Archive · 5 Dec 2025
California Gripped by Dense Radiation Fog
A thick fog called radiation fog or Tule fog is covering a big area in California.
This fog is very dense and can make it hard to breathe, especially for people with asthma.
It also makes driving dangerous because it's hard to see.
The fog happens every year from November to March and affects many cities in California's Central Valley.
The fog forms when the ground gets cold quickly after sunset and traps pollution from cars and factories.
This pollution can make people sick.
The National Weather Service is warning people to be careful when driving because the fog can cause big accidents.
In the past, this fog has caused huge pile-ups on highways, like one in 2007 where 108 vehicles crashed.
Dense radiation fog, also called Tule fog, is covering 20 counties in California's Central Valley, affecting nearly 13 million people.
The fog forms when the ground cools rapidly after sunset, capturing pollutants from cars and factories, which can cause respiratory problems and asthma.
The National Weather Service has issued air quality alerts and travel warnings due to the hazardous conditions caused by the fog.
The fog occurs seasonally from November to March, peaking from December to February, and affects major cities like Sacramento, Fresno, and Bakersfield.
Previous incidents of this fog have led to major accidents, including a 108-vehicle pile-up on Highway 99 in November 2007.
- Who
- Nearly 13 million people in 20 counties of California's Central Valley
- What
- Dense radiation fog (Tule fog) causing respiratory issues and transportation hazards
- When
- Seasonal occurrence from November to March, with peak from December to February
- Where
- Central Valley, California, including Sacramento, Fresno, Bakersfield, Oakland, and Stockton
- Why
- The fog forms when the ground cools rapidly after sunset, capturing pollutants from cars and factories, which can cause health issues and low visibility
- Area Affected
- 20 counties in Central Valley, California
- Population Affected
- Nearly 13 million people
- Fog Type
- Radiation fog (also called Tule fog)
- Seasonal Occurrence
- November to March, peaking December-February
- Major Affected Cities
- Sacramento, Fresno, Bakersfield, Oakland, Stockton
- Health Risks
- Respiratory problems, asthma, and other health issues
- Transportation Risk
- High risk due to low visibility
- Previous Major Accident
- 108-vehicle pile-up in November 2007 on Highway 99