Extreme cold and school closures

Some of the quietest closure mornings are cloudless. No snow is falling. The problem is the air. Prairie and northern boards have long treated extreme cold as a reason to cancel buses and, often, to close schools, because a child waiting at a stop or a diesel fleet that will not start is a safety problem without a flake of new snow. Southern Ontario sees those mornings less often, which is why the model is not equally sensitive everywhere.

What we measure

The forecast field we use is apparent temperature during the commute hours, 5 a.m. to 8 a.m. local time. That is the "feels like" number, combining cold and wind, which is the closest public stand-in for wind chill at the stop. We do not add cold points from the overnight minimum if the morning itself has recovered. A chinook that arrives before the bell is allowed to shrink the score. A cold snap that deepens at dawn is allowed to raise it.

High cold sensitivity, used for Alberta, Saskatchewan, Manitoba, and for saved cities including Calgary, Edmonton, Winnipeg, Ottawa, Thunder Bay, Sudbury, Sault Ste. Marie, and Quebec City, starts adding points at less extreme values and adds more of them. Standard sensitivity waits for a harsher morning. Below −30°C apparent, the bus score gets a small extra bump on top, because equipment and wait times fail before a building does. The exact steps are on the methodology page, generated from the constants in the code.

What we will not pretend

We will not write "boards close at −40" as if it were a statute. Some divisions publish a threshold, and that threshold can change. If yours does, their page is the rule and our curve is a hint that the morning has entered the range where those rules usually matter. Dressing a child properly for a cold stop is necessary on plenty of days that are not closure days. It is not a substitute for the board's call, and the board's call is not a substitute for a coat.

Clear-sky cold also fools people who only watch the snow total. If the closure score is elevated and the snow line says 0.0 cm, read the wind-chill factor before you decide the tool is broken. Compare it with the official forecast for the same hours. If those disagree, believe the official forecast and treat our number as stale.

Cold mornings are also when a remote rural route is least like a downtown walk. The rural setting exists for that gap. Use it if it describes the trip, including on a day when the sky is blue.