air in two years. and for most of the country, it's a sudden and drastic change. in cedar rapids, iowa, a balmy 51 degrees saturday. less than 48 hours later, it was 2 degrees below zero, a temperature swing of more than 53 degrees. >> the wind is just ripping. it makes it ten times worse than it should be. >> reporter: this was chicago a month ago. you won't see any shorts on the street. chicago dropped below zero for the first time in almost two years. and a frozen lake michigan left even this poor dog stranded. this blast came in so quickly, it brought blinding snow. in michigan, whiteout conditions. >> you can barely see in front of you. >> reporter: if you're working outside like these construction workers, there's a real danger of hypothermia, frostbite, even cold air asthma. medical experts tell us breathing freezing air can trigger lung spasms. >> i can't feel my nose or