Extreme low temperatures in the United States due to Polar vortex
Fifty states of the US recorded sub-freezing temperatures in January 2014. The extreme cold wave created shutdown of services. The shivering cold also caused power shortage and adversely impacted daily routine. These low temperatures were not experienced in the region, since the last 20 years. Experts explained that Polar Vortex could be the main culprit behind this brutal cold.
Polar Vortex is a low pressure, upper level (primarily in stratosphere) wind system rotating in anticlockwise direction. Wind Velocity of this system is more than 100 miles per hour. Usually, this vortex remains confined to the Arctic region. As and when the Polar Vortex weakens or splits, these cold winds move out of the Arctic and move towards South. The weakening of the Polar Vortex is attributed to the change in the intensity of the upper level winds that build Polar Vortex.
Polar Vortex may occur anywhere in the Northern hemisphere and is worst in winter, due to more temperature contrast between the Polar Regions and the mid-latitudes.
Polar vortex is often misunderstood for hurricanes or tornadoes. But, these are different phenomena. The areas covered under the influence of the polar vortex are much wider than hurricanes or tornadoes. Polar Vortex lasts for more days than hurricanes or tornado.
As the polar vortex weakened in the arctic region, this weakening forced the cold air to move towards the areas south of the arctic region. Thus, this wind covered entire North America bringing record low temperatures in this region. However, Polar Vortex is touted as only a possible reason. There could be one or more reasons behind the low temperatures in US.
Whether polar vortex itself is an outcome of global warming or not is still a matter of debate.
Image credit: National Oceanic and Atmospheric Administration, hosted at Wikipedia