The visibility in the Central Valley of California can decrease by a quarter-mile down to a few dozen feet within a few seconds, a change that transforms a typical commute into a chain-reaction accident. That sort of unexpected break in sight lines in the San Joaquin Valley near Earlimart preconditioned a huge multi-vehicle collision on Highway 99 where drivers testified to almost no visibility as car crashes piled up on either side of the road.
the movement is known in the winter months of the valley, at which the season was long years ago, but the length has been exceptionally obstinate. Those dwelling between Redding and Bakersfield have spoken of mornings, opening under the same milky lid of low cloud and then remaining so long after the hour at which the sun should have scalded it. “Never lasted this long in my 57 years here,” Greg Clark of Redding said.
The distance covered by what most motorists refer to as “fog” is wide. The common separation of mist and true fog is made by visibility meteorologists: mist can be seen miles, whereas fog can be drawn to within a mile, and in the worst case a yard or two, or to the length of a car. Beneath that mark a road is a maze of incomplete indications–paint on the lanes, reflectors, the dim light of a tail lamp–as the brain continues to attempt to fill in what it cannot validate. It is that imbalance that makes the experience of being in a tule fog feel abrupt to an individual inside the fog: the road may appear manageable until it is not, and one strong brake can spread backwards more quickly than the driver can comprehend what is occurring.
Tule fog is caused when moisture contained in the atmosphere close to the ground cools down to the extent of saturating the air, and the fine droplets are accumulated into a low-lying cloud. In the Central Valley it is exacerbated by geography: mountains border the basin in and peaceful nights invite pooling cold air to rest around the valley floor.
The tenacity of this season has also been based on a bigger atmospheric arrangement. This pattern of a high-pressure has been like a lid forcing the clouds to fall instead of vertical mixing dispersing moisture. Occasionally the valley has been under a shallow, wet, layer above which the higher country a short distance off is significantly warmer, a trait of temperature inversion that sometimes ensures the valley is chilly, wet and miserable.
Availability of moisture is also important. Following an abnormally wet autumn, soils in most of California contained sufficient water to make evaporation go back into the near-surface air. Climate scientist Daniel Swain observed that the amount of precipitation received during the period between September and November 2025 was in the top 10 percent in most of central and southern California, which is a useful component when it comes to forming repeated overnight fog, since winds remain light.
The repetitive pattern that occurs on the road is not about one error, it is more about the accumulation of dangers: the speed that is usual during a clear morning, the distances that were constructed to be driven in the daytime, and the desire to have other vehicles as visual aids. Low-beam headlights, exaggerated spaces between vehicles, and substituting the taillights with the lane lines are constantly reiterated by the safety agencies, as well as it is advised not to pull over in the travel lanes or get out of a car where the other units cannot notice the hazard until it is too late. Another repetitive suggestion is that the fog is to be perceived as dynamic, since the conditions of “sudden wall of white” come without any advance warning.
Tule fog is not simply a weather phenomenon to communities, which rely on Highway 99, Interstate 5, and a network of rural connectors; it is a seasonal aspect that redefines habits. The winter landscape of the valley may be serene through a porch or an orchard edge, and the same low cloud will transform a high-speed traffic-way into a slow and cautious one–a traffic-way that only remains safe when motorists believe that within any moment they may see the view before them disappear.
