Only a thaw can melt crown snow

December 2017 was mild and rainy in almost all of Finland. In many parts of North Karelia, North Savo and Kainuu, precipitation levels for the month were between 70 and 100 mm, when measured as water equivalents. Normal December precipitation levels for the area can be as little as half this amount.

‘For nearly the whole of December, a mild air mass was flowing over the area accompanied by low pressure and precipitation. Snow accumulated in the trees over a long period', says Finnish Meteorological Institute meteorologist Antti Kokko.

According to Mr Kokko, the Finnish Meteorological Institute's weather safety unit became aware of the situation's severity when the number of electricity cuts in the area rose in one day from several thousand to around ten thousand. The first Luova bulletin - a warning about the situation issued to the authorities - was sent on 28 December.

At this point, the crown snow load was especially large in North Karelia. The temperature there, however, then rose briefly to 1.5–2 degrees above zero, which helped the situation. In Kainuu, the first large disruption to the electricity network occurred on 29 December, and the situation there has continued and worsened.

Wet snow accumulates in trees

Finnish Meteorological Institute meteorologist Hannu Valta presents the weather observations from the turn of the year at the Kalliojoki observation station in Kuhmo. Daily snowfall has not been a large, but it has been repeated and steady. The temperature has see-sawed above and below freezing, providing the perfect conditions for crown snow to accumulate.

Crown snow can form in two ways: through frosting, where unfrozen cloud or fog droplets freeze to subzero surfaces, or as sticky snow, where wet falling snow sticks to the trees. Mr Valta explains that the crown snow which accumulated at the end of the year was mostly sticky snow, which builds up best when the temperature is around or slightly above zero and there are wet snow showers and a moderate wind.

The wind can affect in two ways: firstly, it can shake the snow off the tree branches. Secondly, it can break the branches and the trees. For example, the high winds in Kainuu that were seen around Epiphany, with gusts of up to 13m/s, did not help the situation because the snow was so firmly stuck to the tree branches.

Crown snow can be dangerous because it is heavy and can break the trees and electricity lines.

Once every decade

General weather conditions in Finland changed in the week beginning 8 January 2018 with the arrival of a high-pressure front. The areas with high crown snow loads also saw precipitation decrease and temperatures drop below freezing, but the tree branches still sagged under the weight of the snow. This challenging situation continued particularly in Kainuu, where another Luova warning was given on 22 January 2018 because of the high accumulation of snow.

‘The situation is really on a knife edge, such that even small additional snow showers or a strong wind could bring down more trees and cause more electricity cuts', says meteorologist Antti Kokko.

The best remedy for the situation would be for temperatures to rise. Once the temperature is above 2 degrees, snow will start to melt and drop from the trees. The heat from the sun is sufficient to melt the snow in the trees in southern Finland in February and March. In more northern parts, this becomes possible only towards the end of March.

‘The situation in Kainuu could continue for another month if temperatures remain below freezing', meteorologist Hannu Valta estimates.

According to Mr Valta, such difficult crown snow conditions are encountered only around once a decade. He remembers that in South Savo in 2011 there was a similar situation which lasted for a number of weeks. Less serious instances, where less damage is caused, occur many times in a decade.