January 2019 ended with some of the coldest temperatures on record. A polar vortex caused a major cold front to rip across the upper part of the United States, sending temperatures plummeting well below regular levels.
For instance, local weather stations in Chicago recorded wind chills of -55 degrees Fahrenheit. This made the Windy City colder than Antarctica, Greenland, Siberia and parts of Mount Everest and set record low temperatures for the city.
But what is a polar vortex? The term “polar vortex” in the Northern Hemisphere refers to counterclockwise winds gusting around the North Pole. These winds keep arctic air and temperatures swept up in its currents like an upside-down tornado. The polar vortex occurs in the upper troposphere and lower stratosphere, thousands of feet over our heads. In the summertime, the polar vortex maintains an almost-constant belt separating arctic air from more temperate air.
Often in the winter, however, the polar vortex destabilizes at its southernmost points. Subsequently, it sends frigid arctic winds southward over Canada and into the United States for a variety of reasons. Therefore, meteorologists keep watch for destabilizations by monitoring the global position of the vortex’s jet streams. Jet streams are narrow bands of rapidly moving air encircling the planet. Differences in those positions are a vital clue to incoming weather phenomena.
“Inside the vortex, the coldest air on Earth resides,” said Professor Eric R. Snodgrass, Director of Undergraduate Studies for the Department of Atmospheric Studies at the University of Illinois at Urbana-Champaign. “Occasionally, pieces–or lobes–of this vortex shed, break apart and send extremely cold air southward.”
A deceptively similar weather event occurs much more regularly. Most often, suddenly developing cold air troughs at the southern belt of the polar vortex are responsible for many rapid dips in temperature. However, they differentiate from actual lobes of the vortex being shed. “From a global position, cold air troughs appear as southward dips in the position of the jet streams,” Professor Snodgrass said. On the other hand, lobes actually breaking off are far less common.
So how can we tell the difference between a true intrusion of the polar vortex and the less severe event of a jet stream dipping south? “When a lobe of the polar vortex does break off and head south, it manifests itself as a large spinning mass of cold air in the upper troposphere,” Professor Snodgrass said. “When the true polar vortex moves over an area, it may sit and spin high above our heads for several days or more. Normally, troughs of cold air move along much faster and are more transient.”
Due to cold weather, about 1,300 Americans die from hypothermia each year–over two-thirds of whom are men. Regardless of the cause of the frigid temperatures, experts advise taking precautions in the winter. Their suggestions include staying informed about sudden upcoming changes in weather, having a plan before going outdoors in extreme temperatures, keeping a winter survival kit in your car and–especially if you live in the coldest parts of the country–staying sober. Whether owing to a cold air trough or a piece of the polar vortex breaking off, staying safe and being informed save lives. Consequently, analyzing the duration and movement speed of a major cold front can help you identify, differentiate between and prepare for either of these wintry occurrences.
Eric R. Snodgrass contributed to this article. He is the Director of Undergraduate Studies for the Department of Atmospheric Sciences at the University of Illinois at Urbana-Champaign, where he also received his master’s degree. Previously, he earned his bachelor’s degree in Geography from Western Illinois University.