Claim:
A 400-mile blanket of “mysterious” fog containing (nuclear) radiation stretched over California in late 2025.
Rating:
Context
While California’s Central Valley was experiencing a persistent form of “radiation fog” in late 2025, the fog wasn’t radioactive. In fact, radiation fog is the most common type of fog in most of the United States.
Beginning in November 2025, a blanket of persistent clouds lingered over California’s massive Central Valley for multiple weeks, bringing colder than usual temperatures and periods of intense fog to much of the state. The foggy weather began drawing more attention on social media as it dragged on into December, including from those confused by it and from people believing the fog to be more sinister than a typical weather pattern.
Posts on X (archived), TikTok (archived) and Instagram (archived) questioned the “mysterious” fog. Because the fog had been referred to as “radiation fog,” some of the confused people on social media, such as the person in the TikTok post, have wondered if that means the fog was radioactive or had nuclear radiation in it.
While the fog discussed in the posts itself was real and was a type of radiation fog, there was no nuclear radiation in the fog and it could hardly be called “mysterious.”
“Radiation” in this case refers to a process called “thermal radiation,” which is when something emits heat from itself. A common example of thermal radiation is when a person stands near a fire on a cold night and feels the heat radiating from the fire.
In the case of the fog, the sun emits — radiates — its heat to Earth, while the Earth radiates its own heat out and away from its surface. At night, when the Earth continues to radiate heat away from it and the sun isn’t shining down to warm it, the Earth’s surface cools. If there are no clouds to trap the heat the Earth is emitting, surface temperatures may drop quickly overnight, The Weather Channel says. The National Weather Service (NWS) calls this overnight process “radiational cooling.”
Fog forms because the air can’t hold any more water than it already has, according to the Alabama Weather Network. Cooler air generally holds less water than warmer air, so radiational cooling can sometimes cause fog to form overnight. Since the fog is formed by radiational cooling, it’s called radiation fog.
Radiation fog is the most common fog in many areas of the U.S., 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. The Weather Network, a Canadian weather information service, says tule fog is simply radiation fog with a special name. Either way, it’s radiation fog that settles in the Central Valley during the year’s cooler months.
Tule fog, named after a common grass in the region, typically forms after a heavy rain drenches the region during those months, The Weather Network explains. 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 allows a thick radiation fog to persist for longer periods. As explained in a study of tule fog from 2019, between November and March, the region typically experiences two distinct weather conditions: dry days with clear skies and little wind, permeated by the occasional wet winter storms.
The storms soak the ground and fill the air with moisture. At night, that moisture-filled air cools enough for fog to form. Since cooler air is denser than warmer air, the cooler air gets trapped close to the ground. And since California’s Central Valley is surrounded by mountains on almost all sides, there’s nowhere for that trapped, cooler air to go. That means the cool, fog-filled air is likely stuck in place so long as there is little to no wind to mix it up and free it.
Tule fog typically leads to cooler temperatures in the Central Valley, more car accidents as the dense fog limits visibility on the road and better yields for the many crops grown in this region that are eaten nationwide.
And yes, as some of the social media posts have pointed out, pollutants already in the air can also remain trapped in the fog, therefore worsening the air quality. But the mountains surrounding the Central Valley usually trap pollutants anyway, leading to its southern half having some of the worst air quality in the nation. In fact, air quality forecasts for Dec. 10, 2025, when the fog still hovered over the Central Valley, show that the air quality was much better in the Sacramento Valley, which takes up the northern portion of the Central Valley, than it was in the San Joaquin Valley, which takes up the southern portion of the Central Valley.
Tule fog has not been as common or as intense in the last couple of decades as it once was. Some research has suggested that this is because of declining levels of air pollution in California, while other research has linked droughts to the decline in the annual fog. Fog needs particles in the air for the water molecules to attach to, hence why pollution can affect fog frequency and intensity. And if a region is in a drought, it’s not going to have the rainfall needed for the tule fog’s formation in the first place.
Whatever the reason for the decline, the fog has continued to happen and isn’t new, even if it hasn’t been as intense as this episode in awhile. NOAA’s satellite view of the 2025 fog doesn’t look all that different from NASA’s satellite image of the 2005 fog 20 years prior or its satellite image of the 2011 fog more recently. NASA even has a satellite image of a 2020 tule fog episode, albeit that one didn’t blanket the region as fully as the others.
So why are people calling it mysterious? At least one of the more popular social media posts directly references a Dec. 4 Daily Mail article with the headline “Mysterious 400-mile stretch of ‘radiation’ fog blankets 13 million Americans.” Despite the headline, the text of the Daily Mail article doesn’t refer to the fog as mysterious even once. It also explained that the fog “is unrelated to nuclear radiation and generally should not endanger human health.”
This isn’t the first time Snopes has fact-checked ominous claims of “mysterious” fog. In January 2025, Snopes explained the more benign causes of experiences people claimed was caused by “toxic fog.”
