How a combination of a wet winter and a dry summer have fueled constant wildfires in California. (Claritza Jimenez/The Washington Post)

Crews are battling wildfires in Northern California against the force of unimaginable weather conditions this week. Humidity is basically nonexistent and wind gusts are peaking at 50 to 60 mph.

It’s hell on Earth for the firefighters and the tens of thousands of people who live in wine country, all because of a very specific fall weather pattern and scorch-prone ground conditions more than a year in the making.

In the history of California wildfires, this week’s are among the most destructive. By Tuesday morning, the Tubbs and Atlas fires, north of the San Francisco Bay area, had already destroyed more than 1,500 homes and commercial buildings, surpassing the fifth most-damaging fire on record in the state — the “Old Fire” in October 2003.

The most damaging fire on record in the counties affected this week — Napa, Sonoma and Lake — came just two years ago in 2015 with the Valley Fire, which destroyed 1,955 structures, according to Cal Fire.

Each year, thousands of acres and hundreds of structures — homes, business and farms — are burned in California fires.

Typically this season runs from spring to late fall, with October being one of the worst months, historically. This is because, unlike other parts of the United States that experience summertime thunderstorms and rain from tropical storms, the season tends to be dry in California. And wildfires need just three things to start and spread: an ignition source, fuel and dry weather.

“By the end of the summer and into early fall the state’s vegetation is tinder dry,” said Jan Null, a California meteorologist and owner of Golden Gate Weather. Then in the fall, the weather pattern flips to generate hot, dry winds that blast across the already-parched landscape.

“The bottom line is that the culmination of these patterns makes October a particularly tragic month for wildfires in California,” said Null.

High pressure over Great Basin

A big ridge of high pressure is lingering just east of California this month, and winds blow from high pressure to low pressure.

When a big high sets up over the Great Basin — which basically encompasses all of Nevada and half of Utah — air flows east to west in California, which just happens to be from high elevation to low elevation.

At this point, thermodynamics take over.


How hot, dry downslope winds form, like the Diablos and Santa Anas. (The Washington Post)

Downslope winds — Diablos and Santa Anas

When air travels from high elevation to low elevation, three things happen: It gets warmer, winds get faster and humidity plummets.

East-to-west winds sink along with the elevation as it travels from mountains to sea level. The sinking air is compressed due to a pressure increase — something you can feel when your ears pop on a drive out of the mountains.

As air is compressed, it heats up. This is the first law of thermodynamics at work.

The elevation of Reno, Nev., for example, is around 4,500 feet. The elevation of Napa is just 20 feet. That’s a lot of compression, thousands of feet, which leads to 20 to 30 degrees of temperature rise.

During the sinking process, the amount of moisture in the air isn’t changing much, but the temperature is still rising. This creates a large disparity between temperature and moisture, and pushes the relatively humidity very low.

The wind itself also gets faster as it’s compressed, forced over mountains and pushed through canyons. It’s why 70 mile per hour wind gusts are possible on sunny warm days, like the Bay Area experienced on Monday.

Lots of rain last winter = more wildfire fuel

This seems counterintuitive, since rain should suppress wildfires. But from October 2016 to May 2017, record amounts of rain fell in the northern Sierra Mountain region, prompting grass, brush and trees to grow like they haven’t been able to in decades.

The new growth is now serving as fuel for this fire season, made worse by five months in a row of extremely dry weather.

Five months of historically hot, dry weather

Like a pendulum swinging from one extreme to the other, a historic dry spell began after months of unprecedented rainfall.

“California’s Mediterranean climate makes it unique,” Null said. Unlike other regions, “our warmest months coincide with our driest.”

While that tends to be true each year, the contrast was even more stark this summer.

Beginning in May, precipitation all but ceased. Since early July, the region as seen less than 25 percent of its average rainfall. And in early September, new all-time temperature records were set in the Bay Area, which dried vegetation out further. San Francisco set a record in its 150 years of data when it hit 106 degrees on Sept. 1. All-time hot records were set in Salinas and King counties, too.

In all, it ended up being the state’s warmest summer on record.

Given these factors, combined, it may also be the worst wildfire year for Northern California. Until the larger weather patterns shift, conditions will remain ripe for extreme fire behavior.