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.
