What is tulle fog in California’s Central Valley and how does it form?
Explainer on the science, geography, and dangers of Tule Fog in California’s Central Valley
A massive chain-reaction crash involving as many as 150 vehicles closed Central California’s Interstate 99 in both directions Saturday morning, trapped drivers in near-zero visibility, once again blanketed the San Joaquin Valley with dense fog, and sent at least 10 people to area hospitals.
The California Highway Patrol began receiving panicked calls from motorists near Earlimart, Calif., shortly after 8:15 a.m., reporting multiple crashes in the northbound and southbound lanes near 24th Street. Officials said visibility at the time of the crash was 100 to 200 feet, creating what the CHP often describes as a “sudden white wall” phenomenon.
“The drivers encountered heavy fog and were unable to see the brake lights until it was too late,” authorities said.
Dozens of drivers who escaped injury were taken by bus to the Tulare Agriculture Center for evacuation and reunion. As of 1 p.m., Highway 99 remained completely closed to traffic, and authorities said the closure could last into the evening as officers searched the road and tow trucks worked on the path, which was filled with crumpled cars, SUVs and big rigs.
Here’s what you need to know about the tulle fog blanketing California’s Central Valley and the high number of accidents it causes this season.
Highway 99 detoured after Tulle Fog crash in Central California
Both northbound and southbound lanes of Highway 99 are closed in the Earlimart area near Avenue 24.
- Detour – Northbound: Traffic is being detoured on Cecil Avenue.
- Detour – Southbound: Traffic will be detoured on Avenue 48.
A season defined by fog: communities shrouded from November
Saturday’s disaster occurred during the longest and most stubborn Tule fog season the Central Valley has seen in years. Since just before Thanksgiving, millions of Californians from Redding to Bakersfield have been waking up to milky, stationary fog and fog, with the National Weather Service issuing advisories almost daily.
A fog advisory remained in effect for a large area of the San Joaquin Valley on both January 30 and January 31, including areas around Fresno, Visalia, Porterville, Delano, and Bakersfield, with visibility repeatedly dropping below a quarter of a mile and in some cases down to several feet.
A NASA satellite image taken on Dec. 9 shows the valley covered in a cloud of tulle fog stretching for more than 300 miles. This is the result of “perfect” atmospheric conditions that began forming around November 24th and lasted until late January.
According to residents, this year’s fog is unusual.
“It’s never lasted this long in my 57 years here,” Greg Clark of Redding told Record Searchlight.
Meteorologists attribute this unusually persistent layer to a stationary high-pressure system. This high pressure traps cold, humid air close to the ground, preventing daily “burn-off” when the fog often clears by mid-morning.
Other recent fog-related crashes across the Central Valley
The Jan. 31 crash is the biggest so far this winter, but it’s not the only one.
Law enforcement agencies say they responded to dozens of fog-related crashes in January alone, especially during the early morning commute.
Here are some notable incidents around the Central Valley this month:
- A 17-vehicle crash occurred in Fresno County on January 12th. A 61-year-old man died as visibility decreased to 10 to 15 feet along Highway 99.
- Multiple crashes along Interstate 5 Heading north through the valley from Kern County, the National Weather Service said zero visibility conditions “present a very high traffic hazard.”
- Commercial truck collides on Routes 43 and 58 In mid-January, fatal crashes occurred near Highway 99 and Clovis Avenue in Fresno and Kings counties.
- Reduced visibility on Interstate 80 and surrounding roads. The Sacramento Valley experienced repeated fog overnight near river basins and agricultural corridors.
CHP officials say the pattern is clear. It’s an extremely dangerous combination of sudden and severe visibility loss, drivers going too fast for the conditions, and people getting out of their cars on fog-covered roads.
What is Tule Fog and why is it so dangerous?
Tule fog is a type of radiation fog that is formed by a combination of moist ground, cool nights, clear skies, and calm winds. This situation is common in the Central Valley from November through February.
According to forecasters, the fog on January 31st was particularly dangerous:
- High pressure kept the fog fixed on the surface.
- The wind was relatively calm all night
- Cooling overnight deepens the atmospheric “lid” and prevents dispersion.
- Visibility changed from 1/4 mile to tens of feet within seconds
“This is a classic tulle fog setup,” said NWS meteorologist Eric Kurth. “It’s a big bowl full of mist.”
Safety tips: How to drive in fog
For millions of Valley residents, avoiding fog entirely isn’t always an option. But safety officials say good habits can save lives.
If you need to drive:
Please slow down. In dense fog, expect travel times to double or triple.
Use low beam headlights. High beams reflect off water droplets and reduce visibility.
Follow lane lines, not taillights. Looking at road signs can help you stay oriented.
Significantly increases the following distance. Give them a few seconds of reaction time.
When will the fog clear?
Unfortunately, forecasters say the fog cycle is not over yet for the Central Valley.
- Turn on the hazard lights
- Park in a parking lot or rest area if possible
- If you don’t have a safe exit: Pull over to the shoulder of the road as much as possible, apply the brakes, keep your hazard lights on, and take your foot off the brake pedal to avoid pulling over drivers who might mistake your lights for lane markers.
- NOAA Fog Driving Guide: https://www.weather.gov/safety/fog
- Live map of CHP/Caltrans highways: https://quickmap.dot.ca.gov
Mild, mostly dry weather is expected to continue into the weekend, with highs in the mid-60s and lows in the mid-40s. Conditions may improve slightly by late next week as temperatures climb into the low 60s, but tulle fog may continue to develop overnight and into the early morning hours unless a more powerful storm system disturbs the stagnant air mass.
