'All signs do point to warming': What to know about SF's changing summer weather

San Francisco summers have long been infamous for their deep layers of fog and chilly temperatures, even when the rest of the Bay Area is baking hot. But as climate change continues to warm the planet, summer in the city has started to evolve with real summer heat, diminishing fog and more of those rare July and August evenings where you don’t need to carry a jacket.

Since 1970, the average temperature from June to August in downtown San Francisco has warmed 1.1 degrees. And at San Francisco Airport over the same period, it’s warmed by 2.2 degrees, according to Daniel McEvoy, a meteorologist with the National Weather Service. 

The warming may sound minimal, and San Francisco has some natural defenses against extreme heat, but there may still be substantial impacts ahead. 

“The big picture is that all signs do point to warming,” McEvoy told SFGATE. “There’s various levels, maybe a little less so in the downtown area relative to the airport location, but there is definitely a unified warming trend.”

Climate experts told SFGATE that even a small temperature increase is significant. And the shift reflects a global trend.

“Globally, we’ve seen about a 1- or 2-degree temperature change for the whole planet, and that is a big deal,” Eugene Cordero, a meteorology professor at San Jose State University, told SFGATE. “Even a 1- or 2-degree temperature change in San Francisco might be the difference between some parts of the city getting quite warm in the midday or late day and having to turn on their air conditioning, which they don’t have, or the presence of unusually warm evenings.”

Unlike other parts of the Bay Area that are used to scorching summers, San Francisco is particularly vulnerable to “extreme heat events,” or when temperatures climb above 85.6 degrees. With little air conditioning and buildings designed for “cool, coastal temperatures,” hotter temperatures take their toll on the emergency and health services, according to a heat vulnerability study from the city. For example, there were over 1,300 911 calls on Friday, Sept. 1, 2017, when an extreme heat event hit the city and it hit 106 degrees, the highest temperature ever recorded, the study states. The emergency calls were more than double the amount from the previous Friday. It also takes about two weeks for people’s bodies to adapt to extreme temperatures, the study states.

“Even a little bit of heat is too much in San Francisco because our buildings and our bodies are not adapted to handling this heat,” Aritree Samanta, an associate environmental science professor at San Francisco State University, told SFGATE.

In addition to experiencing warmer temperatures over the past 55 years, San Francisco is also facing longer periods of them. Compared to 1970, the city in 2024 had 28 more days of above-average heat, according to a study from Climate Central, a climate research institute.

Some of the warmest parts of the city are called “urban heat islands.” Buildings and large expanses of concrete and asphalt in these areas trap the heat, re-radiate it back and change wind patterns, experts told SFGATE.

“Urban heat islands are very common. They’re almost unavoidable,” Chris Field, an environmental science professor and climate change researcher at Stanford, told SFGATE. “If you measure temperature in a place that doesn’t have a city, and then you plop a city down, the city will make it warmer.”

According to a heat vulnerability map from the city, the neighborhoods most vulnerable to extreme heat are in the eastern half of San Francisco, including the Tenderloin, South of Market, Chinatown, the Financial District, Union Square and portions of the Mission and Portola.

San Francisco is also feeling hotter on summer days as the city’s notorious summer fog thins. Fog keeps California’s coastal climate from becoming hot and dry, Todd Dawson, a UC Berkeley researcher and fog expert, stated in an 2024 interview with the Public Policy Institute of California. But since the 1950s, San Francisco has lost about three hours of fog per day due to climate change, Dawson told the San Francisco Examiner in 2021.

Stanford’s Field said fog, which is created by the contrast between cold water and warm air, has decreased because of warming coastal waters and warmer land evaporating the fog more quickly.

Fog is also the reason why some areas in San Francisco such as the Outer Sunset may be naturally chilly while some, like North Beach, are sunny. Field said fog is a “dramatic driver” of these microclimates in the city, which causes “dramatically different” climates even from block to block.

Even as extreme heat events become more frequent across the Bay Area, San Francisco has an advantage when it comes to hotter weather. Temperatures are still some of the lowest in the entire Bay Area, and the afternoon breeze in particular helps provide some relief. 

“I’m always struck when I hear the daily weather forecast for the San Francisco Bay Area, how the range of high temperatures in the Bay Area is comparable to what you’d expect to range of high temperatures across the entire country,” Field said. 

The coastal cooling in San Francisco happens particularly in the afternoons, which Field called a “summertime phenomenon.” 

“Different areas are experiencing climate change in different ways,” Field said. “Even in the Bay Area, some people might think ‘Oh, the weather is kind of boring.’ But who gets a strong afternoon breeze and who doesn’t, that can cause real relief in terms of temperature difference.” 

Additionally, Jan Null, a consulting meteorologist for Golden Gate Weather Services and former lead forecaster at the weather service, told SFGATE that the breeze is driven as temperatures warm up in interior parts of California. He said San Francisco is a “unique case” because the area gets cool from the ocean.

“We see these almost anecdotal periods where it gets warm, but then it flattens out and it goes back to average,” Null said. 

It’s difficult to predict how this summer will pan out, but San Francisco, along with the rest of the Bay Area, is expected to continue a long-term warming trend, experts said.

“There is good reason to expect hotter than average summers in general with more extreme heat days,” McEvoy said.

According to the National Oceanic and Atmospheric Administration’s Climate Prediction Center, some of the places with above-average temperatures forecast this summer include San Francisco and Northern California. As the city sees intensifying heat, it’s crucial to consider its the health effects, experts said.

“Heat is really a silent killer,” Samanta said. “It’s super important to be attuned to and be mindful of heat days.”

The city’s heat vulnerability study also urges residents to develop a heat preparedness plan for the extreme events and reduce greenhouse gas emissions to mitigate the effects of climate change, which drives the planet’s warming. (A list of San Francisco’s cooling centers can be found on the city’s website.)

Cordero said nationwide, government officials should be allocating more resources to weather services as temperatures rise.

“We should be putting more funding into this, because it really affects our day-to-day life,” he said. “We want to do things like planning, and if you want to forecast a hurricane or you want to forecast a heat wave, if you want to be prepared for this changing weather, you need good scientists.”