The entire continental U.S. is locked in the grip of a “polar vortex”, but just what is a polar vortex and why is it putting the big chill on North American homes and businesses this week? A detailed definition of a polar vortex is pretty complicated and over the heads of most folks without a background in meteorology, but according to NBC news weatherman Al Roker, you can “think of it as a polar hurricane.” That is, a spinning mass of extremely cold air high in the atmosphere, typically rotating counterclockwise around the north and south poles.

Polar Vortex

Plunging temperatures from North Dakota to Texas

It is not uncommon for polar vortexes to dip down as far as northern Canada, but it is very rare for a polar vortex to move thousands of miles south like this vortex has. Particularly large and strong warm air masses building up over Greenland and Alaska this winter are the main reason this polar vortex is moving so far south. Meteorologists report that a polar vortex of this strength only makes it this far south once every 10 or 15 years on average.

Temperatures across the U.S. are dropping to near-record lows. Cities in northern states such as such as Fargo, N.D., are forecasting temperatures as low as 32 below zero, or 21 below zero in Madison, WI. minus 21. When you throw in the wind chill from 30 to 40-mph gusts, it will feel like minus 50 or minus 60 in some areas.

Polar vortex will retreat quickly

The good news is that this polar vortex is expected to retreat relatively quickly. The large amount of warmer air in the more southerly U.S. states will rapidly break down the cold polar air masses, meaning the big chill won’t last for more than three or four days. Temperatures are projected to start moderating by Thursday or Friday. Forecasters are currently anticipating the the 20s and 30s in the Plains and Midwest, while much of the East Coast is expected to be back in the 40s.