CCTV9英语新闻：我们记录的最低气温（在线收听）

Now let's get some of the science behind the cold. What is driving these temperatures and how long is it expected to last. We bring in meteorologist Alex Wilson of US' Weather Channel.

"As even talking about this unusual cold, something you've been hearing a lot is the polar vortex. And how it’s responsible for this cold. Polar vortex is basically, this is digging much further south than usual, so what we're seeing this frigid temperatures is actually air originated from Arctic, that's why it is so cold. In addition to the cold, wind chills. That's why we are dealing with this historical cold, a lot of spots dealing with temperatures over 20 degrees below average," Wilson says.