California authorities have declared that the Thomas wildfire, which has been raging for two weeks in Santa Barbara and Ventura counties, is 45 percent contained.

Is that reassuring or horrifying? It’s a question the state Department of Forestry and Fire Protection just can’t answer.

For the agency known as Cal Fire, the comfort level of the official stages of control is always enigmatic, never more so than in this furious season and with the Thomas conflagration in particular. Santa Ana and sundowner winds, at times exceeding 70 miles per hour, have fanned flames across some 270,000 acres. A Cal Fire engineer died last week battling the firestorm.

“These fires this year have so much potential,” said Scott McLean, a Cal Fire spokesman whose son is a firefighter on duty in the Thomas sector. “It’s just so volatile.”

In Cal Fire terminology, 45 percent containment means that about that much of a hot zone is penned in by physical barriers, either roads, waterways or bulldozed or hand-shoveled clearings.

“The problem is, when winds shift, the line of containment can be breached and embers can create spot fires,” said Max Moritz, a fire specialist for the University of California Cooperative Extension. “That’s what we see with these wind-driven events.”

The dry, hot, powerful Santa Anas — also called devil winds — sweep in from inland deserts regions this time of year. The sundowners arrive in late afternoon or evening, coming from the north. Both phenomena have helped fuel other wildfires in Southern California this month.

“These extreme wind events really challenge most of the norms,” said Moritz, who had to evacuate his home in Santa Barbara. “You can take any tiny bit of ember and whip it back into a real fire.”

A wildfire is considered “out” when it’s fully contained and no hot spots are detected for at least 48 hours, according to the National Wildfire Coordinating Group, which includes federal agencies such as the Interior Department and Forest Service.

For the Thomas, the anticipated date for complete containment is Jan. 7, or 20 days from now. The cause hasn’t been determined.

Containment contours are drawn on the front lines. Firefighters are assigned to branches that are responsible for sections of a blaze — what they call a “span of control” — and within those branches are smaller divisions. Every afternoon, by 3 p.m., each one reports by radio, email or text exactly how much of its area it can confidently say is contained. The reports are penciled onto a map, and the tally is posted on the Cal Fire website.

The containment level may change by a percentage point from one day to the next. McLean said the agency is “very stingy” when it comes to moving the figure up a notch. The slightest ascent, though, is encouraging. “It’s a glimmer of hope.”

The men and women in the field aren’t tracking just flames but also looking at vegetation that can act as tinder and assessing which way winds are blowing. Their aim is to determine what McLean called “the potential for re-ignition.”

Wind is the unnervingly unpredictable factor. Take the Tubbs fire, by far the most devastating of the wine country blazes in Northern California that all told destroyed hundreds of homes and killed more than 40 people in October. Gusts drove it this way and that. “That sucker blew into Calistoga, into Santa Rosa, in less than a day, and then it stopped spreading that way and went south and north from there,” McLean said.

He said he simply couldn’t give a percentage below 100 that should make people breathe easy. Even at 90 percent contained, “there’s always a chance that there could be a sleeper or an ember that goes across the line, and then we’re at it again.”

More than 8,000 firefighters have been deployed, with costs surpassing $123 million, according to Cal Fire. Late Sunday, the agency said 18,000 structures remained under threat; so far, more than 1,000 have been destroyed. Winds are expected to be calmer Monday.

But “we cannot become complacent,” McLean said. “Things can change in a heartbeat, especially with what we’ve been dealing with this year.”