FRESNO (AP) — Some fog creeps in poetically on little cat feet through the Golden Gate. And then there is the prosaic fog of California’s San Joaquin Valley that erupts from the soil in certain wintry conditions to put a stranglehold on the region.

This dense tule fog materializes in a patchwork an average of 35 days each winter on mornings when cold mountain air sinks to the valley’s lowest areas after a rain. On these gray days, visibility is less than 1,300 feet — and sometimes zero, schools start late, workers who can telecommute, commerce grinds to halt, and those who have to go somewhere hug the white line marking the outside edge of the road.

“I could argue it’s the most dangerous kind of fog,” said Jim Andersen of the National Weather Service on Foggy Bottom Road in Hanford, southwest of Fresno. “I’d be hard pressed to figure out another location that gets as bad as this. This is the most dangerous place when it’s fog season.”

Four people died in late November during the first big fog of the season: in one accident, a 90-year-old man was broadsided near Kingsburg when he thought an intersection was clear; in the other incident, a car with three people was struck head-on near Chowchilla by a big-rig when the driver swerved to avoid a minor accident.

Other parts of California get fog, and areas around the Sacramento Delta even get the dense, low tule fog named for the tule reeds along the valley waterways where early settlers first noticed it forms. But experts say that the most dangerous in the state occurs along a 12-mile stretch of Highway 99 south of Fresno. It’s often so foggy that tail lights are impossible to see until it’s too late to react.

In 2002, a crash there on a zero visibility morning sandwiched 81 cars and six tractor-trailer trucks in less than two minutes, killing two. After a 2007 morning pileup involving 108 cars and big rigs killed two, injured 100 more and closed the major north-south transportation artery for half of the day, Caltrans and the Highway Patrol began planning for a warning system.

Now as safety experts brace for December and January, the foggiest months of the year, they knock on their laminate desks when they say that since 2010 the intricate system of six weather stations, 12 cameras, 39 electronic message signs and 41 microwave sensors installed along the dangerous corridor has worked to help avert the deadly pileups.

“The intent was never to stop 100 percent of the accidents. The hope was that 100-car pileups would go to 40 and 40 to 20 and so on. The point was to reduce the numbers,” said Sergio Venegas, the engineer in charge of what he calls the state’s most sophisticated fog monitoring system.

Twenty-two sensors connected to a command center can determine the density of the fog, and others in the roadway tell the speed in each lane of traffic on the highway that sees 100,000 vehicles a day. If traffic suddenly slows, warning signs along the highway instantly alert drivers to slow for foggy conditions ahead, the only system this advanced in the state. Engineers can watch what’s happening in real time on dozens of monitors.

At least that’s what they do when they’re operational. This year state officials were delayed two weeks in firing up the system for the current fog season because copper wire thieves had stolen key parts. On Wednesday a couple of transmitters were not operating, and Venegas fears thieves are responsible for that, too.

Fog and the valley are so intertwined that billboards for roadside restaurants advertise pea soup to drivers and tout that it’s as thick as the fog, which also might serve as another warning for drivers across the foggy valley.

Highway 99 from Merced south to Delano is built along one of the lowest spots in the valley, which is why this significant highway is so notorious for tule fog when conditions are right, says Anderson the meteorologist. The soils that make the region so productive for farming retain moisture after rains. When a high pressure system develops in the valley ringed by towering mountains, it works like a lid to trap cooler air at the ground at night. The resulting temperature inversion allows the fog to form from the soil’s moisture and often catches drivers by surprise.

In 1985, there was a record 16 consecutive days of dense tule fog in Fresno.

“You’re going along with good visibility and get lulled into a false sense of security then suddenly you hit a pocket and, wham, the transition is dangerous,” Anderson said.