On the night of October 8, 2017, winds gusting to 79 mph tore through the North Bay hills north of San Francisco. By morning, the Tubbs Fire had burned through most of the Coffey Park neighborhood in Santa Rosa, and the Atlas Fire was burning east of Napa. The Wine Country fires went on to kill 44 people and destroy nearly 9,000 structures. Many residents woke to flames at their windows and had only minutes to escape. Nothing about that night was truly unusual, because the Diablo winds were doing what they do every fall, only at full strength.
Diablo winds are the most dangerous recurring weather event in the Bay Area. They are not freak occurrences, and they return every fall with conditions in which a single spark can become a regional disaster. Anyone who lives in the Bay Area hills or wildland edges should know what these winds are and when they arrive.
What Are Diablo Winds?
Diablo winds are hot, dry winds that blow downhill from California's high interior toward the Bay Area coast. They begin when strong high pressure builds over the Great Basin and the Sierra Nevada, creating a pressure difference with the lower coastal zone. Air flows from high pressure toward low pressure, which here means west and downhill from the interior toward the bay and the coast. As the air sinks from several thousand feet toward sea level, it is compressed, which heats it and dries it out.
The name comes from Mount Diablo, the tall peak in Contra Costa County in the region these winds affect. They are a northern cousin of Southern California's better-known Santa Ana winds. Both are offshore winds that blow downhill, but Diablo events tend to be shorter and more concentrated in the North Bay and East Bay hills. The winds are strongest on exposed ridges and in passes and canyons that speed up the flow. Examples include Altamont Pass, the Carquinez Strait and the crests of the Diablo Range.
When Do Diablo Winds Occur?
Peak Diablo wind season runs from September through November, and October and early November are the most dangerous weeks. The timing is not a coincidence, because it lines up the strongest winds with the driest vegetation of the year. By October, California has usually gone five to seven months without meaningful rain. Grasses and brush are fully dried out and can catch from a single ember. Moisture in the hillside vegetation is at its lowest point of the year.
At the same time, fall brings the season's first strong high-pressure systems over Nevada and the Great Basin as the weather pattern shifts after summer. Those highs build the inland-to-coast pressure difference that drives Diablo events. Extreme fuel dryness and strong wind together create fire weather unmatched at any other time of year. Diablo winds can also blow in winter and spring, but green vegetation makes the fire risk much lower then.
The Physics: Why These Winds Are So Hot and Dry
The heat and dryness of a Diablo wind do not come simply from air leaving a hot place. Air that sinks to lower elevations is squeezed by the rising air pressure around it, and squeezed air heats up. This process, called adiabatic compression, warms dry sinking air by about 5.5°F for every 1,000 feet it drops. Air sinking from 7,000 feet to sea level gains nearly 40°F from compression alone, before any other heat is added.
The drying is just as dramatic as the heating. Relative humidity measures how much water vapor the air holds compared with the most it could hold at its temperature. Warm air can hold far more vapor than cold air, so relative humidity falls as the air heats, even when its actual moisture stays the same. Air that leaves the high interior at a moderate humidity can arrive near sea level in single digits. It has not lost much water, but it has warmed so much that its moisture is now a tiny share of what it could hold. That air pulls moisture out of plants, soil and skin within hours.
How Diablo Winds Fuel Wildfires
Wildfires behave very differently under Diablo winds than on normal days, for three reasons. First, humidity below 10 percent pulls the last moisture out of plants that were already at their driest. Second, gusts of 50 to 60 mph carry embers a quarter mile or more ahead of the fire, starting new fires in front of the main blaze. A fire that moves at a walking pace on a calm day can outrun a person in strong wind. Third, the same winds make fighting the fire much harder and more dangerous. Aircraft are often grounded in strong gusts, crews cannot work safely downwind of the flames, and wind scatters retardant before it lands.
When very dry fuels, gusty winds and humidity in the teens or lower come together, a fire can double in size every few hours. Diablo wind events often bring all three at once. In 2017, the Tubbs Fire ran about 12 miles in its first few hours, from the hills near Calistoga into Santa Rosa. That pace leaves almost no time for an orderly evacuation unless residents are already prepared to go.
