Weather experts say extreme cold in many parts of the United States this winter has been driven by a polar vortex.
气象专家表示，今年冬天美国许多地方的极端寒冷是由极地涡旋驱动的。
But what is this weather system and how does it work?
但是这个天气系统是什么，它是如何工作的呢？
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.
美国国家海洋和大气管理局将极地涡旋描述为每年冬天在北极上空大气中形成的强烈西风。
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.
这些冷风在北极地区地表以上大约16到48公里处移动，这部分大气被称为平流层。
At times, the polar vortex can weaken, causing masses of cold air to break off and move south.
有时，极地涡旋会减弱，导致大量冷空气中断并向南移动。
A polar vortex can also push warmer air in northerly directions.
极地涡旋也会将温暖的空气推向北方。
The Associated Press, AP, reports that several vortex events have affected parts of the U.S. this winter.
美联社报道，今年冬天，几场涡旋事件影响了美国部分地区。
Experts say weather forces in the Arctic have combined to push colder air to the U.S. and Europe.
专家表示，北极的天气力量共同将较冷的空气推向美国和欧洲。
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.
考虑到风的影响，被称为风寒读数的温度读数在美国中西部部分地区降至零下51摄氏度。
Meteorologists say windchill readings are colder than air temperature readings.
气象学家说风寒读数比气温读数低。
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.
美国国家气象局NWS警告受影响地区的人们，如果皮肤得不到防寒保护，这种低温会导致冻伤和体温过低。
Weather officials also warned that long-term cold temperatures can cause water pipes in or around homes to freeze and burst.
气象官员还警告说，长期的低温会导致家庭内部或周围的水管结冰和爆裂。
The NWS noted that almost every U.S. state, except for Hawaii, California, and Florida, is expected to report low windchill temperatures.
NWS指出，除了夏威夷、加利福尼亚和佛罗里达之外，美国几乎每个州都将报告低风寒温度。
The coldest temperatures were expected in the Midwestern states of Kansas, Nebraska, and Missouri.
最冷的温度预计在中西部的堪萨斯州、内布拉斯加州和密苏里州。
In an explanation on its website, NOAA states that the polar vortex always surrounds both of Earth's poles.
美国国家海洋和大气管理局在其网站上的解释中指出，极地涡旋总是围绕着地球的两极。
The systems strengthen in the winter and weaken in the summer.
这些系统在冬季增强，在夏季减弱。
Every year or so, the polar vortex becomes weaker.
每隔一年左右，极地涡旋就会减弱。
This can cause the system to be pushed off the pole or split into two.
这可能会导致系统被推离极点或一分为二。
This can lead to cold air at the surface being pushed southward and warm air being pushed to the north.
这会导致地表的冷空气被向南推动，而暖空气被推向北方。
NOAA says its weather models are predicting low temperatures for states including Oklahoma, Colorado, Nebraska, Missouri, Illinois, and Iowa.
美国国家海洋和大气管理局表示，其天气模型预测了俄克拉荷马州、科罗拉多州、内布拉斯加州、密苏里州、伊利诺伊州和爱荷华州等州的低温。
Other states likely to have very low temperatures are Wyoming, Montana, North Dakota, South Dakota, Minnesota, Wisconsin, and Michigan.
其他可能有非常低温度的州有怀俄明州、蒙大拿州、北达科他州、南达科他州、明尼苏达州、威斯康星州和密歇根州。
The agency said some storms could bring heavy snow or flooding during that time.
该机构表示，在此期间，一些风暴可能会带来大雪或洪水。
I'm Brian Lynn.
我是布莱恩·林恩。