Why does the U.S. get hit with Arctic air? The media calls it the "polar vortex." Here's everything you need to know—in one short page.
The Arctic Oscillation (AO) is the blasting polar jet stream that circles the Arctic air mass. If it is strong, it keeps all that cold where it belongs—up in the Arctic Circle, making life interesting for the Canadians and Siberians. If it is weak, it lets all the frigid air escape south, and we get hit with a blast of winter misery.
♫ When the wind breaks,
The cold air will fall,
And down will come blizzards,
Snowflakes and all! ♪
When winter arrives, the Arctic Oscillation is the big dog of the weather!
Winds are controlled by air pressure. If the low-pressure areas, particularly the Atlantic’s Icelandic Low or the Pacific’s Aleutian Low, are strong, they make strong winds, a positive AO.
The Positive Arctic Oscillation (strong wind) and the Negative AO (weak wind) Source: National Snow & Ice Data Center
The Arctic air stays pinned to the north and most of the US stays cozy warm. If the low-pressure areas are weak, the winds are too weak and the cold air escapes. The jet stream veers south, bringing storms and the frozen Arctic air follows right behind. Brrrr!
When the polar jet stream veers south, the frigid Arctic air follows it.
These miserable invasions of cold air have a lot of names: Siberian Express, Alberta Express, Saskatchewan screamer, Manitoba mauler and Ontario scary-o. Scientists have climate names which analyze the air pressure and explain where and why the cold air will drop. The positive Pacific North American Oscillation (PNA) makes the jet stream drop into the Great Plains and Midwest. The negative North Atlantic Oscillation (NAO) makes the jet stream drop in the Great Lakes and East.
The Positive PNA and the negative NAO weaken the AO and let the Arctic air drop. Source: NOAA
So, if you love skiing, remember, a powerful AO is not your friend. If you are like the two of us, basking in the desert sunlight, send vitamins north and hope that Arctic wind keeps blowing strong.