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Tule fog can strike with deadly force

When it comes to the San Joaquin Valley, fog doesn't come "on little cat feet" as in Carl Sandburg's poem.

Fog in the Valley comes on big elephant feet -- slamming into the flatlands, causing accidents and days and days of almost zero visibility and cold weather.

In California, the fog that comes to the Valley, from Redding to Bakersfield, is known to locals as tule fog. It got its name because the gray stuff normally forms in the lowest, wettest areas, where tules historically grow.

Tules are freshwater plants found in the Central Valley and have a long history in the lore of the state. The city of Tulare was named after tules, as were tule elk.

According to Modesto Vasquez, a meteorologist with the National Weather Service in Hanford, the fog is formed after the Valley gets enough rainfall that the ground is saturated.

"Once we get cold temperatures, the air can't hold any more water vapor and the water starts to condense, and we get fog," Vasquez said.

Along with wet ground, the fog also needs no wind and no cloudy sky. Because the Valley is surrounded by mountains, the fog gets trapped and can't get out until a low-pressure system, bringing wind, comes and blows the fog out, Vasquez said.

"When it's really foggy here, you can drive up to the foothills and it's clear and sunny," Vasquez said.

The fog can be a killer. In November 2007, heavy tule fog caused a massive pile-up of cars on Highway 99 between Fowler and Fresno. More than 100 cars and 18 big-rig trucks were involved in the accident, which caused two fatalities and 39 injuries. The fog had cut visibility to about 200 feet at the time of the accident.

Low visibility is common in tule fog. It's usually less than an eighth of a mile, but can be as little as a foot. And the visiblity can vary rapidly.

According to the California Highway Patrol, drivers encountering tule fog can take some precautions, including driving with lights on low beams, reducing speed, avoid crossing traffic lanes and traveling with the driver's window open to listen for traffic.

In extremely bad fog, the California Highway Patrol will put pace cars on highways to guide travelers.

Reporter Carol Reiter can be reached at (209) 385-2486 or