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Sub Zero Nights Ahead

We're bracing for another blast of arctic air here across southeast Wisconsin. So far this season, Milwaukee has experienced six nights below zero. Some locations already dropped below this morning.

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And all of us will slip into negative territory tonight and tomorrow night! To make matters worse, the winds will be biting! Wind chill factors will be approaching Advisory criteria overnight.  Here's the time frame for coldest chills ahead:

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A Wind Chill Advisory is issued when chills reach -20° or colder, even with just a light 5 mph wind. If it seems like wind chills used to be colder back in the day, well... you're right. Sort of, at least.

In late 2001, a group called Joint Action Group for temperature Indices was formed with the purpose of improving the old \"Wind Chill Temperature\" and to internationally upgrade and standardize the index for temperature extremes. Here's an article from the National Weather Service on this transition.

http://www.crh.noaa.gov/grb/new_windchill.php

A few highlights on the change include calculating wind speed at the height of a person's face, around five feet from the ground versus 33 feet (the standard anemometer height). The new formula also incorporates modern heat transfer theory (heat loss from the body to its surroundings, during windy days) as well as lowering the threshold for calm wind to 3 mph.

Take a look at the difference from the old and new way of calculating wind chill:

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Before the change, an air temperature of five degrees with a 30 mph used to equal -41° with the old wind chill index, but now the new formula puts that mark at -18°. So in a way, if you used to see more extreme numbers, that is why!

The wind chills across our area will stay below zero now through Friday morning. Keep in mind, frostbite can occur in just thirty minutes with wind chill values at -18° or colder.

Bundle up and stay warm! Highs will recover and approach 30° by Saturday.

I'm meteorologist Rebecca Schuld 

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