Ice fog is a type of fog

Fog

Fog is a collection of water droplets or ice crystals suspended in the air at or near the Earth's surface. While fog is a type of stratus cloud, the term "fog" is typically distinguished from the more generic term "cloud" in that fog is low-lying, and the moisture in the fog is often generated...

 consisting of fine ice crystals

Ice crystals

Ice crystals are a small crystalline form of ice including hexagonal columns, hexagonal plates, dendritic crystals, and diamond dust. The highly symmetric shapes are due to depositional growth, namely, direct deposition of water vapour onto the ice crystal...

 suspended in the air. It can happen only in cold areas of the world since water

Water

Water is a chemical substance with the chemical formula H2O. A water molecule contains one oxygen and two hydrogen atoms connected by covalent bonds. Water is a liquid at ambient conditions, but it often co-exists on Earth with its solid state, ice, and gaseous state . Water also exists in a...

 can remain liquid down to -40 °C (-40 °F). It should be distinguished from diamond dust

Diamond dust

Diamond dust is a ground-level cloud composed of tiny ice crystals. This meteorological phenomenon is also referred to simply as ice crystals and is reported in the METAR code as IC. Diamond dust generally forms under otherwise clear or nearly clear skies, so it is sometimes referred to as...

, a precipitation

Precipitation (meteorology)

In meteorology, precipitation In meteorology, precipitation In meteorology, precipitation (also known as one of the classes of hydrometeors, which are atmospheric water phenomena is any product of the condensation of atmospheric water vapor that falls under gravity. The main forms of precipitation...

 of sparse ice crystals falling from a clear sky.

In the United States

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

Alaska

Alaska is the largest state in the United States by area. It is situated in the northwest extremity of the North American continent, with Canada to the east, the Arctic Ocean to the north, and the Pacific Ocean to the west and south, with Russia further west across the Bering Strait...

, since the temperature frequently drops below -40 °C (-40 °F) in the winter months. Pogonip only forms under specific conditions, the humidity

Humidity

Humidity is a term for the amount of water vapor in the air, and can refer to any one of several measurements of humidity. Formally, humid air is not "moist air" but a mixture of water vapor and other constituents of air, and humidity is defined in terms of the water content of this mixture,...

 has to be near 100% as the air temperature

Temperature

Temperature is a physical property of matter that quantitatively expresses the common notions of hot and cold. Objects of low temperature are cold, while various degrees of higher temperatures are referred to as warm or hot...

 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

Shoshone

The Shoshone or Shoshoni are a Native American tribe in the United States with three large divisions: the Northern, the Western and the Eastern....

 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 "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

Jack London

John Griffith "Jack" London was an American author, journalist, and social activist. He was a pioneer in the then-burgeoning world of commercial magazine fiction and was one of the first fiction writers to obtain worldwide celebrity and a large fortune from his fiction alone...

 described Pogonip which happened to the main characters, killing one of them.

In Siberia

In Siberia

Siberia

Siberia is an extensive region constituting almost all of Northern Asia. Comprising the central and eastern portion of the Russian Federation, it was part of the Soviet Union from its beginning, as its predecessor states, the Tsardom of Russia and the Russian Empire, conquered it during the 16th...

 ice fog is relatively common, especially in Sakha Republic, Russian Federation

Russia

Russia or , officially known as both Russia and the Russian Federation , is a country in northern Eurasia. It is a federal semi-presidential republic, comprising 83 federal subjects...

. The city of Yakutsk

Yakutsk

With a subarctic climate , Yakutsk is the coldest city, though not the coldest inhabited place, on Earth. Average monthly temperatures range from in July to in January. The coldest temperatures ever recorded on the planet outside Antarctica occurred in the basin of the Yana River to the northeast...

is famous for its ice fogs.