The coldest blast of air this winter, and perhaps in years, is gripping a large part of the northern U.S.   Wind Chill alerts are in effect through Thursday from the Dakotas, down through Missouri, and over to Pennsylvania.  When the air temperature and wind values are combined, it will feel like 25 to 60 below zero!

Overnight lows into Wednesday morning will be brutal.  28 below zero in Minneapolis.  21 below zero in Chicago.  And 22 below zero in Des Moines.

But when the wind blows, the wind chill values will kick in.  And the resulting windchill values will be unbelievably cold.  In Duluth, it will feel like 59 below zero!  Chicago will feel like negative 47 degrees.  Buffalo will feel like 22 below zero.  Frostbite can set it in a few minutes under such conditions.

The temps and wind chills will be so brutal, that many school districts will be closed on Wednesday.  Public schools in Buffalo, Chicago, Columbus, Detroit, Des Moines, Fargo, and Grand Rapids will all be closed on Wednesday.  And with similar conditions expected on Thursday, many of these school districts may opt to remain closed for another day.

When determining school closures in the winter, many factors are taken into account.  Usually, a team of school officials will confer on the following variables:

• Amount and Type of Precipitation
• Temperature/Wind Chill
• Visibility
• Status of Clearing Roads
• Status of Clearing Sidewalks
• Refreeze of Melted Snow/Ice from Previous Day
• Blowing Snow and Continued Snow in the Forecast

With regard to temperatures and wind chill values, the key factor is whether or not it's safe for students to wait at the bus stops in the morning.  School districts have different criteria when it comes to determining this.  Many districts don't have a hard-set temperature cutoff for delaying or cancelling school.  But as a general rule, if students could get frostbite by standing outside for ten minutes, school would likely be cancelled.

And school districts in rural areas have different factors to consider versus school districts in urban areas.  For example, in rural areas, many students have to walk farther to bus stops and wait at these locations longer than students in a city.  So weather can impact students in rural areas much more. Many districts have their delay and cancellation criteria outlined online.  Parents and students can review school policy to get a better idea of how decisions are made. For WeatherNation:  Meteorologist Matt Monroe