It’s become a routine part of winter. That bulging mass of frigid air roars down from the North Pole and parks over a vast expanse of the nation, bringing with it bitter, record-breaking cold. But that unwelcome arctic visitor is not the polar vortex, even though that’s what some folks call it, although it is directly related to it.
The polar vortex is mass of low pressure that hovers at about 10,000 feet above the North Pole and the hurricane-like winds that surround it. The winds are generated by air rushing from areas of high pressure to the extreme low pressure system that deepens at the North Pole in the winter. The greater the difference in the degree of pressure, the pressure gradient, the stronger the wind blows. These frigid winds blow and howl fiercely across the desolate ice plains. Because the earth is rotating, those winds circle rapidly, not unlike a polar hurricane.
The polar vortex, that broad expanse of swirling wind and low pressure, acts like a circular wall trapping the cold air in a tight rotating mass. As long as the pressure gradient is high, the winds blow hard and the cold air is trapped at the pole.
So what causes the mass of cold air to drift south in the winter? It all has to do with the atmospheric temperature changes.
When summers are warmer than usual in the arctic, there’s more ice melt. When winter rolls around, there’s less ice to begin with, so less ice will build up as the days grow dark and cold. If the wintertime temperatures are even a fraction of a degree warmer, even less ice builds up and the polar vortex of low pressure doesn’t reach its full strength. When the pressure gradient between the polar vortex and the high pressure systems further south is low, the power of the circulating winds are lighter. The circulating wall holding the cold air in place weakens and the arctic air mass if free to spills south.
This is why when atmospheric temperature rise and the ice packs at the poles diminish, we sometimes experience much harsher, colder winters.