On most summer days, the Bay Area stays cool thanks to the marine layer and the afternoon sea breeze. Cool ocean air flows inland every afternoon, and even the hottest inland valleys usually top out in the 90s. Every few years, that protection breaks down under a heat dome. The marine layer shrinks or vanishes, the sea breeze fades, and the whole region heats up at once. Livermore can climb past 110°F, and San Francisco, where summer afternoons usually stay in the 60s, can top 100°F. These events have a specific cause, and understanding it explains why they feel so unusual here.
What Is a Heat Dome?
A heat dome is a strong, slow-moving area of high pressure high in the atmosphere that parks over a region for days. Under high pressure, air sinks toward the ground. Sinking air is squeezed by the heavier air below it, and that compression warms it by about 5.4°F for every 1,000 feet it descends. The sinking air also keeps clouds from forming, so the sun beats down on the ground from a clear sky.
Because the pattern lasts for days, the heat builds on itself. Each afternoon the ground absorbs more sunlight, and each night less heat escapes because the air above stays warm. Temperatures rise a little more every day until the pattern breaks. The high pressure acts like a lid on a pot, holding the hot air in place over the region.
A Bay Area heat dome begins when the ridge of high pressure builds over California or the Great Basin. It pushes aside the usual flow of cool air from the ocean. The sinking air squashes the marine layer until it is too thin to reach far inland, and the sea breeze weakens or disappears. Sometimes the wind even turns around and blows from the hot interior toward the coast. When those offshore winds are strong and dry, they are known as Diablo winds, and they can drive the humidity down into the teens or lower.
Bay Area Heat Dome Characteristics
The most extreme heat lands on the inland valleys. Livermore, Concord and Walnut Creek can reach 105°F to 115°F in the worst events, and Livermore hit 116°F in September 2022. The coast heats up too, which is what makes these events feel so strange. San Francisco reached 106°F on September 1, 2017, its hottest day on record, at a time of year when its normal high is near 70.
Heat domes are especially risky here because the region is not built for heat. Many homes, especially near the coast, have no air conditioning, since most summers never require it. Older buildings are designed to hold warmth in, which helps on foggy days but traps heat during a long hot spell. When temperatures stay near or above 100°F for several days, homes can stay dangerously hot overnight, and the power grid strains as demand for cooling jumps.
The people most at risk include older adults, young children, people with health problems and people without housing. California heat waves have proven deadly before, including in July 2006. During that heat wave, official counts put the statewide death toll at more than 140. Later studies linked several hundred more deaths to the heat, most of them in the Central Valley. Cooling centers, checking on neighbors and drinking plenty of water all help reduce the toll. The Bay Area heat wave guide covers what to expect in each part of the region.
A heat dome rarely arrives by surprise, because forecast models usually show the ridge building five to seven days ahead. The National Weather Service issues a Heat Advisory when hot weather could harm sensitive groups. It issues an Extreme Heat Warning when the heat becomes dangerous for everyone. When one of these is posted, plan outdoor work and exercise for early morning, close blinds during the day and open windows once the evening air cools.
Heat Domes and Wildfire
A heat dome also raises wildfire danger, because days of heat and low humidity dry out grass, brush and trees. The August 2020 heat dome showed how quickly that can turn into disaster. Moisture from the remains of Tropical Storm Fausto drifted north over the hot, dry air and produced thunderstorms that brought lightning but little rain. Over a few days in mid-August, thousands of lightning strikes started more than 500 fires across Northern California.
Several of those fires grew into the SCU, LNU and CZU Lightning Complexes, which burned across the hills around the Bay Area. Their smoke gave the region some of the worst air quality in its history. Weeks later, on September 9, the smoke from these and other fires turned Bay Area skies a deep orange.
The Bay Area's most destructive fires usually need one more ingredient, which is strong offshore wind. The 1991 Oakland Hills Fire and the October 2017 Tubbs Fire in Santa Rosa and Atlas Fire in Napa were all driven by Diablo winds in the fall. Those events were not heat domes in the summer sense, but they combined warm, very dry air with gusty winds that spread fire faster than crews could respond. When a fall heat spell and Diablo winds arrive together, fire danger in the Bay Area is at its peak.
Deep Dive: Why Does a Heat Dome Get Hotter Each Day?
A heat dome builds on itself because both the ground and the air remember the previous day's heat. Sinking air under the ridge keeps the sky clear, drying soil stops cooling the surface and warm air aloft never mixes away at night. Each morning starts warmer than the last.
How Do Forecasters See the Ridge?
Forecasters track heat domes on maps of the 500-millibar level, the height where air pressure is about half its sea-level value, roughly 18,000 feet up. Warm air takes up more room than cool air, so a warm column pushes that level higher. The tallest bulges on those maps mark the ridges where the hottest air sits.
Under a strong ridge, air high in the atmosphere slowly converges and sinks. It warms by compression as it comes down, which keeps clouds from forming and lets full sunshine reach the ground day after day. The sinking also presses down the marine layer along the coast, so the sea breeze that normally cools the Bay Area weakens.
Why Does the Heat Build Day After Day?
A 2014 study in Nature Geoscience by Diego Miralles and colleagues explained how the worst heat waves build up. They studied the 2003 European and 2010 Russian heat waves using weather balloons, satellites and models. Daytime heat came from warm air carried in from elsewhere, from ever-drier ground and from warm air mixed down from above.
As the soil dried out, less of the sun's energy went into evaporating water, so more went into heating the air. The team also found that the day's heat did not escape overnight. It stayed stored in a layer of warm air kilometers deep, starting several hundred meters above the ground. The next morning, the growing daytime mix tapped that stored heat again.
California's dry summers make that feedback strong, because by late summer most of the inland hills have little water left to evaporate. That helps explain why the worst Bay Area heat waves often come in late summer and early fall. During the September 2022 heat wave, downtown Sacramento reached 116°F on September 6, its hottest temperature on record, and Livermore hit 116°F on two days in a row.
Heat Dome Frequency and Climate Change
Heat domes have always happened in California, but a warming climate is making the hottest ones hotter. The June 2021 heat dome in the Pacific Northwest pushed Portland to 116°F and shattered records across the region. Climate scientists concluded that heat that extreme would have been virtually impossible without human-caused warming.
For the Bay Area, the marine layer still offers strong protection on most summer days, but that protection has limits. As the background climate warms, each heat dome starts from a slightly higher baseline, so the hottest days can climb above what the region has seen before. Shaded spaces, cooling centers and homes that stay cool at night are becoming part of life, even in one of the mildest climates in the country.
