With the polar vortex making its return this week in the U.S., many schools will likely face the decision to close or not.
In Michigan, temps won't reach above zero on Wednesday, with wind chills likely to be near -35 degrees for multiple days.
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So, how cold does it have to be for schools to close?
The rule most school districts follow is: if the wind chill is -25 or lower, schools will close.
However, many other variables can force this rule to be flexible. For example, if there's snow or ice with the wind chill.
The -25 degree benchmark is likely because of frostbite concerns. Frostbite can occur starting around -20 degrees.
Track: Latest school closings
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