11. When weaker, the current is easily disrupted, allowing arctic air masses to frequently plunge south. 12. There is no further evidence of major arctic air masses headed into the U . S. 13. Meanwhile, a mainly dry arctic air mass will linger from the northern Plains to New England. 14. The pressure over North America has been abnormally low, preventing the building of Arctic air masses . 15. Monday night, however, an exceptionally cold Arctic air mass offset the moderating influence of the Pacific. 16. Obviously, the Arctic air mass that gripped the Middle West was very cold and very dry. 17. Arctic air masses form over the snow-covered tundra of northern Canada where sunlight is dim or absent.18. An advancing Arctic air mass will enter the border states from western New York to North Dakota. 19. Arctic air masses approaching from the interior of the state are typically modified by the surrounding waters.20. However, the Arctic air mass that arrived Friday will moderate somewhat as winds from the southwest develop.
