The Mission’s Summers Come Briefly, Typically in Autumn

The Mission’s hottest month isn’t July. It’s September.

Heat here is rare, short-lived and late. Here’s how it plays out, hour by hour.

01The season

Summer shows up in September

Most of the country peaks in July. The Mission doesn’t.

September and October bring 56% of the year’s hours above 80°F. June through August, the months most people call summer, bring just 30%.

Blame the fog machine. In summer, intense inland heat creates a pressure difference that pulls cool Pacific air east across the city. The hills west of the Mission block much of the fog that blankets the Sunset, but ocean-cooled air still gets through and keeps even sunny afternoons mild.

By early fall, that engine winds down. Inland temperatures ease and the westerly wind weakens. Fog thins, coastal water warms slightly, and now and then a warm, dry offshore wind replaces the ocean breeze. September alone brings nearly a third of the year’s hot hours. October adds another quarter.

02Year to year

There’s no such thing as a typical year

7 hours in 2018. 175 hours in 2020.

The 82-hour average hides huge swings. Of 18 complete years, only five landed within 30 hours of it. The other 13 were either mild, under 50 hours, or hot, over 120.

So what makes a hot year? Usually one or two heat waves. The eight days from September 30 to October 7, 2024, brought 80 of that year’s 129 hours. The 2017 Labor Day heat wave brought 54 of 139.

One heat wave can deliver more than half of a year’s heat.

Month by month, the last five years show the same thing. Most of each year’s heat lands in one or two tall bars, usually in September or October.

03The 2026 season

2026: hot, on a strange schedule

99 hours and counting.

That’s more than 11 of the other 18 full years on record. But 2026 got there in an odd way.

It started early. A warm spell in mid-March brought 28 hours at 80°F or above, the second-earliest start to a heat season on record. Then summer barely showed up. June, July and August added just 15 hours combined. September and October made up the difference with 53 hours, including a 93°F reading late on the afternoon of September 9.

04The peak

How hot it gets comes down to a few days

The year’s hottest reading usually lands in the low 90s. It has ranged from 84°F to 105°F.

The difference is offshore wind. When high pressure parks over the Great Basin, the wind turns around and blows from the interior toward the coast. That air warms as it sinks downhill, and it shuts off the sea breeze that normally caps the afternoon. Whether that setup arrives, and how strong it gets, decides the year’s peak.

The record is 105°F, on September 1, 2017. It came in the middle of a Labor Day heat wave that set an all-time high of 106°F downtown. The next day reached 102°F.

All ten of the hottest days on record fall between mid-May and early October, the window when offshore winds can break through.

San Franciscans weren’t ready. By early afternoon, parts of the city had passed 100°F, and residents took to Twitter to joke and complain about the heat.

From 100°F to 60°F in three days.

The Mission’s third-hottest day on record was Sunday, September 6, 2020, at 100°F. Downtown set a record for the date that afternoon. Three days later, smoke from wildfires across Northern California turned the sky orange. Forecasters had expected parts of the Bay Area to reach the 80s. Instead, the Mission topped out at 60°F.

05Time of day

Heat here is an afternoon thing

Noon to 5 p.m. That’s when 71% of the Mission’s hot hours happen.

On a typical hot day, the first 80°F reading comes around noon and the peak around 3 p.m. The sun is highest near 1 p.m., but the ground and air keep gaining heat into the afternoon. Then the sea breeze returns and temperatures drop fast. Mornings almost never reach 80°F, even during heat waves.

06Microclimates

The sunny side of the city

On hot days, the Mission runs about 14°F warmer than the Sunset.

The Sunset faces the ocean, so fog and sea breeze reach it first and stay longest. Between the ocean and the Mission stand Twin Peaks and the ridge running south from Mount Sutro, about 900 feet high. The ridge blocks much of the fog, and the air that spills over warms as it sinks.

Even downtown, less than a mile away, runs about 3°F cooler than the Mission on hot days.

07After dark

Warm nights are almost unheard of

23 nights in 20 years.

That’s how often the Mission stayed above 70°F all night, about once a year. Even on the hottest days, the city usually cools off after dark, when the sea breeze brings ocean air back. Most warm nights came during heat waves, when offshore winds kept blowing through the night.

08The full record

Twenty years, day by day

Look for a pattern and you won’t find much of one.

Some years run hot in spring, others only in fall, and a few barely register. The years since 2017 have averaged about 13 more heat hours than the decade before. But one heat wave can swing a year by 50 hours or more, and twenty years is too short to call that a trend.

The longer downtown record tells a clearer story. Mission Local found that average temperatures there have risen about 2°F compared with the years before 1980, and that September 2022 had the city’s fifth-hottest start since 1921.

09The rest of the country

A year of Mission heat is a week in Phoenix

82 hours a year. Phoenix gets that in about a week. Houston, in under two.

New York gets about eight times as many hot hours as the Mission. Even Seattle, famous for mild summers, gets more than twice as many.

That mildness shapes how people live. Only about half of San Francisco-area homes have air conditioning, among the lowest shares in the country. Nationwide, more than 9 in 10 do. Most years, that is a fair bet. The trouble comes on the handful of days when it isn’t.

Most years, the Mission doesn’t need air conditioning. On a few days, it badly does.