Ice fog is a type of fog consisting of fine ice crystals suspended in the air. It can happen only in cold areas of the world since water can remain liquid down to -40 °C (-40 °F). It should be distinguished from diamond dust, a precipitation of sparse ice crystals falling from a clear sky.[1]
In the western United States, ice fog is commonly known as pogonip.[2] 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.[3]
Supposedly, western Native Americans called it "white death" because they believed the crystals got into their lungs and caused death. (source: Print version of the Reno Gazette Journal, Jan. 9, 2011, page 2a "photo of the week.")
In The Old Farmer's Almanac, in the calendar for December, the phrase "Beware the Pogonip" regularly appears. In Smoke Bellew Jack London described Pogonip which happened to the main characters, killing one of them.