One of the most dangerous aspects of the rainy season throughout the San Joaquin Valley is the Tule fog. It forms on clear nights when the ground is moist and the wind is near calm. On nights like this, the ground cools rapidly. In turn, the moist air above it cools and causes water vapor to condense. Once it has formed, the air must be heated enough to either evaporate the fog or lift it above the surface so that visibilities improve.

The visibility in Tule fog is often less than 1/8th of a mile, about 600 feet, but can be less than 10 feet. Visibility can vary rapidly in any area, with sudden decreases to near zero in only a few feet. It is situations like these that often lead to multi-car accidents where one car follows another into a fog bank.

Another area prone to fatal accidents is intersections across major roads or heavily traveled roads, where the cross traffic does not have to stop.

Fog blankets portions of the San Joaquin Valley

December 18th was one of many days when Tule Fog
blanketed portions of the San Joaquin Valley

Some tips for driving in Tule fog include:

Drive with your lights on, even during daylight hours. But use the low beam only. High beam lights decrease your visibility by reflecting more fog back to you.

Reduce your speed. If you are late to your destination, then just be late. It is far better to be late a few minutes due to fog than to be late by several hours or days via a trip to the hospital because of an accident.

Listen for traffic you cannot see.

Avoid intersections where cross traffic does not stop.

Be patient. Do not pass lines of traffic.

Do not stop on a freeway or heavily traveled road unless absolutely necessary. If you must stop, get away from your vehicle to avoid personal injury.

If possible, postpone your trip until the fog lifts, usually by late morning.

Finally, be sure to heed instructions in those areas where the CHP is providing escorts.

Always read and obey the overhead traffic and weather information on signs along California highways.