By Jill K. Applegate

The season of tule fog is descending on the Valley. Soon it will enshroud us, wrap our homes, and imperil our commutes. Here’s a look at how this particular flavor of pea soup is concocted, its history, and how to navigate it.

Foggy For Days

Cool temperatures and little sunshine give tule fog its staying power, allowing it to cling low to the ground for days at a time. The warm air above acts like a blanket that holds the humid fog down to the valley floor, according to WiseGeek.com.

Ground Cloud

Fog has the same structure as clouds—tiny water droplets suspended in the air and dropping in super slow-motion due to their small size, according to the Sacramento Bee. When the air warms up the water is still there, but it turns from a liquid to a gas and lets us see that car in front of us again.

Low Visibility

Tule fog is responsible for more auto accidents in California than any other weather condition, according to BayNature.org.

Behind the Wheel

If you can avoid driving in tule fog, the California Highway Patrol says ‘Put that trip off!’ Other advice from these traffic-safety experts: Choose low beams over high beams, which reflect light off the fog and actually decrease visibility. And slow down—the fog obscures visual cues that let you judge how fast you are going, so you feel like you’re driving slower than you actually are.

Central Marshland

Tule fog is named after the tule reed, a marsh plant found in the Central Valley before its floodplains were drained and turned into fertile cropland, according to BayNature.org.

Legendary Mist

According to Answers.Yahoo.com, the tule fog got its name because it lies so close to the ground—just high enough to cover the tule plants in the marshes and lakes (or so says local lore). The Indians, it is said, used the fog as cover, to move unseen in the low-lying mist.

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