Ice Fog
An important characteristic of the arctic and subarctic environment, especially in winter, is the stillness of the air. Aircraft pilots in particular notice the change that winter brings as their craft speed steadily along, instead of bouncing around through summer's turbulent air.
As the sun retreats to near or below the horizon, less heating of the ground surface and the near-surface air occurs. If the sky is clear, the earth radiates its heat energy to the frigid reaches of space and then cools the air in contact with it. Cold, stagnant air near the ground results, often inverting the normal trend for decreasing air temperature with increasing altitude. Sometimes extreme inversions develop. At Fairbanks, where hills surround the city to further hamper air movement, the near-ground inversions are among the world's most extreme, as much as 16°F (9°C) each 100 feet (30 meters) of altitude.
The stagnation and horizontal layering of the air creates spectacular mirages and some effects that are less pleasing. Industrial pollution from urban areas of the northern hemisphere finds its way into the arctic where it hangs suspended in multiple reddish-brown layers to signal the passersby that they haven't entirely escaped civilization.
Of immediate concern to residents of northern cities is the trapping of man-made pollutants by the steep inversions such as occur at Fairbanks. Most are concerned with one particular pollutant, ice fog, because they can see it, or more precisely, because of it they cannot see vehicles on the streets, or land at the local airport.
Ice fog forms from water vapor expelled into the air by people breathing, but mostly from water vapor ejected into the air from automobiles and smokestacks. Compared to warm air, cold air is able to hold very little water vapor. Air at room temperature, if saturated, can contain about 20 grams of water vapor per cubic meter, but air at -40°C can hold a maximum of only 0.1 grams, 200 times less. When air is cooled to the point of saturation, excess water condenses into either liquid or ice crystals, depending upon the temperature and also upon the presence of other particles which help supercooled water droplets to turn into ice crystals. [Supercooled water is that remaining liquid even though its temperature is below 32°F (0°C)]. In clean air the resulting ice fog may not form until the temperature falls to -40°C, but if the air is dirty, the fog of tiny spherical, block-like or platelet ice crystals can start to develop at temperatures as warm as - 20°C.
In a way, ice fog is but a warning of conditions that also trap more lethal urban pollutants. The stagnant air within the near-ground inversion also traps carbon monoxide, nitrogen and sulfur oxides, lead and hydrocarbons. Even tiny amounts of carbon monoxide are bad, especially for young children, since prolonged exposure can permanently retard their mental processes. However, the ice fog particles perhaps combine, as do liquid water droplets, with other pollutants to create obnoxious or dangerous acid compounds.
The air in a place such as Fairbanks can be so stagnant and the inversions so severe (inversions of at least some degree occur here approximately 240 days each year) that the city's pollution becomes trapped in a comparatively tight box of small volume. It is for this reason that scientists say that this particular city is so susceptible to pollution and that control of pollution sources is essential. They point out that though Fairbanks has a population two hundred times less than Los Angeles, the levels of the pollution in the two cities is sometimes comparable.
