High over the pole spins a great vortex of frigid air, ordinarily locked in place through the winter by the polar jet stream. But now and again that swirl weakens and buckles, and a lobe of the deepest Arctic cold spills southward across the continent. When it reaches the northern plains, the region endures some of the harshest cold in the country — days on end at temperatures far below zero.

When the Arctic Comes to Visit

These are the spells the plains measure themselves by: the mornings when the air hangs still and impossibly cold, when ice fog forms over the towns, when metal grows brittle and diesel gels and the whole country seems to hold its breath. Life narrows to the essentials, and the community checks on its elderly and its livestock with special care.

From the almanac's reckoning:

• The polar vortex is a swirl of Arctic air normally locked over the pole.
• When it buckles, deep cold spills south onto the plains.
• Such spells can hold temperatures far below zero for days.

The vortex passes, as it always does, and the cold relents, but not before it has reminded the plains of exactly where they sit on the map. The almanac counts the polar vortex among the great tests of the plains winter — the visit of the Arctic itself.