What Is Freezing Fog?

This winter weather phenomenon can be both beautiful and dangerous.

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.

The U.K.'s Met Office explains, "Liquid needs a surface to freeze upon. When droplets from freezing fog freeze onto surfaces, a white deposit of feathery ice crystals is formed. This is referred to as rime; rime is a characteristic of freezing fog and is often seen on vertical surfaces exposed to the wind."

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, an Anglicized version of a Shoshone word ("payinappih") that means "cloud."

When freezing fog occurs, it can cause ice buildup on roads (also known as black ice), creating dangerous driving conditions. Drivers should be especially careful on bridges and overpasses, as these will freeze first because they have no ground insulation. Always drive slowly in such conditions and leave plenty of distance between your vehicles and others. Be careful in transition zones, e.g. if you're traveling from sun into fog.

Since freezing fog will freeze on any surface, it often builds up on power lines and causes power outages, particularly if exposure is prolonged. It can freeze on airplanes and cancel or delay flights until de-icing procedures occur. It can turn sidewalks into slippery hazard zones for pedestrians.

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% 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.

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  1. Merriam-Webster, "Pogonip."