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