Many visitors book a July trip to San Francisco expecting California sunshine, and they are surprised. Along the coast and around the bay, the warmest month is September, not July. Downtown San Francisco averages a high of 66.3°F in July and 70.2°F in September, the warmest of the year, and October is nearly as warm at 69.8°F. This is not a fluke but a pattern that repeats every year, driven by the ocean and the fog it creates.
Is September Really Hotter Than July? The Numbers Say Yes
The pattern has held for decades at the downtown San Francisco station, where NOAA's 1991-2020 normals put the average high at 66.5°F in June and 66.3°F in July. August rises to 67.9°F, September to 70.2°F and October holds at 69.8°F, so July is actually a shade cooler than June. September also brings the city's hottest days of the year. Of the 34 complete years from 1991 to 2025, 14 had their hottest day in September, more than any other month. The all-time record of 106°F was set on September 1, 2017.
Fog explains the gap between the summer and fall months. In NOAA's HRRR model analysis from 2014 to 2026, fog covered the Outer Sunset in about 54 percent of July's 7 AM hours. The share was 46 percent in August, 38 percent in September and 22 percent in October. The coast does not clear all at once, but each month after July lets in more sun.
Inland, the calendar works differently. Valleys such as Walnut Creek and Concord run 15 to 25°F warmer than the coast in summer and peak in July and August. Concord averages about 88°F in both months, then eases to the mid 80s in September and the upper 70s in October. So while San Francisco warms in September, the inland valleys are already cooling. The temperature gap across the bay is at its smallest of the warm season.
Why Summer Fog Keeps July Cooler Than You Expect
July is cooler than September because of the marine layer. June, July and August are the peak fog months in the Bay Area. Heat building inland pulls cool ocean air toward the coast, and that moist air forms fog as it cools over the cold water. The fog covers the coast from early morning into the afternoon and blocks the sun. It keeps the air cool, so San Francisco feels more like a spring day than midsummer.
Even when the fog burns off by noon, the morning fog has already cost the day several hours of sunshine. The coastal air never warms the way it would on a clear day. People who live in the Outer Sunset or near the water in Berkeley know this well. A sunny July afternoon there is still cooler than a September afternoon, even when the sky looks the same.
How the Ocean Warms Slowly and Triggers Fall Clearing
The ocean off San Francisco stays cold all year, kept that way by the California Current and coastal upwelling. Its temperature still shifts by several degrees through the seasons, though. NOAA's buoy off San Francisco shows the water at its coldest in April and May, about 52.5°F, and at its warmest in September, about 58°F. The ocean responds slowly, so its warmest point comes two to three months after the longest days of summer.
By September, the ocean has absorbed enough summer warmth to run a few degrees warmer, and that small change matters. At the same time, the inland valleys are starting to cool. Together, those shifts shrink the contrast between the cool coast and the hot interior, which weakens the pressure difference that pulls marine air ashore. With less force driving it, the fog season winds down and largely ends for the fall.
When Upwelling Weakens and the Fog Machine Shuts Off
Coastal upwelling is the other half of the story. Through spring and summer, steady northwest winds blow along the California coast. They push surface water offshore, and cold water from deeper down rises to replace it. That upwelling keeps the ocean off San Francisco near 57°F even in August, while the interior often tops 95°F.
In late August and September, the seasonal winds shift and the northwest winds weaken. Less cold water reaches the surface, so the sun can finally warm the upper layer of the ocean without fresh cold water constantly replacing it. The fog loses its most reliable source of cold water. The result is clearer skies and warmer temperatures, the fall warmth that longtime Bay Area residents think of as their real summer.
What Indian Summer Actually Means in the Bay Area
The term “Indian summer” usually means an unseasonably warm spell in fall, often after the first frost. The Bay Area's version is a regular season rather than an occasional spell. Here, the stretch from September into October is Indian summer, when the city finally delivers on California's sunny reputation. Downtown San Francisco averages about 70°F, while the inland valleys sit in the mid 80s in September and ease into the upper 70s in October. Mornings are cool and crisp, afternoons are warm and clear, and the summer fog is mostly gone.
This is when San Francisco's outdoor restaurants fill up without diners wearing three layers. The beaches at Alameda and the Marin Headlands become pleasant places to sit. The Golden Gate Bridge can stay in view from Crissy Field for days at a time. Locals see it as their reward for a foggy summer that many tourists mistake for the city's best season.
Deep Dive: Why Does the Coast's Warmest Month Lag So Far Behind the Sun?
Sunshine peaks at the June solstice, but almost nowhere is warmest in June. Temperatures keep rising as long as a place takes in more heat each day than it loses. The more heat a place can store, the longer that takes, and the foggy, ocean-cooled coast stores a great deal.
What Sets the Length of the Lag?
A simple model used in climate textbooks treats a place as a single store of heat. Sunlight fills it, and the store leaks heat back out faster as it warms. The key number is the time the store takes to adjust to a change in sunlight, which climate scientists call its response time.
By our calculation with that model, a place that adjusts within about a month peaks about four weeks after the solstice. That fits inland California, where Sacramento's warmest month is July. A place with a response time of about 100 days peaks about two months after the solstice. The lag can never exceed three months, however large the store becomes.
San Francisco's September peak sits close to that three-month limit. Its weather is tied to the ocean, and the ocean mixed layer stores far more heat than the soil does. That large store warms slowly through summer and reaches its own peak late, at about 58°F offshore in September.
Why Does Fog Stretch the Lag Even Further?
The coast does not just store heat slowly. Its summer weather also blocks much of the heat it could receive. From May through August, the northwest winds that drive upwelling bring cold water to the surface, and the marine layer shades the coast through much of the day.
In effect, the coast's strongest sunshine arrives during the months when the fog and cold water hold it back. As the North Pacific High weakens in late summer, upwelling slows and the fog retreats. More sunshine then reaches the ground and the water at the same time that the ocean is near its warmest. That combination arrives in September, well after the inland valleys have peaked.
The pattern shows up in the averages on this page. Downtown San Francisco averages about 66°F in July and about 70°F in September. Sacramento follows the inland pattern, with its warmest month in July and September already cooling. The same sun drives both places, but the ocean and the fog set the calendar on the coast.
When to Visit San Francisco for Warm Weather
If you want warm, clear weather in San Francisco, plan your visit for September or early October. Inland, the hottest weather comes in July and August, so fall is also the milder time to visit the valleys. If you visit the city in June, July or August, pack layers and expect cool mornings and plenty of fog, especially near the coast. Many visitors still love summer in San Francisco, because the city is beautiful and lively, but it is not the warm California trip they pictured.
A useful rule of thumb is that coastal Bay Area seasons run about two months behind the calendar. June, July and August feel like spring, while September and October feel like summer. Once you know the pattern, you can plan around it with confidence, because the fall warmth returns reliably every year as the fog season fades.
