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Submitted by Tim Buckley on Wed, 01/19/2011 - 11:00am.
As our cold weather has relented somewhat, the winter is just getting started way up north. The morning lows in parts of the Dakotas and the Upper Midwest were downright impressive as Arctic air is in full effect. Check out some of these numbers.
THAT's how bad it is during the winter in the nation's icebox! Widespread temperatures in the minus teens, and even minus twenties in the Northern Plains. The bad part is this isn't even that out of the ordinary. Try a place like Aberdeen, South Dakota whose morning low of -24 didn't come close to breaking the record for the date of -36.
But why - and how - does it get this cold? A combination of ideal conditions have to come into play before you can get temperatures this far off the charts.
In this case we have a large area of High Pressure which has slid south into the U.S. from Canada. High pressure is an important part of the equation as it provides a lot of the conditions needed for the cold. Most importantly, you need to have clear skies to have a chance at cooling off this much. Why?
At night, the Earth is constantly giving off heat into the atmosphere in the form of longwave raditiation. This is called radiational cooling. These waves leave the Earth's surface and then rise through the sky (heat rises!). On a clear night, the heat escapes unobstructed into the sky. But when clouds are present, some of this radiation is absorbed by the clouds and even reflected back toward the surface. This is why clouds keep us warmer at night. With high pressure, generally sinking air prevents clouds from forming so we have the clear skies needed to be cold.
As you can see - the coldest temperatures Wednesday morning were found where the skies were clear. Compare the -24 in Aberdeen to the +1 in Minneapolis. Clear skies vs. cloudy skies (and maybe some urban heat island too!).
Believe it or not, even a humid air mass will act in a similar way to cloud cover, with water vapor absorbing and reflecting some radiation at night. So - it's best to have little to no humidity in order to cool down most quickly. This is usually the case in the winter months, especially in this part of the country with high pressure.
Finally, light winds are a big key to getting as cold as possible. Windy conditions allow the air to mix throughout the lower levels of the atmosphere. At night, this mixing allows slightly warmer air above the surface to mix fully downward and keep the ground warmer that it would be on a calm night. In this case, the winds are extremely light to even calm across the Plains.
So in the end - a perfect storm for cold weather. If you've learned anything, it's that you probably don't want to live in this part of the country during the winter if you can't stomach the cold.
But you probably knew that already!
By: Tim Buckley