What Is Freezing Fog?

freezing fog landscape in a forest with bare trees covered in ice

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During winter months, it's not uncommon to see freezing-fog warnings on your local forecast, but what exactly does the term mean? Fog typically forms when there's cool air over a warm, moist surface such as a body of water or damp soil. However, freezing fog occurs when the air temperature is below freezing and the water droplets in the fog become supercooled.

Supercooled water droplets stay in liquid form until they come into contact with a surface they can freeze on. Therefore, any object that freezing fog comes into contact with will become coated in ice, often creating stunning landscapes.

Freezing fog can occur anywhere the air temperature drops below freezing, but it's most common in mountain areas and is most likely to form at night when heat escapes from the atmosphere.

In the Western U.S., freezing fog often occurs in mountain valleys and may be referred to as pogonip, a Shoshone word that means "cloud."

When freezing fog occurs, it can cause ice buildup on roads, creating dangerous driving conditions, especially on bridges, which will freeze first because they have no ground insulation.

Because freezing fog will freeze on any surface, it often builds up on power lines and can cause power outages.

Freezing fog differs from ice fog, which is composed of tiny crystals instead of water droplets. Specific conditions are necessary for ice fog to form — humidity has to be near 100 percent as the air temperature drops well below freezing.

Typically, the temperature must be 14 degrees Fahrenheit or colder for ice fog to occur, which is why it’s rarely witnessed outside of polar or arctic regions.