Why San Francisco’s Hottest Day of the Year Hasn’t Happened Yet

While much of California experiences temperature spikes during the summer months, San Francisco typically has its hottest day of the year in the fall, according to decades of climate data.

Most of the United States experiences its hottest day in July or August. But atmospheric circulation patterns and the Pacific Ocean change the timeline later for San Francisco and other coastal communities in California.

This map shows when the hottest day of the year occurs across California, with later dates shown in darker shades of orange and red. The data comes from the U.S. Climate Normals 1991 to 2020, a record of the country’s typical climatic conditions over a 30-year period, including daily maximum temperatures. In locations where multiple days shared the highest temperature, a date in the center of the range was selected.

The hottest day of the year in San Francisco is usually October and around 70 degrees Fahrenheit – mild compared to other nearby areas and the state as a whole. The city has seen highs in the 80s to 100s in recent years, but this 70-degree high is based on a 30-year average.

People and pets gather on a hot afternoon at Crane Cove Park in <a class=San Francisco in 2021.”/>

People and pets gather on a hot afternoon at Crane Cove Park in San Francisco in 2021.

Gabrielle Lurie/The Chronicle 2021

Localities farther from the coast generally experience higher annual peaks. Their hottest days are also earlier. Fresno, for example, normally sees high temperatures near 100 degrees Fahrenheit at the end of July.

Likewise, most places in the continental United States have their hottest days in the summer, when solar heating is highest in the northern hemisphere. Along the west coast, however, other factors come into play.

A high atmospheric pressure system in the northeast Pacific Ocean – the Pacific High – moves air towards North America. Winds are pushing water up off the coast of Northern California, bringing cold water from lower depths to the surface.

“We have all this cool water – the air in there is getting colder,” said Jan Null, a meteorologist with Golden Gate Weather Services. The air is so cool that the moisture condenses to form water droplets, producing the clouds and fog that engulf San Francisco during “Fogust”.

This marine layer acts as a form of natural air conditioning.

Downtown San Francisco shrouded in a blanket of fog in September 2021.

Downtown San Francisco shrouded in a blanket of fog in September 2021.

Stephen Lam/The Chronicle 2021

“It brings all this nice, cool air along the coast,” Null said. “The further inland you go, the less of that impact.”

Low clouds provide additional cooling power by keeping sunlight at bay.

“Cool ocean helps support these types of clouds, and then these types of clouds help keep it cool,” said Rachel Clemesha, a climatologist at UC San Diego‘s Scripps Institution of Oceanography. “There are positive feedbacks.”

But this cooling becomes less powerful in September and October when the Pacific High moves south. High pressure is also developing over the Great Basin, an area east of the Sierra.

This movement causes the air to push the marine layer away from the coast.

“You turn off the air conditioning, it starts to warm up,” Null said.

San Francisco surfers Ramsey Cook and Kristen Abberley rest at sunset at Rodeo Beach in Sausalito in 2020.

San Francisco surfers Ramsey Cook and Kristen Abberley rest at sunset at Rodeo Beach in Sausalito in 2020.

Scott Strazzante/The Chronicle 2020

This high pressure also produces the Santa Ana and Diablo winds, which bring additional heat as they descend into coastal areas in the fall. This hot air increases the risk of destructive wildfires.

Scientists have linked global warming to the frequency and intensity of consecutive hot days – or heat waves. But how climate change affects specific weather activity, such as global changes in cloud cover, remains an area of ​​active research.

“We’re still trying to figure it out,” Clemesha said.


Jack Lee is a writer for the San Francisco Chronicle. Email: jack.lee@sfchronicle.com

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