3.1.6 Arctic Stratus (Clouds) and Arctic Haze
The Arctic Ocean is often blanketed in low-lying clouds called arctic strati, which sometimes descend to surface level to become fog. The average monthly frequency of arctic stratus formation in the Arctic Ocean ranges 20-40% from region to region in the winter months from November to April and 70-80% by region in the summer months from June to September. In the center of the Arctic Ocean the phenomenon is especially pronounced, with average cloud frequency in the 80-90% range from June to August, of which some 70% consists of arctic strati. In the era of visual aviation, the region was impassable in two-thirds of the summer months: an average of 18 days in June, 23 days in July and 22 days in August. In the winter, the number of impassable days fell to one third of each month.
In the spring, the Arctic is often covered in a smog called Arctic haze. Whereas on clear days in the Arctic the horizon is visible at a distance of 200km, when the Arctic haze settles in visibility is cut to 3-8km. When samples of the Arctic haze were analyzed, the results revealed that the haze is the consequence of human activity, containing large quantities of sulfuric acid, soot and organic materials. In the spring, the concentration of fine particles suspended in the atmosphere is equal to that in many urban areas. Because precipitation is low from winter to spring in the Arctic Ocean, the atmosphere is unable to clean itself through the precipitation cycle during this time. It is only in the summer months of July and August, when the mist and precipitation increase, that that the arctic haze lifts, leaving the Arctic air clean through the summer and fall.
The significance of this pattern for NSR navigation is that, in the narrow straits between islands and in shallow bays, NSR shipping will depend on radar and satellite guidance for much of the year. Many routes follow low-lying coastlines, and in summer visibility is poor on many days.
Problems such as these that are associated with the Arctic atmosphere must be understood in terms of the behavior of aerosols in the polar atmosphere. Aerosols, generated by human activity and carried to the polar regions, are likely to form in the atmosphere when the density of gases surpasses the critical pressure required for solidification; this can occur in conditions conducive to increasing the gas density or susceptible to cooling to extremely low temperatures. It is this latter condition that causes the condensation of aerosols in the Arctic.