Why ice is harder on schools than snow
Snow can be plowed, shoveled and driven through slowly. Ice takes away grip entirely. A school bus is heavy, stops slowly and needs traction on hills, curves and intersections. A glaze of ice on the road turns a normal route into a risk, and the same glaze coats sidewalks, parking lots, bus steps and school entrances.
Ice also does damage that snow does not. Tree limbs and power lines can sag or snap under frozen weight, causing outages and blocked roads. A district that is already worried about getting buses out may also have to think about schools with no power or heat.
Freezing rain, sleet and black ice
- Freezing rain
- Rain that falls as liquid and freezes on contact with surfaces at or below freezing. It builds a smooth, clear coating and is the most disruptive winter precipitation for transportation.
- Sleet
- Small ice pellets that bounce on impact. Sleet makes roads slick and can accumulate like gritty snow, but usually causes less glazing than freezing rain.
- Freezing drizzle
- Very fine drops that freeze on contact. Amounts are tiny, but even a thin film can make roads dangerous.
- Black ice
- A thin, nearly transparent layer of ice on pavement that is hard to see. It often forms when melted snow refreezes overnight or when temperatures fall after a wet evening.
Timing and refreezing: the morning risk
Ice is as much about timing as totals. Rain that falls on warm roads in the evening may be harmless until temperatures drop after midnight and puddles and wet pavement freeze. A forecast that shows temperatures dipping to or below freezing right before sunrise is a warning sign for the morning bus run.
Road salt also becomes less effective as temperatures fall well below freezing, so treated roads can still be slick on very cold mornings. Crews may also be stretched across a whole region during a widespread ice event.
How the calculator treats ice
This snow day calculator checks the hourly forecast for freezing rain and freezing drizzle between 6 PM the evening before and the end of the morning commute. It adds points for the number of icy hours and for the amount of precipitation that falls during them, with a cap so extreme totals do not break the scale. If freezing precipitation is forecast during the 5 to 9 AM bus run, it adds extra points for timing.
The result is that even a light event can push your snow day chance well above what snowfall alone would suggest. The breakdown under your result shows an "Freezing rain / ice" bar so you can see how much it contributed.
Winter weather alerts worth watching
In the United States, the National Weather Service issues ice storm warnings, winter storm warnings and winter weather advisories for freezing rain and other wintry conditions. Other countries have their own alert systems, so look for your national weather service's wording. As a rule, any warning that mentions ice accumulation overnight or through the morning should make you take a snow day chance seriously.
Staying safe on ice days
- Stay off the roads if you can. If you must drive, slow down, leave extra distance and avoid sudden braking.
- Watch your step on sidewalks, porches and parking lots. Walk slowly in short strides, and treat shiny wet-looking pavement as ice.
- Stay well away from downed power lines and anything touching them.
- Charge phones and prepare flashlights in case the power goes out.
- Do not use grills, generators or camp stoves indoors, since they can produce carbon monoxide.
Ready to check your own forecast? Try the snow day calculator.