The current cold is due to a weather phenomenon called a polar vortex. Although the name might bring to mind tornadoes and thunderstorms, a polar vortex involves neither. Rather, a polar vortex is essentially a region of cold air that typically stays over the poles, one over the north pole and one over the south pole. (The USA is affected by the one over the north pole.)

In meteorology (study of the atmosphere and weather), "vortex" actually refers to the air currents (a particular set of jet streams) which circulate around the poles and hold the cold air in place. Note that the formation of a polar vortex is not the cause of the unusual cold in the northeast. These vortices are always over the poles and it is only when a portion is pushed down to lower latitudes that the US experiences the surge of cold, Arctic air. This occurs when the containing vortex weakens, as this allows warm air to move toward the pole and cold air to move toward the equator.

Recently, the northeastern and upper mid-western regions of the US have been in what we call a polar vortex. According to the government's official website, a polar vortex is an area of low air pressure and cold temperature surrounding either the north or south pole (there are two polar vortexes).

The reason that it is called a vortex is that the air circulates the poles. When the vortex is stable, the cold air is somewhat confined to a smaller area around the pole. When the vortex becomes less stable, the cold air expands outward. Take the northern polar vortex, for instance: Itis less stable during the winter (in the Northern Hemisphere), and will therefore expand and send cold air south to the regions closest to the North Pole in the Northern Hemisphere. This cold air from the vortex causes sharp temperature drops in the northern United States.

The extremely cold weather seen in the northeast right now is due to the polar vortex. The polar vortex is a low pressure area that usually stays in the North Pole. However, during winter, it expands and the low pressure area creeps further down, bringing cold, rain, and snow to Canada and the upper parts of the US. This year, the polar vortex weakened and the jet stream shifted, pushed the polar vortex even lower down and caused the unusually cold temperatures in the northeast.

The northeast got hit by a cold front that was positioned in such a way that it is north of the polar/tropical airmass boundary, which means that it stays cold and does not warm up. This is a familiar weather phenomenon and happens fairly regularly. This will change when the airmass boundary moves north due either to the jet stream pushing a warm front over it, or as the seasons change.

It has been suggested that these extremely cold winters are due, ironically, to global warming, since warmer temperatures globally could cause the airmasses to move differently, which would result in some areas getting colder even as the world as a whole became warmer. This is possible, but far from certain. Climate change is really only visible over decades. On a year-to-year basis, the differences in weather are much larger than the long-term effects of climate change, and this cold snap could be just weather.