Weather experts say extreme cold in many parts of the United States this winter has been driven by a polar vortex . But what is this weather system and how does it work ?
The National Oceanic and Atmospheric Administration (NOAA ) describes a polar vortex as strong westerly winds that form each winter in the atmosphere above the North Pole .
These cold winds move about 16 to 48 kilometers above the surface in the Arctic area in the part of the atmosphere called the stratosphere .
At times , the polar vortex can weaken , causing masses of cold air to break off and move south . A polar vortex can also push warmer air in northerly directions .
The Associated Press (AP ) reports that several vortex events have affected parts of the U .S . this winter . Experts say weather forces in the Arctic have combined to push colder air to the U .S . and Europe .
Temperature readings that take into account the effects of wind , called windchill readings , fell to minus 51 degrees Celsius in parts of the U .S . Midwest . Meteorologists say windchill readings are colder than air temperature readings .
The U .S . National Weather Service (NWS ) warned people in affected areas that such low temperatures can cause frostbite and hypothermia if skin is not protected from the cold . Weather officials also warned that long -term cold temperatures can cause water pipes in or around homes to freeze and burst .
The NWS noted that almost every U .S . state – except for Hawaii , California and Florida – is expected to report low windchill temperatures . The coldest temperatures were expected in the midwestern states of Kansas , Nebraska and Missouri .
In an explanation on its website , NOAA states that the polar vortex always surrounds both of Earth’s poles . The systems strengthen in the winter and weaken in the summer .
Every year or so , the polar vortex becomes weaker . This can cause the system to be “pushed off the pole or split into two .” This can lead to cold air at the surface being pushed southward and warm air being pushed to the north .
NOAA says its weather models are predicting low temperatures for states including Oklahoma , Colorado , Nebraska , Missouri , Illinois and Iowa . Other states likely to have very low temperatures are Wyoming , Montana , North Dakota , South Dakota , Minnesota , Wisconsin and Michigan .
The agency said some storms could bring heavy snow or flooding during that time .
I’m Bryan Lynn .
Bryan Lynn wrote this story for VOA Learning English , based on reports from The Associated Press , Reuters , Agence France -Presse , NOAA and NASA .
northerly –adj. in, to or from the north
frostbite – n. a condition that results in extreme cold causing injuries to body parts such as fingers and toes
hypothermia – n. a serious sickness caused when a person’s body becomes too cold
windchill – n. a temperature reading that takes into account the cooling effect that wind has on the human body and that is lower than the actual air temperature
◆ ◆ ◆
Word highlighting is estimated until aligned timings are added.