Heat stress in San Francisco
Cool and foggy most of the year; rare heat spells hit hard because few homes have air conditioning.
Light precautions for intense activity. Keep water handy.
Wet-bulb globe temperature via the Liljegren (2008) method. This page is cached and refreshes in the background, so the live badge below can be a few minutes ahead of it.
View San Francisco on the live mapReading heat stress in San Francisco
Wet-bulb globe temperature (WBGT) is the heat measure that actually tracks danger to the body. Unlike the plain air temperature on your phone, it folds in humidity, wind, and direct sun: the factors that decide whether your sweat can still cool you. It’s the standard the military, athletic associations, and occupational-safety bodies use to call when outdoor activity is no longer safe.
That matters in San Francisco because cool and foggy most of the year; rare heat spells hit hard because few homes have air conditioning. The number above is the live outdoor WBGT for the city right now; tap through to the live map to inspect any neighborhood, scrub the forecast hour by hour, and see the 7-day outlook.
How hot San Francisco usually gets
Across 2015–2024, San Francisco averaged a 67°F daily high in summer: 173rd hottest of the 174 metros we track, and September is typically the warmest month.
Heat arrives in bursts here rather than settling in, with only about 2 days a year at or above 90°F.
Humidity is the real story: summer averages about 79%, so sweat evaporates poorly and heat stress runs well above what a 67°F afternoon suggests.
The ceiling is a long way above the routine: the hottest day in this sample hit 101°F, about 34°F past a normal summer afternoon.
Daily high temperatures and humidity from ERA5 reanalysis, 2015–2024 (hottest day in the sample: 2017-09-02). These are climate averages for context: the reading at the top of this page is the live one.
For school sports, see our California heat policy page.
Heat in San Francisco: common questions
How hot does it usually get in San Francisco?
Across 2015–2024, San Francisco averaged a 67°F daily high through summer, with about 2 days a year at or above 90°F. September is typically the hottest month, and the hottest day in that sample reached 101°F. Figures are from ERA5 daily reanalysis.
When is the hottest time of year in San Francisco?
September has the highest average daily high in San Francisco. Humidity is the real story here: summer averages about 79%, so sweat evaporates poorly and heat stress runs well above what the temperature alone suggests. Use the forecast on this page to plan around the specific days that matter.
What is the WBGT in San Francisco right now?
The reading at the top of this page is the current wet-bulb globe temperature for San Francisco, California, computed from live weather by the Liljegren (2008) method and labeled with how recently it updated. For an exact reading at your own neighborhood, plus an hour-by-hour forecast, open the live map.
Why is WBGT more useful than the temperature for San Francisco?
Air temperature ignores humidity, wind, and sun: the things that decide whether heat is dangerous. San Francisco's climate (cool and foggy most of the year; rare heat spells hit hard because few homes have air conditioning.) is exactly why a single "feels-like" or thermometer reading can understate the real risk. WBGT captures all of it in one number.
When is heat most dangerous in San Francisco?
September is the peak month in San Francisco, and within a day heat stress is highest in the early-to-mid afternoon when sun load is greatest. Dangerous days are uncommon here, which is its own hazard: 2 days a year reach 90°F, and people are least acclimatized when they arrive. Use the forecast on this page to find the window that works.
Put San Francisco’s live heat stress on your site
A self-updating WBGT badge for schools, teams, camps, and local sites: it always shows the current reading and links back here.
<iframe src="https://wetbulbtracker.com/embed/san-francisco-ca" width="340" height="150" style="border:0" title="Live WBGT: San Francisco, CA" loading="lazy"></iframe>Paste it into any website. The badge updates itself and links back here. Add ?theme=light for light sites.