The central US has taken on the brunt of cold temperatures brought on by the polar vortex. On Feb. 15, many cities located in the nation’s heartland, from the Canadian to Mexican border, exhibited record-breaking cold air temperatures and frigid sub-zero wind chills. The vortex also unleashed winter storms with watches, warnings, or advisories in at least 40 states, CNN reports.

According to the National Oceanic and Atmospheric Administration (NOAA), the polar vortex is a large area of low pressure and cold air surrounding both of the Earth’s poles. The vortex’s air moves in a counterclockwise direction, keeping the air close to the poles. In the northern hemisphere, the polar vortex becomes less stable and expands during the winter. This sends chilly Arctic air south in the United States, where it is not typically seen.

Unlike the central U.S., New Jersey had a milder experience of the polar vortex with slightly colder temperatures and some snow. This pales in comparison to the record-breaking cold temperatures the heartland endured. Overall, much of the U.S. experienced cold temperatures, but it is important to remember that this won’t last forever, and warm spring temperatures are coming soon.