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Heavy fog rolls past the Golden Gate Bridge and San Francisco skyline.

Salesforce Tower puts on a light show as the fog returns after a brief storm in San Francisco.

The recent heat wave could have been much worse for San Francisco, but it was spared because of a familiar phenomenon: fog. The Bay Area’s coastal fog machine is so intense during much of the summer that residents often refer to this month as “Fogust.”

San Francisco’s famously chilly summers are caused by sharp temperature differences that trigger rapid condensation along the coast — a.k.a. fog. The Bay Area can see temperatures up to 30 degrees lower than the nearby Sacramento Valley. This gradient fuels San Francisco’s summer fog production.

Hot air from the Sacramento Valley is pushed away from the coast and rises into the atmosphere, leaving a gap. Cold air from the Pacific then rushes in to fill that void, driving the natural AC felt in the Bay Area. The sun heats up this leftover air, and the cycle repeats.

Foggy skies and a persistent sea breeze shield the coast from summer heat, while the veil fades and temperatures increase farther inland.

But the weather is about to change.

Higher temps and less moisture in the air are on tap next month. And it’s all because of a shift of the winds.

Starting in September, the Diablo winds will sweep into the Bay Area, bringing blistering hot air from the Sacramento Valley. The cool blanket of Pacific air that’s made for a mild summer will be swapped out.

What drives this cycle? Big picture: It’s winds moving, cooling and heating the air that travels around California.

But how does so much cold air from the Pacific Ocean rush back to the coast in time to complete the feedback loop? It’s all made possible by a weather system called the Pacific High.

The Bay Area’s coastal fog machine brings plenty of clouds to the region during the summer, but a change in the winds means the cool, moist weather is about to change.

The clockwise flow of the Pacific High drives winds 5,000 feet above California. That flow brings south winds to the Bay Area that drive thunderstorms during the summer months. It also propels cold air from the Pacific Ocean back to the coast, keeping the cycle of fog production going.

But this entire pattern begins to switch around Labor Day, when the Pacific High starts to move south for the winter.

The Pacific High’s new position means that its clockwise flow suddenly reverses, swapping the southern winds with northern winds. These are the Diablo winds, and they’re generally warmed and dried by the Sacramento Valley as they make their final approach from the Diablo Range into the Bay Area.

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So, instead of hot air being pushed away from the region, the Diablo winds do the opposite. It’s no wonder that San Francisco’s hottest month is September, and that we see some of the most destructive fires ignite around this time.

This year, we’re in better shape compared with some recent summers. In recent years, Napa, for example, has seen almost no rain in June and July. This year, the county saw 0.27 inches of rain, placing it above average for the first time in the 2020s. This year’s fog has also been incredibly healthy along the Pacific coast and San Francisco Bay, making for a decent amount of drizzle in June and July.

Fog season tends to last until the end of August, and the Diablo winds usually begin wafting into the Bay Area around Labor Day. The latest projections from the European and American weather models are all leaning toward this normal transition between the two seasons.

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We should see balmy temps in the 70s linger in the San Francisco Peninsula and neighboring communities through much of September and October, while the Oakland hills and Santa Cruz Mountains will occasionally climb into the 80s.

And while the extended weather models don’t see any anomalies that would suggest extremely above-average temperatures just yet, we know from recent years that heat waves in September and October aren’t out of the question.

But thanks to this summer’s robust Fogust moisture and the normal transition between summer and fall, the Diablo winds are going to be met by a more heat- and fire-resilient Bay Area.

Heat wave and fire concerns will generally begin to decrease by November. The Sacramento Valley will finally cool off, and the Diablo winds will begin to funnel cooler air into the Bay Area in time for Thanksgiving.

Gerry Díaz is The San Francisco Chronicle’s newsroom meteorologist. Email: gerry.diaz@sfchronicle.com Twitter: @geravitywave

Gerry Díaz is The San Francisco Chronicle's first ever Newsroom Meteorologist.

He previously served as a meteorologist for the National Weather Service Bay Area, working from the agency's Monterey office. In that position he led an initiative aimed at increasing outreach to Spanish speaking communities during extreme weather events.

Most recently he worked as a meteorology specialist for utility Southern California Edison.

Díaz enjoys hiking through California's national parks and shooting panoramic photography, including of the Central Coast.