Query: “During the sunless winter, a heatwave raised concerns that the polar vortex may be eroding.

Relevant passages:
Stratosphere <=SEP=> The stratosphere ( -LSB- ˈstrætəˌsfɪər , _ - toʊ - -RSB- ) is the second major layer of Earth 's atmosphere , just above the troposphere , and below the mesosphere . About 20 % of the atmosphere 's mass is contained in the stratosphere . The stratosphere is stratified in temperature , with warmer layers higher and cooler layers closer to the Earth . The increase of temperature with altitude is a result of the absorption of the Sun 's ultraviolet radiation by the ozone . This is in contrast to the troposphere , near the Earth 's surface , where temperatures decreases with altitude . The border between the troposphere and stratosphere , the tropopause , marks where this temperature inversion begins . Near the equator , the stratosphere starts at 18 km ; at mid latitudes , it starts at 10 - and ends at 50 km ; at the poles , it starts at about 8 km . Temperatures vary within the stratosphere with the seasons , in particular with the polar night ( winter ) . The greatest variation of temperature takes place over the poles in the lower stratosphere ; those variations are largely steady at lower latitudes and higher altitudes .

Winter 1985 cold wave <=SEP=> The Winter 1985 cold wave was a meteorological event , the result of the shifting of the polar vortex further south than is normally seen . Blocked from its normal movement , polar air from the north pushed into nearly every section of the eastern half of the United States and Canada , shattering record lows in a number of areas . The event was preceded by unusually warm weather in the eastern U.S. in December 1984 , suggesting that there was a build-up of cold air that was suddenly released from the Arctic , a meteorological event known as a Mobile Polar High , a weather process identified by Professor Marcel Leroux .

2013–14 North American winter <=SEP=> The 2013 -- 14 North American winter refers to winter as it occurred across the continent from late 2013 through early 2014 . While there is no well-agreed-upon date used to indicate the start of winter in the Northern Hemisphere , there are two definitions of winter which may be used . Based on the astronomical definition , winter begins at the winter solstice , which in 2013 occurred on December 21 , and ends at the March equinox , which in 2014 occurred on March 20 . Based on the meteorological definition , the first day of winter is December 1 and the last day February 28 . Both definitions involve a period of approximately three months , with some variability .  __ TOC __

Early 2014 North American cold wave <=SEP=> The 2014 North American cold wave was an extreme weather event that extended through the late winter months of the 2013 -- 2014 winter season , and was also part of an unusually cold winter affecting parts of Canada and parts of the north-central and upper eastern United States . The event occurred in early 2014 and was caused by a southward shift of the North Polar Vortex . Record-low temperatures also extended well into March .   On January 2 , an Arctic cold front initially associated with a nor'easter tracked across Canada and the United States , resulting in heavy snowfall . Temperatures fell to unprecedented levels , and low temperature records were broken across the United States . Business , school , and road closures were common , as well as mass flight cancellations . Altogether , more than 200 million people were affected , in an area ranging from the Rocky Mountains to the Atlantic Ocean and extending south to include roughly 187 million residents of the Continental United States .

Polar vortex <=SEP=> A polar vortex is an upper level low-pressure area lying near the Earth 's pole . There are two polar vortices in the Earth 's atmosphere , which overlie the North , and South Poles . Each polar vortex is a persistent , large-scale , low pressure zone that rotates counter-clockwise at the North Pole ( called a cyclone ) , and clockwise at the South Pole . The bases of the two polar vortices are located in the middle and upper troposphere and extend into the stratosphere . Beneath that lies a large mass of cold , dense arctic air . The vortices weaken and strengthen from year to year . When the vortex of the arctic is strong it is well defined , there is a single vortex and the arctic air is well contained ; when weaker , which it generally is , it will break into two or more vortices ; when very weak , the flow of arctic air becomes more disorganized and masses of cold arctic air can push equatorward , bringing with it a rapid and sharp temperature drop . The interface between the cold dry air mass of the pole and the warm moist air mass further south defines the location of the polar front . The polar front is centered , roughly at 60 ° latitude . A polar vortex strengthens in the winter and weakens in the summer due to its dependence on the temperature difference between the equator and the poles . The vortices span less than 1,000 kilometers ( 620 miles ) in diameter within which they rotate counter-clockwise in the Northern Hemisphere , and in a clockwise fashion in the Southern Hemisphere . As with other cyclones , their rotation is driven by the Coriolis effect .   When the polar vortex is strong , there is a single vortex with a jet stream that is `` well constrained '' near the polar front . When the northern vortex weakens , it separates into two or more vortices , the strongest of which are near Baffin Island , Canada and the other over northeast Siberia . The Antarctic vortex of the Southern Hemisphere is a single low pressure zone that is found near the edge of the Ross ice shelf near 160 west longitude . When the polar vortex is strong , the mid-latitude Westerlies ( winds at the surface level between 30 ° and 60 ° latitude from the west ) increase in strength and are persistent . When the polar vortex is weak , high pressure zones of the mid latitudes may push poleward , moving the polar vortex , jet stream , and polar front equatorward . The jet stream is seen to `` buckle '' and deviate south . This rapidly brings cold dry air into contact with the warm , moist air of the mid latitudes , resulting in a rapid and dramatic change of weather known as a `` cold snap '' .   Ozone depletion occurs within the polar vortices -- particularly over the Southern Hemisphere -- reaching a maximum depletion in the spring .