How the Halk-Index works

In short

6 rules are tested against the weather in your municipality. Each rule that applies adds its points, the points are summed and the total is kept within 0–100. The index is our own estimate, not an official warning from SMHI (the Swedish weather service) or Trafikverket (the Swedish Transport Administration). Always follow official warnings and what you see on the road.

The six rules

• Damp road surface below zero (+40). Road temperature below 0 °C and the surface damp, wet or snow-covered, or ice measured on the road by a Trafikverket road weather station. Water on a road below zero freezes. It is the most common way to black ice, so it weighs the most.
• Precipitation and sub-zero air (+35). Precipitation in the past 24 hours (at least 0.2 mm) and air temperature below 0 °C. Rain or wet snow followed by frost gives icy roads, and freezing rain turns to ice the moment it lands.
• Cold road under mild, humid air (+35). Measured by a Trafikverket road weather station: road at 0 °C or colder, air above 0 °C and humidity above 85 %. Only tested where there is a station, never on an estimate. The road has cooled below zero while the air above it is mild and humid. The moisture can freeze straight onto the asphalt as a thin, clear film of ice, while the car thermometer shows plus degrees.
• Trafikverket reports slippery roads nearby (+35). Trafikverket’s road condition report for a state road within 25 km gives ice, snow, slush or frost, or warns of slippery roads. Reports older than a day do not count. It is an observation of the road, not a forecast. If the road authority reports ice nearby, there can be ice where you drive even when the weather in town does not suggest it.
• Road colder than the dew point (+20). Road temperature below the dew point and below +1 °C. The air then deposits moisture on the road, as frost or a thin film of ice, without any rain.
• Wind dries the road (−5). Wind above 8 m/s and a wet road. A fresh wind dries a wet road faster, leaving less water to freeze.

The positive rules can add up to 165 together, but the index stops at 100.

The levels

• 0–30: Low. Little in the weather suggests ice. Bridges, shade and leftover ice can still be slippery.
• 31–60: Elevated. One of the heavy rules applies: ice may occur in patches.
• 61–100: High. Several signs of ice at once: expect slippery roads.

Each level always has its own icon and name, so it never depends on colour alone.

Where the numbers come from

• Weather forecast: SMHI’s open forecast and MET Norway’s (the source behind yr.no), blended, for the municipality’s largest town. Precipitation in the past day comes from SMHI’s analysis.
• Road surface: measured by Trafikverket’s nearest road weather station (VViS) when there is one within 20 km with a fresh reading. Most stations measure the temperature only; where nothing measures the surface, it is estimated from the weather.
• Road reports: Trafikverket’s own road condition reports for state roads within 25 km.

Data licences: SMHI and MET Norway CC BY 4.0, Trafikverket CC0. The index is recalculated every half hour. A few large mountain municipalities also show extra forecast points for their resorts, each with its own index.

What it cannot tell you

The index describes one point per municipality, not every road. Hills, bridges, shade and forest can differ from the town. It does not know whether a road has been salted or ploughed. How often it agrees with Trafikverket’s reports is published on our accuracy page, and every change to the formula is logged in the changelog (both in Swedish).

See today’s index for your municipality on the ice risk pages, or the full method in Swedish.