At least five people have died in the United States after a winter storm hit a large swathe of the country over the weekend, leading to mass school closures, dangerous road conditions and power cuts. Some 60 million people are under weather alerts across 30 states. Seven of them have declared emergencies: Maryland, Virginia, West Virginia, Kansas, Missouri, Kentucky and Arkansas. The National Weather Service has warned that severe thunderstorms, with the possibility of tornadoes and hail, would also take place in some regions over the next few days. The extreme weather has been caused by the expansion of the polar vortex southwards. Here is a look at what exactly polar vortex is, when it causes extreme cold, and if global warming is impacting it. What is the polar vortex? The polar vortex is a large area of low-pressure and cold air that swirls like a wheel around both of the Earth’s polar regions. There are two types of polar vortex: tropospheric and stratospheric. The tropospheric polar vortex occurs at the lowest layer of the atmosphere — it extends from the surface up to about 10 km to 15 km — where most weather phenomena occur. The stratospheric polar vortex occurs at around 15 km to 50 km high. Unlike the tropospheric polar vortex, the stratospheric polar vortex disappears during the summer and is the strongest during the autumn. When does the polar vortex cause extreme cold? The US, parts of Europe, and Asia experience chilly winds when the polar vortex at the North Pole weakens travel from its usual position. “As this system weakens, some of the cold, arctic air can break off and migrate south, bringing plenty of cold air with it. Areas as far south as Florida may experience arctic weather as a result,” according to a report in the National Oceanic and Atmospheric Administration (NOAA) SciJinks. That happens because when the polar vortex is strong and stable, it keeps the jet stream travelling around the planet in a circular path. The jet stream is a narrow band of strong wind in the upper levels of the atmosphere which plays a key role in keeping cold air north and warm air south. “Without that strong low-pressure system, the jet stream does not have enough force to maintain its usual path. It becomes wavy and rambling. When high-pressure systems get in its way, a collection of cold air pushes south, along with the rest of the polar vortex system,” the report added. Is climate change impacting the polar vortex? Scientists are still researching the precise impact of climate change on the polar vortex, and if rising temperatures are making the low-pressure system buckle more frequently. Some researchers believe that as the poles are getting warmer at a faster rate than the rest of the Earth, the polar vortex and jet stream are becoming weaker. Warmer temperatures make it easier for the polar vortex and jet stream to get disrupted. “It makes sense that the polar vortex tends not to be as strong due to global warming because the planet isn’t warming uniformly. It’s warming more at the pole, overall decreasing the strength of the polar vortex and the jet stream and making it more susceptible to being dislodged and sent our way,” Steven Decker, the director of the Meteorology Undergraduate Program at Rutgers University, said last February in a press statement.
