Washington, Feb. 1 -- The U.S. Midwest has been gripped by the lowest temperatures it has seen in years. An unusually cold Arctic air mass, called a polar vortex, is responsible for the severe temperatures, which in many areas have plunged well below 0 degrees Fahrenheit (-18 degrees Celsius). NASAs Atmospheric Infrared Sounder (AIRS) instrument aboard the Aqua satellite captured the polar vortex as it moved southward from central Canada into the U.S. Midwest from Jan. 20 through Jan. 29, read the NASA website. The lowest temperatures are shown in purple and blue and range from -40 degrees Fahrenheit (also -40 degrees Celsius) to -10 degrees Fahrenheit (-23 degrees Celsius). As the data series progresses, you can see how the coldest purple ...