San Francisco's summer fog does not come and go at random. When it thickens, when it pulls back and when it clears for the day all follow a pattern driven by heat in the Central Valley. Understanding that pattern tells you when to be in the city for sunshine and when to head inland for warmth. It also explains why July plans for the middle of the day are more reliable than plans for early morning or late afternoon.
What Is the Central Valley Thermal Low?
California's Central Valley is a long, flat basin that heats up quickly on summer days. Afternoon highs there often reach the mid 90s to around 100°F in June, July and August. As the ground heats, the air above it warms, expands and rises. That rising air lowers the pressure at the surface, and the valley develops what meteorologists call a thermal low, an area of low pressure created by strong heating.
Air flows from higher pressure toward lower pressure, and that difference in pressure drives the wind. The thermal low sets up a strong pressure difference between the cool, dense air over the Pacific and the hot, light air inland. That difference is the engine that pulls marine air, fog included, through the Golden Gate and into the Bay Area each afternoon. The hotter the Central Valley gets, the stronger the pull.
The Daily Fog Cycle in San Francisco
From June through August, San Francisco's fog follows a remarkably consistent daily pattern. Fog usually forms or spreads overnight as the air cools, and by early morning the marine layer has often pushed deepest into the bay and the city. The Inner Sunset, Outer Sunset and Richmond are often completely fogged in at 7 or 8 AM. Downtown and the Mission may be only partly cloudy at the same hour.
As the morning goes on, two things start to happen at once. The sun warms the ground and the air near it, slowly eating away at the bottom of the fog layer. At the same time, the Central Valley heats up again and the thermal low strengthens. Neighborhoods sheltered behind the hills, such as Noe Valley and the Castro, often break into sunshine first, while the western neighborhoods stay gray.
In many neighborhoods, the fog burns off between 10 AM and 1 PM. You can watch it happen as the cloud base rises, the view at street level opens up and patches of blue sky appear. By early afternoon on most summer days, at least the eastern and southern parts of the city are in sunshine. The Outer Sunset and Outer Richmond may stay under the marine layer through most of the afternoon.
Why the Fog Returns Every Afternoon
Many visitors expect a sunny afternoon to stay sunny, but in many neighborhoods the fog comes back. By 3 to 5 PM, the Central Valley reaches its peak heat and the thermal low is at full strength. The pressure difference between ocean and interior is at its greatest, and it pulls a fresh surge of marine air through the Golden Gate. This afternoon push is driven more by wind than the overnight fog, and it arrives with the famous westerly winds of 15 to 25 mph.
That gives many summer days the same shape. Morning fog gives way to a short sunny window around midday, then clouds and wind build again in the afternoon. The Inner Sunset can go from clear at noon to thick fog by 4 PM. The Embarcadero may have lovely afternoon light while the Haight is already gray. Each neighborhood's pattern depends on its height, its position relative to the Golden Gate and how exposed it is to the westerly wind.
Fog Burn-Off Varies Dramatically by Neighborhood
The fog does not clear evenly across San Francisco. Those differences are one of the defining parts of life in the city, and they come down mostly to shelter and distance from the ocean.
The western neighborhoods, including the Outer Sunset, the Outer Richmond and the area around Ocean Beach, sit low and directly in the path of the incoming marine air. They clear latest and get the fog back first, and on many summer days they never clear at all. The Inner Sunset and Inner Richmond, farther from the beach, often clear a little earlier.
The Castro, Noe Valley and the Mission sit on the sheltered east side of Twin Peaks, out of the direct path of the marine air. These neighborhoods clear fastest and are often warm and sunny while the western half of the city is still gray. Hilltops can poke above the fog on days when the marine layer is shallow. Bernal Heights, on the city's sunnier east side, often clears early. Twin Peaks itself, though, is frequently wrapped in fog, because it stands directly in the marine air's path.
In summer, the Sunset District can be 10 to 15 degrees cooler than the Mission District, only about 3 to 4 miles away. The Mission is sheltered from the direct marine flow, so the gap is not a random quirk. It follows predictably from where each neighborhood sits relative to the ocean and the hills.
Deep Dive: What Actually Burns the Fog Off?
Fog does not burn in any literal sense. Sunlight warms the air in and under the fog until it can hold its water as vapor again, and the droplets evaporate. How long that takes depends mostly on how thick the fog is and how much new marine air keeps flowing in.
How Much Energy Does It Take to Clear the Fog?
Fog typically holds about 0.2 grams of liquid water per cubic meter. By our calculation, a fog layer 1,000 feet thick then holds about 60 grams of water above each square meter of ground. Evaporating that water takes about 150,000 joules per square meter. Warming that whole 1,000-foot column of air by just 1°C takes more than twice as much, about 370,000 joules.
Most of the work, in other words, goes into warming the air rather than drying the droplets. The air has to warm enough that it is no longer saturated. A thick fog deck also reflects much of the morning sunlight back to space, so only part of the sun's energy reaches the layer. Thicker fog needs more warming and lets less sunlight through, which is why deep marine layers clear so slowly.
One forecasting rule of thumb, from a National Weather Service Sacramento technical note, adds about one hour after sunrise for every 1,000 feet of cloud height. The rule was written for valley stratus, but the same physics applies on the coast.
How Does the Fog Break Apart?
Sunlight works on the fog from two directions. Some of it passes through the cloud and warms the ground, which heats the air from below and lifts the cloud base. Droplets inside the cloud also absorb some sunlight, which offsets the cooling at the cloud top that keeps the layer stirred overnight. As that stirring weakens, dry air from above the inversion mixes in and thins the cloud from the top.
The fog clears first where it is thinnest and where the ground warms fastest. That is usually over land east of the hills, then the bay shore, then the western neighborhoods. Over the ocean, the cold water keeps cooling the air from below, so the fog bank offshore often never clears at all.
Burn-off timing matters well beyond picnics. As MIT reported in 2000, summer stratus cut peak arrivals at San Francisco International Airport from 55 to 30 per hour. Its Lincoln Laboratory counted about 70 such mornings each summer. The fog typically cleared by about 10:30 AM, though sometimes as early as 8 AM or as late as 2 PM. The lab built a forecasting system for the airport that combined instruments measuring the height of the inversion and the strength of the sunlight at the ground.
When Does the Fog Stay Gone All Day?
A fully sunny summer day in San Francisco needs one of a few setups. The North Pacific High can weaken enough that the marine layer thins and pulls back. The Central Valley heat can ease, which weakens the pressure difference that draws marine air inland. Or offshore winds can develop and push the marine layer out to sea instead of letting it flow ashore.
Those setups are more common in September and October than in July and August. By September, the North Pacific High weakens, the Central Valley cools and the thermal low loses strength. Sunny windows last longer, and the afternoon fog is less likely to return. That is why September is San Francisco's warmest month on average, with October close behind. The city can reach the 70s or even 80s on fall days that would have been stuck near 60 under July fog.
For more reliably sunny summer days, Oakland and the East Bay are a good alternative. Marine air does reach Oakland and Berkeley, but it has thinned and warmed by the time it crosses the bay. The fog there usually clears earlier than in San Francisco. Farther inland, beyond the Berkeley hills, towns like Walnut Creek usually stay clear all day. On a typical July afternoon when the Outer Sunset sits in the upper 50s, downtown Oakland is often in the low to mid 70s and sunny.
