FOG has sometimes had a sinister reputation. Mediaeval fogs were dreaded as the embodiment of an unhealthy dampness, as catalysts for rheumatic aches and pains, "and as evil vectors for every" kind of ague and fever, and even today, the Nevada pogonip recalls many of these ancient fears.
When fog exists in sub zero temperatures, a white opaque deposit known as rime may be seen to build up on the windward side of obstacles such as clothes lines, shrubs or garden fences. Fog in these circumstances is known as "freezing fog". It is composed of water droplets that are "supercooled", continuing to exist in the liquid state at temperatures well below the normal freezing point of water. A supercooled water droplet, however, quickly freezes given the opportunity when, for example, it comes into contact with a solid object, and this explains the build up of rime at rates of up to half an inch a day on the windward of any obstacles.
Sometimes, however, when the temperature is very low excess water vapour in the atmosphere condenses into ice crystals rather than into tiny droplets of water. Such a fog is common in wintertime on the eastern slopes of the Sierra Nevada Mountains in northern Nevada, and is known locally as the pogonip, a Shoshone Indian word that means "white death". As with rime, the ice crystals drifting in the air adhere to trees and fences, often forming spectacular patterns on any surface with which they come in contact, and some say that colliding crystals can be heard to tinkle gently as they jostle with each other.
The pogonip, however, has traditionally been seen as rather sinister. A report in the American Meteorological Journal in 1887, for example, stated, "To breathe the pogonip is death to the lungs, and when it comes, the people rush" for cover. When it ascends from the valleys its chill embrace is so much feared by the Indians, who are predisposed to infections of the lungs, that they change their camp if apprised by the atmosphere conditions that the dreaded fog is imminent."
But observers in more recent times declare that the pogonip is harmless. It is likely that its sinister reputation arises from the prevalence of tuberculosis in the late 19th century, and the fact that breathing harsh, cold air, whether containing little particles of ice or not, probably exacerbated existing conditions of the lungs.