Why is California so chilly this summer?

Observer

02-08-2025

San Francisco is seeing the coldest start to a summer in over two decades, and nobody has felt the chill more than the people tasked with painting the Golden Gate Bridge. Fred Mixon, paint superintendent for the bridge, said his team has used heaters on most days this summer to warm up the steel before applying the International Orange paint that defines the bridge's iconic vermilion hue. When the steel is cold, the paint doesn't adhere as well and can flake off after it dries.
'This is the first time in a long time that we've used the heaters in summer,' he said. 'We are using them most days this summer, whereas in prior summers we might use them for a day or two all season.' San Francisco, where 70F degrees can be considered a hot summer day, is known for its cool summers, but this year, June and July were even chillier than normal. The average afternoon high downtown was 63.7F degrees, 2.7F degrees below normal, making it the coldest since 1999. Because of the city's proximity to the Pacific Ocean and the blanket of fog that often hugs this part of the California coast, temperatures tend to stay cool there, even as inland areas roast in the hot sun and triple-digit heat. This summer, that nip in the air has been felt up and down the coastline of Northern and central California and into the Sacramento Valley.
Just an hour south of the city, San Jose saw its 10th coolest start to summer, with records going back more than 100 years. The average high was 76.9F degrees, compared with the normal 81.2F. In July, the average afternoon high in Napa, the epicenter of California's wine industry, was 5 degrees below normal. Vintners noticed it, as the grapes in the Napa Valley softened and changed color later this summer than in recent years. This stage in grape growing is visual and easily observed, with red grapes going from green to purple, and white grapes from greenish to golden. 'Often we'll start to see this in early July around Independence Day,' said Lorenzo Trefethen, third-generation vintner of Trefethen Family Vineyards. 'This year we didn't start seeing it until two weeks later.'
The story was the same in Sacramento, which saw only one day above 100 last month. In 2024, the city had 21 days of 100-plus degree heat in July. Michael Nielson, who oversees the 40-acre garden surrounding the state Capitol in Sacramento and other gardens in the capital, said the cooler weather has meant he has been using less water, and flowers have been blooming longer, especially in the rose garden, with its 750 plants. 'Last year, new buds would start to break and the color and the petals would fade and start to crisp three times as fast, even scorching the blooms,' Nielson said. 'We're not seeing the leaf burn and the blossoms drying and losing their color faster this year.'
California is a vast state, and the climate varies widely from region to region in the summer, but generally the coastal areas are mild and dry in the summer, with frequent morning fog and afternoon sunshine. Inland areas, across the Central Valley and into the mountains and desert areas, are dry and warm, often blazing hot. The warming usually comes from a seasonal area of high pressure that develops in the Four Corners region — where the state lines of Utah, Colorado, New Mexico and Arizona meet — and builds into California. That high pressure brought some warmth to Southern California in June, but has mostly been absent in July.
This summer, the more dominant weather feature has been a low-pressure system sitting off the coast. That system 'has been pulling cool air into our region for much of the summer,' said Brian Garcia, a meteorologist with the National Weather Service office in Monterey, California. While a warming trend is expected in California next week, Garcia said he expected the influence from the low pressure, and the cooler temperatures that accompany it, to continue in August.
One morning toward the end of July in San Francisco, a rare summer rain fell, and people walked around the city's Financial District with puffy jackets and umbrellas. This sort of weather can be startling to visitors from the East Coast. Regina Spektor, a musical artist who grew up in New York City, performed at the Masonic Auditorium in San Francisco that night. 'Happy summer,' she said when she greeted the audience, with air quotes around the word 'summer.'