Polar vortex is 'spinning backwards' above Arctic after major reversal event

The polar vortex circling the Arctic is swirling in the wrong direction after surprise warming in the upper atmosphere triggered a major reversal event earlier this month. It is one of the most extreme atmospheric U-turns seen in recent memory.

In the past, disruptions to the polar vortex — a rotating mass of cold air that circles the Arctic — have triggered extremely cold weather and storms across large parts of the U.S.. 

The current change in the vortex's direction probably won't lead to a similar "big freeze." But the sudden switch-up has caused a record-breaking "ozone spike" above the North Pole.

The polar vortex is most prominent during winter months and extends into the stratosphere — the second layer of the atmosphere up to around 30 miles (50 kilometers) above the surface. The vortex spins counterclockwise with wind speeds of around 155 mph (250 km/h), which is around the same speed as a Category 5 hurricane, according to the U.K. Met Office. A similar vortex also encircles Antarctica during the southern winter.

Polar vortices occasionally reverse temporarily. These events can last for days, weeks or months and are caused by sudden stratospheric warming (SSW), when the temperatures in the stratosphere climb by as much as 90 degrees Fahrenheit (50 degrees Celsius) in the space of a couple of days, according to the Met Office. 

Related: 'One of the biggest on record': Ozone hole bigger than North America opens above Antarctica

The sudden warming is caused by "planetary waves" in the atmosphere — compression waves formed when air rises into a region of different density and is pushed back downward by the force of Earth's spin. This process disrupts or reverses the vortex flow.