The 9pm central time observations tonight (12/14/10) show 3 degrees with partly cloudy skies in Rockford, IL. In the mean-time, it's 24 degrees in Munising, MI... Some 300 miles to our north. The main reason it's warmer up there? It's snowing! Yes, in situations where an arctic airmass is sitting over the Midwest, clouds and snow can save your temperature from "falling through the floor!"
Of course, clouds act like a blanket at night, and inhibit radiational cooling. But if snow is falling at a given location, it can be warmer still because snow formation is an exothermic process. This means the creation of snow releases heat into the surrounding atmosphere. Here's a brief explanation why:
Snow forms when droplets of water vapor (in gaseous form, of course) condense onto a dust particle or some other microscopic material in the air. The water quickly goes from liquid to solid phase in temperatures below freezing. This phase change from liquid to solid actually releases a small amount of heat into the atmosphere.
Light snow is forecast to move into the Stateline Wednesday night and Thursday morning. Along with the snow, our high temperatures will warm into the low to mid-20s, and our lows will moderate into the teens on Wednesday night. This is still below average, but better than the teens for highs and sub-zero lows we've seen over the last 36 hours!
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