San Francisco's Secret Summer, Explained by a Fog Snob
If you live in San Francisco, you've noticed it. The mornings are bright. The afternoons are warm. Your hoodie is sitting on a chair, confused. The fog has left the building.
Welcome to San Francisco's "secret summer," the few weeks each fall when the city finally warms up. Everyone else is thrilled. We're refreshing the weather app, hoping for a little gray.
Let’s get into San Francisco Fog.
San Francisco fog season runs from roughly June through August, with fog tapering off in September. July is the foggiest stretch, averaging about 14 to 16 foggy days, according to SFBayWeather. By September, that drops to about 4 to 6.
That's why visitors show up in July wearing shorts and leave wearing a souvenir sweatshirt. The summer fog is real, and it's cold.
Why is September hotter than July in San Francisco?
It comes down to the ocean. Summer fog forms when warm, moist air moves over the cold California Current and cools to its dew point, as Wikipedia's San Francisco fog entry explains. Hot inland valleys pull that marine layer through the Golden Gate like a vacuum.
By late summer, three things change:
- The ocean warms up. The Pacific takes a couple of months to absorb summer heat, so the water is warmest in early fall.
- The winds calm down. The northwest winds that pull cold water up from the deep weaken by late August.
- The contrast fades. With a warmer ocean and cooler inland valleys, there's less pull to drag fog onshore.
The result: coastal San Francisco averages around 66°F in July and about 72°F in September, with October often just as warm. Our warmest weather arrives right when the rest of the country is pulling out sweaters.
How Fog Snobs survive the sunny season
We're not anti-sun. We're just pro-fog. Here's how we get through it:
- Chase the early mornings. Fall fog still sneaks in at dawn along Ocean Beach, Lands End, and the Outer Sunset. Set an alarm.
- Walk the Crosstown Trail. It's 17 miles across the city through neighborhoods with very different microclimates. Somewhere along the way, there's usually a bit of gray.
- Dress for both. It can be 72° at noon and 55° at 6pm. A midweight crewneck works for both.
- Wear the forecast. Our Sunset crewneck, "Always Foggy, Sometimes Sunny," was made for exactly this time of year.
What to wear in San Francisco in October
October in San Francisco means warm afternoons and cool evenings, sometimes on the same block. Pack layers:
- A cotton long-sleeve crewneck for afternoons in the park
- A heavy sweatshirt for when the wind picks up after sunset
- A cap for the sun you weren't expecting
Shop the full Certified Fog Snob collection, grab a tote and fill it with your FOG SNOB crewneck and an embroidered hat.
When does the fog come back?
The classic summer fog returns with late spring and early summer, as inland heat builds again. Until then, winter brings its own gray days, with rain, low clouds, and inland valley fog.
So enjoy the sun. Go to the beach. Eat outside. Then put on something cozy and remember: the fog always comes home.
See you in the fog.
