Why is it so cold? How the polar vortex brings record low temperatures and winter storms

The polar vortex's blast of Arctic air from Canada brought the season's coldest weather to the central U.S. The vortex is forecast to provide frigid weather with "a free ride," according to AccuWeather senior meteorologist Brett Anderson, as it continues to bring a big chill to much of the Midwest and into the East for most of the week.

The polar vortex is a gigantic circular upper-air weather pattern in the Arctic that envelops the North Pole. It's a normal pattern that is stronger in the winter and tends to keep the coldest weather bottled up near the North Pole. The jet stream usually pens the polar vortex in and keeps it there, but at times, some of the vortex can break off or move south, bringing unusually cold weather down into the U.S., Europe, and Asia.

The central U.S. has experienced record low temperatures over the past week as the polar vortex continues its icy grip on the region. Looking ahead, yet another winter storm is forecast for portions of the central and eastern U.S. over the next several days. 

The wind chill index is a guide to help us understand how dangerous low temperatures and strong winds can cause frostbite. To determine the wind chill temperature on the chart below, locate the relevant wind speed at the top and the outside temperature on the right; you will find wind-chill temperature where they meet:

Weather experts say wind chills can cause frostbite on exposed skin in 20 minutes.

Subzero wind chills will be common throughout the week, so be prepared if you must venture out. Wind chill is based on the rate of heat loss from exposed skin because of  wind and cold.

Black ice is a winter driving danger, especially at night and early in the morning. It often forms on bridges and overpasses. How black ice forms:

SOURCE National Weather Service; AccuWeather; Weather Prediction Center and USA TODAY research. Contributing: Doyle Rice and John Bacon, USA TODAY