Ice on the roads develops as a result of cold and wet conditions.
Danger levels slippery roads
These warnings apply primarily to the road conditions, whilst differentiating between the following hazards:
- Slippery conditions due to packed snow: Snow that has frozen solid on the roads (up to 4 cm thick).
- Black ice: Wet roads which subsequently freeze over as a result of clear skies at night.
- Icy roads: Cold or cooled precipitation (freezing rain/drizzle/fog precipitation) that falls onto a road surface with a temperature below 0 degrees, thus freezing solid instantly.
Please note: warnings for slippery conditions due to ice or snow are not issued foraltitudes above 800 m.
Danger level 1 (minimal or no danger)
Slick road warnings are only issued in levels 2 - 4.
Danger level 2 (moderate danger)
Impact
Local or widespread risk of skidding.
Recommended action
Information on recommended action can be found here: What to do when the roads are slippery
Description of measurement and prognostic values
- Packed snow (less than 4 cm, road temperature of < 0 degrees)
- Black ice (wet roads that have frozen over)
- Icy roads (freezing rain)
Danger level 3 (significant danger)
Impact
- Sudden, acute danger of skidding.
- Icy roads to the extent of being impassable.
- Disruption to air travel.
- Branches breaking and falling as a result of their ice load.
Recommended action
Information on recommended action can be found here: What to do when the roads are slippery
Description of measurement and prognostic values
Freezing rain: up to 2 mm
Danger levels 4 (severe danger)
Impact
- Sudden, acute danger of skidding.
- Road and air travel brought to a standstill.
- Disruption to rail travel.
- Falling branches or trees as a result of their ice load.
- Interruption of electricity supply and/or telephone network, particularly in regions that are not easily accessible.
Recommended action
Information on recommended action can be found here: What to do when the roads are slippery
Description of measurement and prognostic values
Freezing rain: over 2 mm (total precipitation volume freezing on the ground)
Danger levels 5 (very severe danger)
It is not possible to make a distinction for larger deposits of precipitation. Widespread ice formation on roads over a sustained period is a rare occurrence in Switzerland.
Notes:
Warnings of slippery conditions due to ice or snow are not issued for altitudes above 800 m.