Snow is a nuisance, but black ice kills. An invisible layer of ice on tarmac multiplies braking distance by ten. It forms when liquid water, rain or drizzle, meets a surface at zero degrees or below, or when vapour in the air freezes directly on a road colder than the air.
Why this indicator matters
The trap is the air temperature: the air can show +1 °C while the ground is at -2 °C, especially after a clear night, on bridges and in shaded areas, which cool from underneath. Freezing rain, for its part, is supercooled rain that freezes on impact, even on ground that is barely cold.
For snow, the total decides: one centimetre melts on warm ground, five centimetres disrupt traffic, fifteen block it. The start time matters just as much, because an evening fall makes compacted snow freeze overnight.
How to read it
The box adds up the snow expected over forty-eight hours, in centimetres, and looks for hours of likely black ice: precipitation with an air temperature between -3 and +1 °C, or ground below zero, or a freezing rain or drizzle code in the forecast.
The sentence gives the time range of the snowfall or black ice and the driving advice that goes with it. It stays silent in dry weather and above zero, day or night, rather than repeating advice that does not apply.
The thresholds used on this site
Every sentence in the “Today” box comes from a numerical rule. Here are the rules for this indicator, exactly as they are applied in the code.
- The indicator speaks up from 1 cm of snow over 48 h; notable at 5 cm; heavy at 15 cm.
- Black ice likely: precipitation of at least 0.1 mm per hour with air between −3 °C and 1 °C, or ground below 0 °C.
- Freezing drizzle or freezing rain codes in the forecast also trigger the indicator.
A worked example: Toronto
In Toronto, with the forecasts of 2026-11-02 08:13, this indicator stays silent: none of the conditions above is met at the moment. The accumulated snow over 48 h is 0.0 cm.
Its limits
Ground temperature is a model forecast: a bridge, a tunnel or a forest road will depart from the average. Maintenance crews grit before the event, which can prevent black ice where the calculation predicts it. Snow totals depend strongly on altitude, and a town's point forecast says nothing about the state of a neighbouring pass.
Frequently asked questions
Why is black ice possible when it is above zero?
Because the air temperature is measured at two metres and the ground can be colder, especially after a clear night. A bridge also cools from underneath.
What is freezing rain?
Rain whose drops stay liquid below zero and freeze on impact. It is rare, but within minutes it coats roads, wires and branches in a layer of ice.
How much snow disrupts traffic?
From just a few centimetres on cold ground, summer tyres lose their grip. Beyond five centimetres, slowdowns and stoppages become frequent.