The Midwest and East Coast have recently been experiencing weather that has reached a record all-time low. This freezing weather is known as a polar vortex. A polar vortex is a strong area of low pressure that is usually located around the Arctic throughout the winter. Counterclockwise winds around the vortex occasionally push waves of intensely cold air into the United States. A vortex is strongest during the winter, and usually weakens or even disappears in the summer. It tends to move very slowly or even stay stationary, and its position determines what part of the United States the Arctic air will invade.

From Atlanta, Georgia, to Green Bay, Wisconsin, the country saw the coldest temperatures that it has experienced in decades. Green Bay’s 35-year-old record was overcome by a frigid -18 °F. On Jan. 6, Babbitt, Minnesota was the coldest place in the country at −37 °F. New York City’s Central Park broke a 118-year-record when the temperature there dropped to 4 °F. Central Park’s previous record of 6 °F had stood since 1896.

The cold even turned deadly for some: authorities reported at least 21 cold-related deaths across the country since Sunday, including seven in Illinois and six in Indiana. At least five people died after collapsing while shoveling snow, while several victims were identified as homeless people who either refused shelter or did not make it to a warm refuge soon enough to save themselves from the cold.

In an area extending far enough to include roughly 187 million Americans in the Continental U.S., this phenomenon can be understood to result from the rapid melting of polar sea ice, which replaces white, reflective ice with dark, absorbent open water. As a result, the region has heated up faster than other parts of the globe. Overall, this whole epidemic can be traced back to global warming.