Radiation Fog: A 643-Kilometer Belt Of Mist Lingers Over California's Central Valley

A giant blanket of radiation fog has smothered California’s Central Valley for several days and is expected to linger for days more. But fear not, the fog has nothing to do with nuclear meltdowns or the splitting of atoms.

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What is radiation fog?

Radiation fog is a perfectly normal type of fog that’s most common when things get chilly in the fall and winter. It develops on clear, calm nights when the ground cools rapidly, chilling the air just above it. Once that air reaches saturation, moisture condenses into millions of tiny droplets, creating a thick mist that hangs in the air. 

It’s called radiation fog because it forms through radiational cooling, not because it contains ionizing radiation.

Each fall and winter in parts of California, a special form of radiation fog can linger for days or weeks at a time, known as tule fog. It’s the result of cool, moist air from the Pacific close to the surface. This creates high humidity, especially after recent heavy rain, which provides plenty of moisture to fuel the fog and help it persist.

Tule fog in California’s Central Valley

This is exactly what’s been happening over the past week in the Central Valley of California. Following a downpour of heavy rain in late November, the region's damp valleys have been invaded by a cloak of fog. Trapped in the low-lying terrain with no strong winds or weather systems to stir it up, the fog is expected to linger for several days.

The National Weather Service (NWS) forecast said on Thursday, December 4, that there’s a large outgrowth of the Tule Fog across the Central Valley. It started to emerge last week in late November and is due to hang around for some time. 

“Expect another round of Tule Fog impacts tonight into Friday morning, mainly across the interior Bay Area with potential impacts in the North Bay valleys,” reads the NWS forecast.

Satellite images show it hovering over the Central Valley, like a big, fluffy, white sausage. The patch of tule fog covers a vast area, nearly 643 kilometers (400 miles) long, stretching from Bakersfield to north of Sacramento, near Red Bluff and Redding, according to the Washington Post.

Back on the ground, tule fog can reduce visibility to as little as 3 meters (10 feet) in severe conditions, making driving extraordinarily difficult. In response, several NWS offices have issued Dense Fog Advisories, warning motorists to use extra caution and consider delaying their journey if possible.

Indeed, the seasonal fog has been known to cause tragic incidents on the highways. In November 2007, tule fog was responsible for a massive 108-car pile-up on the Northbound California State Route 99 near Fresno, killing two people. 

It isn’t all negative news, however, as tule fog plays an important role in California’s agriculture. The blanket of cool, moist air helps crops such as almonds, pistachios, cherries, apricots, and peaches to enter and stay in their necessary winter dormant period. This cold-induced dormancy allows the plants to save energy and produce buds and flowers properly in the spring.

Many farmers and scientists are worried that the Central Valley is undergoing a significant dip in wintertime fog based on year-on-year statistics, but this week's weather shows that it’s still part of the region’s seasonal rhythm.