•  Weather experts are predicting an extremely cold January and February for the northeastern United States, much of northern Europe and parts of Asia. The reason being given is the polar vortex.
• In this decade, the polar vortex has also been blamed on extremely cold weather in the United States in 2014 and the infamous ‘Beast from the East’, the blast of cold weather that blew from Siberia towards western Europe and the UK in February and March of 2018.

What Is the Polar Vortex?

• The polar vortex is part of a low-pressure system, and just like other lows this one spins as winds blow in toward the lower pressure at the centre of the system.
• Due to Earth's spin and a phenomenon called the Coriolis effect, these systems spin counter clockwise in the Northern Hemisphere and clockwise in the Southern Hemisphere.
• As the North Pole tilts away from the sun in the winter months, bathing it in 24 hours of darkness, temperature gradients form between higher and lower latitudes. These gradients drive winds, leading to a pattern of low pressure that circulates counter clockwise around the pole.
• The polar vortex is technically in the stratosphere, the middle layer of Earth's atmosphere. That area of low pressure is about 20 miles (32 kilometers) above the Earth's surface, Furtado said. The vortex affects daily weather when it drops down into the troposphere, the lowest layer of the atmosphere.

Strong Polar Vortex

• Strong is the more common state of the polar vortex. When the polar vortex is strong, this creates strong low pressure in the Arctic region. Because of the pressure difference between the Arctic and mid-latitudes, air flows into low pressure and this confines the cold air to high latitudes closer to the Arctic. Therefore it is often mild across the Eastern US, Europe and East Asia during winters when the polar vortex is strong.
• During strong polar vortex, the air flow is fast and in a direction from west to east.
• Low pressure in the Arctic region is referred to as the positive phase of the Arctic Oscillation (AO), which is also know as the North Atlantic Oscillation (NAO).

Weak Polar Vortex

• When the polar vortex is weak or “perturbed”, the flow of air is weaker and meanders north and south (rather than west to east). This allows a redistribution of air masses where cold air from the Arctic spills into the mid-latitudes and warm air from the subtropics is carried into the Arctic. This mixing of air masses also favors more storms and snow in the mid-latitudes.
• During a weak polar vortex, high pressure occurs in the Arctic region and is referred to as the negative phase of the Arctic Oscillation (AO). Air flows away from the high pressure Arctic. The north to south direction of the polar vortex carries cold Arctic air into the mid-latitudes of Eastern US, Europe and East Asia. Therefore it is cold across the Eastern US, Europe and East Asia during winters when the polar vortex is weak.