wind.
How Do Scientists Track Upwelling?
In 1973, NOAA fisheries scientist Andrew Bakun published an upwelling index that estimated the offshore flow from air pressure maps. Fisheries researchers used it for decades, because upwelling also carries nutrients that feed plankton and fish. Its weakness was that it ignored ocean currents and relied on rough wind estimates.
In 2018, Michael Jacox and colleagues at NOAA introduced a newer measure, the Coastal Upwelling Transport Index. It uses an ocean model that includes both the wind-driven flow and the ocean's own currents. It reports daily upwelling for each degree of latitude along the West Coast from 1988 onward. These indices show the same seasonal pattern this page describes, with upwelling that strengthens in spring, stays strong through summer and fades in fall.
The rising water is cold because it comes from below the thermocline, the boundary between sun-warmed surface water and the colder ocean beneath. That deeper water has not seen sunlight for a long time, and much of it arrived with the California Current from the far North Pacific.
Why Doesn't the Ocean Off San Francisco Warm Up in Summer?
The ocean off San Francisco does not warm up in summer because upwelling keeps replacing the warming surface water with cold water from below. In most of the world, coastal water peaks in late summer after months of sunshine. Off Northern California, August water is often not much warmer than May water.
That cold surface keeps the contrast with the warm air above it strong all summer, so sea fog keeps forming. The fog season peaks in July and August because inland heat is at its peak, not because the ocean changes.
The pattern shifts in September and October, when the North Pacific High weakens and northwest winds ease. Upwelling slows, and the ocean surface finally warms a little. Inland heat eases at the same time, so the summer fog fades. September and October become the warmest, clearest months on the Bay Area coast.
