What is a polar vortex, the weather event causing winter storms in the US?

The expanding of the polar vortex – an area of cold air around the Arctic – southwards is causing cold spells in the US.

The United States has been experiencing severe winter storms, which is likely to affect more than 60 million people in the country’s east. The ongoing cold spell, which may bring down the mercury to as low as -50C (-60F), is caused by the expansion of the polar vortex – an area of extremely cold, rotating air around the Arctic – southwards.

Sub-freezing temperatures can extend as far south as the Gulf Coast and Florida Peninsula in the US.

The polar vortex primarily affects countries situated in the Northern Hemisphere’s mid to high latitudes. These regions are particularly susceptible to extreme cold weather events.

The polar vortex spins anticlockwise around the North Pole with wind speeds of about 155mph (250km/h).

There are two types of polar vortexes – tropospheric and stratospheric.