What is tule fog?

Tule fog is a thick, ground-hugging fog that forms in California’s Central Valley during the late fall and winter months. This fog is known for reducing visibility to just a few feet, creating hazardous travel conditions and dramatically changing the landscape.

You'll often see a thick fog forming over California's Central Valley this time of year on the WeatherRadar. It's called tule fog, and it's a type of radiation fog that forms when specific weather conditions come together. After winter rain saturates the valley floor, clear skies and calm winds allow heat to escape from the ground at night. This cooling causes the moist air near the surface to condense into tiny water droplets, creating dense fog.

Because the Central Valley is surrounded by mountains, cold, moist air can pool easily, making the region especially prone to tule fog. Once established, the fog may persist all day.

While tule fog plays a role in the region’s climate and agriculture, it is also one of California’s most dangerous weather phenomena, contributing to multi-vehicle accidents and travel disruptions each winter. Keep an eye on the WeatherRadar to spot when this weather phenomenon is in the forecast.