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The cold temperatures in the polar regions cause air to descend, creating the high pressure (a process called subsidence), just as the warm temperatures around the equator cause air to rise instead and create the low pressure Intertropical Convergence Zone.
The warming of the air near to the ground leads to rising movements and the atmosphere becomes less stable. One of these patterns is the air pressure difference between the Arctic and mid-latitudes: the Arctic oscillation with the Azores highs and Iceland lows known from the weather reports. If this difference is high, a strong westerly wind ...
That means arctic air is blasting over the US, while pressure changes and the motion of the polar vortex whip up high winds and create a perfect recipe for wintry weather.
"Cold arctic air gripped western Europe in the first three weeks of December. Two major snowstorms, icy conditions, and frigid temperatures wreaked havoc across much of the region...The harsh winter weather was attributed to a negative Arctic Oscillation, which is a climate pattern that influences weather in the Northern Hemisphere.
An area of high pressure that originated in the Arctic has plunged southward across the region, providing the cold air to support snow, and moisture from the Pacific Ocean and Gulf of Mexico will ...
At the 60th parallel, the air rises to the tropopause (about 8 km at this latitude) and moves poleward. As it does so, the upper-level air mass deviates toward the east. When the air reaches the polar areas, it has cooled by radiation to space and is considerably denser than the underlying air. It descends, creating a cold, dry high-pressure area.
Pipes exposed to "severe cold" including outdoor hose bibs, swimming pool supply lines, and water sprinkler lines. (If you have swimming pool or sprinkler supply lines, they needed drained too.)
Upper flow which was meridional all the way up to the Arctic Circle provided a direct path for cold air advection to occur southward to the United States. The average temperature of the contiguous United States was 17.9 °F (−7.8 °C), the coldest day since January 12–13, 1997 and the 40th-coldest day in the United States since 1900.