CNN The Lead With Jake Tapper January 6, 2014 21:05:00
>> i'm very sorry that you can see a river steam in chicago in the winter. that's the difference between how cold the air is and how relatively warm the water is, which is probably about 34. there's a big enough difference that it looks like steam. arctic vortex, it's a low pressure dip in. the jet stream. it happens sometimes. there's a polar jet stream almost all the time. here's the north pole. it goes back and forth and keeps all the cold air bottled up here. until one of the troughs decides to do that. then all of a sudden all that cold air that's supposed to be up here gets released down...
Source: archive.org