It’s a hazard every fall, winter and spring. Dense fog in the Valley drops the visibility on the roads down to nearly zero feet, and it’s a leading cause of accidents in the Central Valley.

During the last two months of 2021, the Valley experienced weeks on end of dense fog, while the foothills above had sunshine and far warmer weather.

We have several more months to go in our rainy season, and it’s a good time to review fog and how to stay safe while driving with low visibility.

Tule fog is our local name for radiation fog – one of several categories of fog. Radiation fog forms when air near the ground cools overnight. The temperature keeps falling until it reaches the dew point. The dew point is the temperature the air must be cooled to in order to reach 100% relative humidity – meaning the air cannot hold any more water vapor. The water vapor condenses into liquid form, and we have a cloud, or fog, at the surface.

The surrounding air above will continue to cool, and the fog layer gets deeper and grows higher. In the winter, nights are longer, so there is more time for a deep layer of dense fog to develop.

Several ingredients are needed for the atmosphere to be primed for fog development. We need moisture at the surface from recent rain. We also must have little to no wind. Wind creates turbulence and disrupts fog formation.

Lastly, we need a clear sky. This will allow the heat from the day to escape up into the atmosphere and for temperatures to cool more. Clouds help to trap the heat of the day and can prevent temperatures from dropping.

The Central Valley is a favored location for the fog, since we are in a fairly sheltered Valley, and fog can settle undisturbed. This fog can persist for weeks if high pressure is overhead and the weather pattern is stagnant. Sunshine doesn’t always break through the fog, keeping temperatures chilly. Oftentimes, the Central Valley is cooler than the sunny surrounding foothills and mountain areas above the fog.

If you must travel in the fog, leave early. You’ll want to drive slowly to reach your destination safely. Don’t rely on daytime running lights. Turn your headlights on so your tail lights are also on. Cars can completely disappear in the fog, especially white and silver colors. Having both front and back lights on will help you be more visible.

Use your low beams, not your high beams. The brighter and higher high beams bounce off of the fog’s water droplets and essentially are reflected back to your eyes.

Increase your following distance in case of sudden movements of the car ahead of you. If the fog is extremely dense, spot the lines on the road for guidance.

If you can not drive safely, find a parking lot to pull into. If there isn’t one, turn on your flashing hazard lights, and pull well off the road. Turn off your lights so other drives don’t mistake you for the line of traffic on the road.

We have a few more months of our rainy season to go, and likely more fog on the way. Please stay safe out there.