Fact Check: Is California’s thick 'radiation fog' causing health problems?

SACRAMENTO, CALIFORNIA: In late 2025, widespread claims circulated online about a massive blanket of radiation fog covering California’s Central Valley. 

These claims suggested the fog was mysterious and potentially harmful. But how much of this story is actually true?

Claim: A 400-mile stretch of 'mysterious' fog containing nuclear radiation covered California in late 2025

Starting in November 2025, thick clouds settled over California's expansive Central Valley for several weeks, resulting in cooler-than-normal temperatures and frequent dense fog across much of the region.

As the fog persisted into December, it gained increased attention on social media, both from curious observers and from individuals who interpreted it as something more ominous than an ordinary weather phenomenon.

Users on social media raised questions about the fog. The fog was labeled as “radiation fog” in some social media posts leading to speculations whether it was actually radioactive or contained nuclear radiation.

One user posted on X claiming, "Mysterious 400-mile stretch of 'radiation' fog blankets 13 million Americans. How long can you hold your breath?"

Another user posted a video on Instagram with the same claim, "The mystery fog is back. Just like clockwork"

The claim was also shared on TikTok, spreading rapidly across the platform and attracting attention from users who questioned the nature and safety of the fog.

Fact Check: False, California radiation fog is a natural weather phenomenon

Although the fog mentioned in the posts was real and classified as a type of radiation fog, it did not contain any nuclear radiation and was not truly “mysterious”, making the claim false.

In this context, radiation refers to a process called 'thermal radiation', which occurs when an object emits heat from its surface.

For the fog, the sun radiates heat to the Earth, while the Earth simultaneously radiates its own heat away from the surface. At night, when the sun is no longer warming the ground, the Earth continues to lose heat.

If clouds are absent to trap this escaping heat, surface temperatures can drop rapidly overnight. The Weather Channel explains this, and the National Weather Service (NWS) refers to this nighttime process as "radiational cooling."

Radiation fog is the most common fog in many areas of the US, according to the Alabama Weather Network. The NWS notes that tule fog, a special kind of radiation fog, occurs each winter in the Central Valley of California. 

Normally, radiation fog dissipates during the day as the temperature rises or winds pick up to mix the atmosphere, but the Central Valley's geography and climate allow a thick radiation fog to persist for longer periods. 

The fog has continued to happen and isn't new, even if it hasn't been as intense as this episode in a while.

NOAA’s 2025 satellite images closely resemble NASA’s views of similar fog events in 2005, 2011, and even 2020, showing the phenomenon is not new.