Anyone who lives in or drives through the San Joaquin Valley during the late fall or winter months will inevitably have harrowing experiences with Tule fog. Not only does Tule, or ground, fog develop in the Great Central Valley of California but also in the coastal and inland valleys — especially after it rains.
And here’s why:
The first rains of the season moisten the bone-dry ground of summer. When much of the Earth’s heat is radiated out to space, usually on clear, windless nights, it cools the moist and denser layer of air near the valley’s floor.
Fog is water vapor that has condensed onto microscopic particles in the air and formed a cloud at ground level. When air temperature decreases, relative-humidity levels increase and the valley begins to take on a grayish tint. When the air has reached its dew-point temperature, the relative humidity is at 100 percent. Water droplets become visible to the naked eye. At that point, Tule fog can rapidly develop. Each cubic meter of fog contains between .05 and 0.5 grams of liquid water.
The air in the inland valleys usually contains more particulates — or pollution — than the wind coming off the Pacific Ocean. Consequently, the fog is often thicker than fog forming along our coastline. Visibility can suddenly decrease to near zero in only a few feet. Vehicles following too close to each other and traveling too fast into the great gray unknown can lead to massive chain-reaction mishaps. During periods of tranquil weather, the San Joaquin Valley Tule fog can last for days on end as it settles between the Sierra Nevada to the east and the coastal range to the west under a strong temperature inversion layer.
The air above the inversion layer is drier, warmer and, hence, lighter. In fact, the air above the inversion layer can be as much as 50 degrees warmer than the air below it. Like a cover over a bed, this further serves to trap the cold and heavier air within the valley. At this time of the year, the sun is low in the southern sky, and much of its light is reflected back out to space off the top of the fog layer.
Tiny amounts of sunlight penetrate the fog and warm the valley floor. Afternoon high temperatures may only reach the upper 30s with little temperature change through the night.
Thankfully, storms coming out of the Gulf of Alaska often mix out the temperature inversion layer, producing clearer skies and warmer afternoon temperatures.
According to the California Highway Patrol, here are some tips for driving in foggy conditions:
▪ Drive with lights on low beam. High beams will reflect off the fog, creating a “white wall” effect.
▪ Reduce your speed, and watch your speedometer. Fog creates a visual illusion of slow motion when you might actually be speeding.
▪ Avoid crossing traffic lanes.
▪ Travel with the driver’s window partially open. Listen for traffic.
▪ Watch for CHP pace cars to guide you.
▪ If your car is disabled or you can’t continue, pull well onto the shoulder and turn off lights. Move away from your vehicle. Consider postponing your trip until the fog lifts.