Famous Bay Area Fires Driven by Diablo Winds
Many of the Bay Area's worst wildfires were Diablo wind fires. The 1991 Oakland Hills Firestorm killed 25 people and destroyed 2,843 single-family homes and 437 apartment and condominium units in the hills above Oakland and Berkeley. Hot, dry northeast winds gusting to about 65 mph drove the fire so fast that some residents could not escape on foot. Whole neighborhoods burned in the time it takes to pack a car. It remains one of the deadliest urban wildfires in California history.
The October 2017 Wine Country fires were the most destructive Diablo wind event on record. The Tubbs, Atlas and Nuns fires ignited within hours of each other on the night of October 8, with gusts of 65 to 79 mph on the ridges. The Tubbs Fire destroyed more than 5,600 structures in Sonoma County, making it the most destructive fire in California history until the Camp Fire a year later. In late September 2020, the Glass Fire, driven by the same kind of winds, burned through Napa and Sonoma communities that were still recovering from 2017. The pattern repeats each fall, with dry offshore winds turning ordinary fires into disasters.
None of these fires needed an unusual spark to start. Most began from ordinary causes such as power lines or equipment, and a few causes were never firmly established. What made them catastrophic was the condition of the land and the air when the spark landed.
Deep Dive: What Makes Diablo Winds So Strong and So Dry?
A Diablo wind combines a large pressure difference with the shape of the land. High pressure over the Great Basin pushes air toward lower pressure off the California coast. As that air pours over the Bay Area hills, it speeds up on the downhill side and dries out as it warms.
Why Does the Wind Speed Up on the Downhill Side?
When stable air flows over a ridge, it can set up waves in the atmosphere downwind, much like water flowing over a rock in a stream. Under the right conditions, those mountain waves push fast air from higher up down onto the slopes. A 2018 San Jose State University thesis by Bowers found that Diablo winds combine small downslope winds with these larger mountain waves interacting with the terrain.
That is why the strongest gusts hit ridgetops and the upper slopes on the downwind side, while nearby valleys can stay much calmer. A University of Washington study of Diablo winds also found large differences from place to place, tied to exposure, elevation and mountain-wave acceleration. During the October 2017 North Bay fires, KQED reported gusts of 96 mph and 108 mph on mountain peaks near Geyserville.
Timing makes the danger worse. Researchers at the Desert Research Institute, led by Craig Smith, built a climatology of these winds from 18 years of weather station data in 2018. They found that Diablo-like winds tend to blow overnight and into the early morning, in fall, winter and spring. The Tubbs Fire started on a Sunday night in October 2017 and spread while many people slept.
How Dry Does the Sinking Air Get?
The air itself does not lose much water as it sinks. What changes is how much water it could hold, which rises steeply as it warms. Relative humidity is the ratio of the two, so it falls fast.
By our calculation, consider air at 5,000 feet over the interior, at 45°F with a dew point of 10°F. Its relative humidity is about 24 percent. If it sinks to sea level, compression warms it by about 27°F to around 72°F, and its dew point rises only a few degrees. Its relative humidity falls to about 11 percent, less than half of where it started.
Grass and twigs swap moisture with the air around them within an hour or two, so they dry out almost as fast as the humidity falls. Fire scientists call these the fastest-drying fuels. On a long Diablo wind night, they become ready to burn by morning.
How to Stay Safe During Diablo Wind Events
The National Weather Service issues Red Flag Warnings when very low humidity, strong gusty winds and critically dry fuels come together. A Red Flag Warning means conditions are in place for any fire to spread out of control. During a warning in September, October or November, treat it as the first stage of an emergency and be ready to leave.
Preparation starts with a go-bag packed before fire season begins, not after a warning is issued. Know two evacuation routes from your home, because a fire can close one before you reach it. Keep your phone and a portable battery charged, since utilities often shut off power during Diablo wind events to keep downed lines from starting fires. Sign up for your county's emergency alerts so evacuation warnings reach you directly. The hardest part is leaving as soon as the order comes, without waiting to see the flames. The 2017 fires spread faster than most people imagined, and residents who left immediately were the ones most likely to escape safely.
Diablo winds are a permanent part of the Bay Area's climate. Hotter summers are leaving fuels drier at the start of each fall wind season, so the fire seasons these winds drive may grow more dangerous. The region enjoys some of the best weather on the planet for most of the year. That bargain includes several weeks each fall when the weather turns dangerous, and the best protection is to be ready before those weeks arrive.
