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What is an ice day?
An ice day is a school closure, delay or bus cancellation caused by ice rather than snow. The roads may carry no snow at all; the danger is a glaze on pavement, sidewalks and bus steps that salt and plows handle poorly. Ice days are common along the freezing line — the US South and Mid-Atlantic, the Ohio Valley, and in Canada across southern Ontario, Quebec and the Maritimes.
Enter the freezing rain, any plain rain, the morning temperature and the overnight low from your forecast. The calculator shows the ice pressure, whether conditions are freezing, a refreeze risk estimate and the ice-day chance. Snow is deliberately left out, so you can see what the ice alone does. It does not fetch live weather or closure announcements.
Kinds of winter ice
- Freezing rain — rain that freezes on contact with surfaces at or below 0 °C. The most dangerous kind: it builds a clear glaze.
- Freezing drizzle — the same process with tiny drops. Amounts are small, but even a light glaze makes untreated roads slippery.
- Ice pellets (sleet) — drops that freeze before landing. Choose “Sleet / mixed”.
- Refreeze or flash freeze — plain rain that freezes when the temperature drops after dark. Enter the rain and the overnight low.
- Black ice — a thin, transparent layer that looks like wet pavement. Describe it under “Morning commute conditions” as icy roads.
How ice is counted
- Ice accretion you enter counts in full.
- Rain counts as ice only for a freezing condition: all of it for “Freezing rain”, “Ice”, “Snow + ice” and “Rain + freezing temperature”, half for “Sleet / mixed”, none for plain “Rain”.
- Refreeze: when at least 1 mm of rain (not already counted as ice) is followed by an overnight low of −1 °C or colder, 25% of that rain (at most 2 mm) is counted as possible ice.
The ice points in the model, before timing, roads and settings:
| Freezing precipitation | Ice points |
|---|---|
| 0.2 mm (0.01 in) | 8 |
| 0.5 mm (0.02 in) | 15 |
| 2 mm (0.08 in) | 30 |
| 6 mm (0.24 in) | 50 |
| 12 mm (0.47 in) | 65 |
How much ice closes school?
There is no fixed amount, and each district or board decides. As a guide to how forecasters see it: the US National Weather Service issues its Ice Storm Warning in many areas for about a quarter inch (6 mm) of ice or more. Schools often react to far less — a thin glaze at bus time can be enough to stop buses.
Examples for a typical suburban school: 4 mm of freezing rain overnight at −1 °C with icy roads gives a weather pressure of 52.6, a 42% ice-day chance and a 42% chance of a delay or cancelled buses. 1 mm with roads that are only wet by morning gives 1% and 6%.
Why the timing of ice matters
Freezing rain that ends by midnight gives salt trucks the night to work; freezing rain at 6 a.m. meets the buses. Temperature is the other clock: ice that forms at −1 °C can melt by mid-morning, while ice followed by a deep freeze stays for days. Place the storm on the timeline and describe the roads, and both count.
Ice days in Canada
In Ontario and the Maritimes an ice morning usually means buses cancelled, not schools closed. For board rules — Toronto, Ottawa, Hamilton, Halifax, PEI and more — use the Canadian snow day calculator with the “Freezing rain” condition.
Limitations
No calculator knows the temperature of a particular road, whether it was pre-treated, or how much of the rain ran off before freezing. Freezing rain forecasts are among the hardest in winter weather: a one-degree error in the warm layer aloft turns ice into rain or sleet. Treat the estimate as a reason to watch the official forecast and wait for your district’s announcement.
Frequently asked questions
What is an ice day?
An ice day is a school closure or delay caused by ice rather than snow — usually freezing rain or freezing drizzle glazing roads, sidewalks and bus stops, or rain that freezes when the temperature drops overnight.
How much ice does it take to close school?
There is no fixed amount, and districts decide locally. A thin glaze can be enough to stop buses because salt and plows work poorly on ice. For reference, the US National Weather Service issues its Ice Storm Warning for about a quarter inch (6 mm) of ice or more in many areas.
Does this measure ice on the roads?
No. It uses the figures you enter. Ice accretion you type counts in full; plain rain counts as ice only if you choose a freezing condition, or partly when the overnight low is cold enough for it to refreeze.
What is a flash freeze?
A flash freeze is when wet roads freeze quickly as the temperature drops below 0 °C, often after rain the evening before. The calculator flags it when at least 1 mm of rain (not already counted as ice) is followed by an overnight low of −1 °C or colder, 25% of that rain (at most 2 mm) is counted as possible ice.
Does snow count on the ice day calculator?
No — this page scores ice, cold and road conditions only, so you can see what the ice alone does. For snow and ice together, use the snow day calculator with the “Snow + ice” condition.
Does it fetch the forecast?
No. This calculator uses the weather conditions you enter. It does not fetch live weather, alerts or school closure announcements.
Is freezing rain worse than snow for schools?
Often, yes. Buses cannot stop safely on glaze, sidewalks and parking lots are hard to treat, and ice can bring down branches and power lines. A small amount of freezing rain can close schools that would stay open in several inches of snow.
Who decides if school closes for ice?
Your school district or board — and in much of Canada, the student transportation consortium for buses. Weather services forecast the ice but do not close schools.