Slipperiness and pedestrian weather

The Finnish Meteorological Institute’s weather warning for pedestrian helps the public better prepare for extremely slippery conditions. Typically, there are five to twenty days per year per region. However, slippery and locally extremely slippery conditions occur much more frequently.

It is extremely slippery when:

• Snow falls on ice.

• Rain falls on ice, or a layer of water forms on the ice due to melting.

• Snow becomes hard-packed slippery after heavy snowfall, and the temperature hovers around or just below freezing.

• Slush that has frozen into hard and uneven ice.

When it is extremely slippery, in addition to caution, one needs shoes with good grip and aids such as anti-slip devices.

Pedestrian weather warning tells the cause and timing of slipperiness.

In the picture, the temperature is initially above freezing and it rains. Then it freezes, and the wet sidewalks turn to ice. The sidewalks become slippery. Snow falls on the icy sidewalks, making the ice even more slippery and hiding it from view. At this point, the conditions are extremely slippery.

In the picture, first the sidewalks become icy and slippery. The temperature rises above freezing, and rain falls on top of the ice. The ice does not melt under the water, making it extremely slippery. Similarly, water can appear on top of the ice if the surface layer of the ice melts. Once the ice has melted completely, the surface becomes non-slippery again.

Slipping accidents are expensive

In Finland, around 50,000 slipping accidents occur each winter due to ice. According to VTT (the Technical Research Centre of Finland), these lead to costs of 2.4 billion euros, covering healthcare expenses, sick leave, and loss of well-being.

• People under 30 slip the most

• People aged 50–60 most often require medical care.

• The risk of hip fractures increases for those over 70.

You are more likely to slip if you are not paying attention to walking, for example, when looking at your phone. Good grip footwear is also important on slippery winter surfaces.

Pedestrian conditions are assessed using the pedestrian index

The warning is determined using a pedestrian index. The pedestrian index is calculated based on a model that describes conditions on pedestrian and cycling paths. It assesses whether there is ice, water, snow, or frost. The calculations utilize:

• Weather observations from the past four days.

• Observations of current weather conditions, such as temperature.

• Weather forecasts for the next two days

The model has been developed in collaboration with the Finnish Institute of Occupational Health, using slipperiness measurements.