In mid-August 2020, a powerful heat dome settled over California and set off a chain of disasters that lasted for weeks. Inland Bay Area cities broke heat records, and the state's power grid ran short, leading to its first rolling blackouts in nearly 20 years. Then a burst of dry lightning started hundreds of wildfires across Northern California. Three of those fires grew into some of the largest the Bay Area had ever seen, and their smoke hung over the region into September. A second heat wave over Labor Day weekend and the orange sky of September 9 followed. Together, these events made 2020 a turning point in how the region thinks about extreme heat.
The Meteorology of the 2020 Heat Dome
The August heat began when a strong ridge of high pressure built over the Great Basin and the Southwest and spread west over California. Under high pressure, air sinks and warms as it is squeezed. The ridge was unusually strong and slow to move, so the weather systems that normally end heat waves were held off for days. The marine layer, which usually cools the coast and sends cool air inland each afternoon, shrank to a thin layer along the shore. Without that daily cooling, inland temperatures climbed day after day, and nights stayed warm.
The heat peaked from August 14 to 19, when San Jose reached 103°F on August 16 and beat its previous record for the date by five degrees. Livermore hit 108°F the next day, and many other inland towns broke or tied daily records. Farther south, Death Valley reached 130°F on August 16, among the highest temperatures ever reliably measured on Earth.
Power ran short as well. On the evenings of August 14 and 15, demand for air conditioning stayed high just as solar power faded with the setting sun. Neighboring states were short of power in the same heat, so California could not import enough. The state's grid operator ordered utilities to cut power in rotating blocks, the first rolling blackouts in California since the energy crisis of 2001. Hundreds of thousands of homes and businesses lost power, including many in the Bay Area.
The Lightning Siege
On August 15 and 16, moisture from the remains of Tropical Storm Fausto drifted north over the overheated air. The combination of hot air near the ground and moist air above set off thunderstorms across Northern California, including over the Bay Area. Most of the rain from these storms evaporated before it reached the ground, but their lightning did not. This kind of storm is called dry lightning, and it is especially dangerous in late summer, when the hills are tinder dry.
For many Bay Area residents, the storms were the strangest part of the week. Thunderstorms are rare here, and lightning before dawn is rarer still. In the early hours of August 16, people across the region woke to flashes and rumbling thunder. The storms went on for hours but brought only a few drops of rain. By morning, new fires were already burning in the hills around the bay.
Over three days, more than 10,000 lightning strikes hit Northern California and started more than 500 fires. Firefighters were stretched far beyond their resources, and many fires burned for days with little or no crew working on them. The outbreak soon became known as the Lightning Siege.
Three groups of fires around the Bay Area grew into huge complexes. The SCU Lightning Complex, in the hills east of San Jose, burned more than 396,000 acres. The LNU Lightning Complex in the North Bay burned more than 363,000 acres, destroyed homes near Vacaville and forced evacuations across Napa, Sonoma and Solano counties. The CZU Lightning Complex in the Santa Cruz Mountains burned about 86,000 acres. It destroyed more than 900 homes, forced more than 75,000 people to evacuate and damaged the old-growth redwoods of Big Basin Redwoods State Park.
Smoke from the fires spread across the region for weeks. Air quality swung between moderate and unhealthy from day to day. Schools and businesses already strained by the pandemic struggled to keep indoor air clean, and many people stayed inside with the windows shut.
Labor Day Heat and the Orange Sky
The heat came back over Labor Day weekend, with the fires still burning. On September 6, downtown San Francisco reached 100°F, smashing the record for the date by five degrees, while Oakland hit 102°F and Gilroy 112°F. Three days later, on September 9, smoke from huge fires across Northern California and Oregon spread above the marine layer. The Bay Area woke to a dark orange sky, and streetlights stayed on through the middle of the day.
The orange light came from the way smoke filters sunlight. The thick smoke layer scattered and absorbed most of the blue light, leaving mainly red and orange to reach the ground. Because the smoke sat above the cool marine layer, the air near the ground stayed cleaner than the sky suggested. Many monitors showed only moderate readings that morning. Photos of the Golden Gate Bridge and the downtown skyline under that sky spread around the world.
Lessons and Legacy
The weeks from mid-August to mid-September changed how California prepares for extreme heat. State energy officials ordered more backup power and battery storage so the grid could handle hot evenings, when solar output drops. Fire agencies and utilities put more effort into cutting brush, burning dead fuel under controlled conditions and shutting off power when fire weather is extreme. Cities and counties also expanded cooling centers and heat warnings.
The deeper lesson is about how disasters stack up. The heat alone would have been serious, and so would the fires, the smoke or the power shortages on their own. Together, they overwhelmed many of the systems people rely on in an emergency. Climate scientists call this a compound extreme event, in which several hazards strike at once and do far more harm together than any one would do alone. The summer of 2020 has become a reference case for that kind of event in California.
