A blast of Arctic air gripped the vast middle of the United States on Monday, bringing the coldest temperatures felt in two decades, causing at least four deaths, forcing businesses and schools to close and canceling thousands of flights.

 

The weather threatened to briefly curtail some oil production in the severe cold described by some meteorologists as the "polar vortex" and dubbed by media as the "polar pig."

 

Temperatures were 20 to 40 degrees F (11-22 degrees C) below average in parts of Montana, North and South Dakota, Minnesota, Iowa, Wisconsin, Michigan and Nebraska, according to the National Weather Service.

 

More than half the flights at Chicago's O'Hare International Airport were closed as fuel supplies froze, leaving crews unable to fill aircraft tanks. The afternoon temperature in Chicago was minus 12 Degrees Fahrenheit (minus 24 degrees C), making it colder than in areas of Antarctica, parts of which hovered around 32 F (0 C) during the southern hemisphere summer.

 

The northeast saw unseasonably mild weather and rain, but authorities warned travelers to expect icy roads and sidewalks on Tuesday.

 

In oil fields from Texas to North Dakota and Canada, the severe cold threatened to disrupt traffic, strand wells and interrupt drilling and fracking operations.

 

It also disrupted grain and livestock shipments throughout the farm belt, curbed meat production at several packing plants and threatened to damage the dormant wheat crop.

 

Some 4,392 flights were canceled and 3,577 delayed, according to FlightAware.com, which tracks airline activity.

 

Many airlines could not allow their ground crews to remain outdoors for more than 15 minutes at a time. There were hundreds of cancellations by airlines including United, Southwest, and American.