In the western United States, ice fog is commonly known as pogonip. It occurs very rarely during cold winter spells, usually in deep mountain valleys. Ice fog can be quite common in interior and northern Alaska, since the temperature frequently drops below -40 °C (-40 °F) in the winter months. Pogonip only forms under specific conditions, the humidity has to be near 100% as the air temperature drops to well below 0 °C (32 °F), allowing ice crystals to form in the air. The ice crystals will then settle onto surfaces.
The name pogonip is an English adaptation of the Shoshone word meaning "cloud" (payinappih). The English-speaking settlers who encountered this unpleasant and sometimes scary phenomenon when they went out West in the 1800s needed a word for it and they borrowed it from local populations.
Supposedly, western Native Americans called it "frozen death" because it took so many lives from upper respiratory infections.
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