What wind chill is
The National Weather Service defines wind chill with a formula in degrees Fahrenheit and miles per hour: 35.74 + 0.6215T − 35.75V^0.16 + 0.4275T·V^0.16, where T is the air temperature and V is the wind speed. The NWS says wind chill temperature is only defined for temperatures at or below 50 °F and wind speeds above 3 mph. It gives one example: a wind chill of −19 °F, from 0 °F air and a 15 mph wind, can freeze exposed skin in 30 minutes. Our wind chill calculator uses exactly this formula.
NWS cold alerts changed in 2024
On October 1, 2024 the National Weather Service replaced the Wind Chill Watch, Warning and Advisory with the Extreme Cold Watch, Extreme Cold Warning and Cold Weather Advisory. The change reflects that dangerous cold does not need wind. The thresholds are still set by each local office. Omaha lists an Extreme Cold Warning for wind chill or actual temperatures of minus 30 °F or colder and a Cold Weather Advisory for minus 20 to minus 29 °F. Raleigh issues its warning for apparent temperatures of 0 to −5 °F or colder.
Why there is no school closing number
A Minnesota story on cold closures found no common cutoff. St. Paul Public Schools said it has no magic number, and that it uses a NOAA wind chill chart to estimate how long exposed skin would take to reach frostbite. The same story reports that one district, Atwater-Cosmos-Grove City, started school two hours late at a wind chill of −30. The factors the story lists are bus ridership, whether buses will start reliably, winter clothing and road conditions, plus the effect on families and learning time.
Delay or closure for cold
A delay can sometimes help on a cold morning by moving the wait at the bus stop later and into daylight. It does not help when the cold lasts all day or the buses will not start. Stoughton’s page notes that NWS Extreme Cold Watches and Warnings are issued when the forecast calls for −30 °F wind chills, and it does not say that wind chill alone triggers a closure.
How the calculator treats cold
The calculator adds cold points only when wind chill reaches one of three steps that depend on how snowy your area usually is: for “often” areas 15, 25 and 35 °F below zero, for “sometimes” areas 0, 15 and 25 below, and for “rarely” areas 10 above, 0 and 10 below. These steps are our assumptions, loosely informed by the NWS examples above, and are not district rules.
Sources and further reading
- National Weather Service: Wind chill chart and formula
- National Weather Service: Cold watch, warning and advisory products revised (October 2024)
- National Weather Service Omaha: Winter products and criteria
- National Weather Service Raleigh: Weather criteria (winter storm, advisory and extreme cold)
- MPR News (2014): When temps plummet, how do school districts decide when to close?
- Stoughton Area School District (Wisconsin): Inclement weather
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