• Extreme cold wave has spread through US Midwest, caused by blast of arctic air which is known as 'polar vortex' event.

According to US National Oceanic and Atmospheric Administration (NOAA).

(1) It is low pressure area, surrounded at both polar regions.

(2) When polar vortex is strong it remains at the polar region.

(3) When it is weak, it sends cold air south.

(4) According to NOAA, many times during winter polar vortex expands as the "breaking off" of the part of vortex.

(5) Strong vortex keeps jet stream in circular path, so that cold air at the top and warm air at bottom.

• Global temperature has risen by 0.8 degree Celsius since 1880.
• The Arctic has warmed over twice the average.
• Temperature difference between north pole and relevant southern area has reduced.
• Jet streams travelling upward causes main vortex to disrupts.