ABSTRACT. A strong and persistent air temperature inversion at ground level, resulting from outgoing radiation, is a feature of Arctic and sub-Arctic regions. In sheltered valleys the lowest air layers become extremely stable and prone to pollution. Temperature gradients of 30°C per 100 m are common in the lowest 50 m. The stable air structure extends 1 or 2 km upward from the surface and permits transport of pollutants from mid-latitudinal sources in sheet-like layers across the Arctic with negligible vertical dispersion. This produces widespread Arctic haze, most effectively in winter and early spring during cold, dry periods when the air is stable and well-stratified, and ending in April when conditions become turbulent and moist. Considering the distances involved, the concentration of pollutants in the haze seems surprisingly high, but it is a natural consequence of exceptionally stable air. Urban haze is most intense during December and January, when the inversions are undisturbed even at midday: pollution levels in a small community like Fairbanks during winter can equal or exceed those of large industrial, urban centres with populations two orders of magnitude larger, such as Detroit, Los Angeles or New York. Local winter problems become especially acute at latitudes north of 60° in continental regions such as Siberia, interior Alaska and Canada.