A field programme on light conditions in ice-covered lakes and optical properties of lake ice was performed in seven lakes of Finland and Estonia in February–April 2009. On the basis of irradiance measurements above and below ice, spectral reflectance and transmittance were determined for the ice sheet; time evolution of photosynthetically active radiation (PAR) transmittance was examined from irradiance recordings at several levels inside the ice sheet. Snow cover was the dominant factor for transmission of PAR into the lake water body. Reflectance was 0.74–0.92 in winter, going down to 0.18–0.22 in the melting season. The bulk attenuation coefficient of dry snow was 14–25 m–1; the level decreased as the spring was coming. The reflectance and bulk attenuation coefficient of snow-free ice were 0.1–0.4 and 1–5 m–1. Both were considerably smaller than those of snow cover. Seasonal evolution of light transmission was mainly due to snow melting. Snow and ice cover not only depress the PAR level in a lake but also influence the spectral and directional distribution of light

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Sea ice is a key climate factor and it restricts considerably the winter navigation in severe seasons on the Baltic Sea. So determining ice conditions severity and describing ice cover behavior at severe seasons are necessary. The ice seasons severity degree is studied at the years 1982 to 2016. A new integrative characteristic named the weighted ice days number of the season is introduced to determine the ice season severity. The ice concentration data on the Baltic Sea published in the European Copernicus Programme are used to calculate the maximal ice extent and the weighted ice days number of the seasons. Both the ice season severity characteristics are used to classify the winters with respect of severity. The ice seasons 1981/82, 1984/85, 1985/86, 1986/87, 1995/96 and 2002/03 are classified as severe by the weighted ice days number. Only three seasons of this list are severe by both the criteria. We interpret this coincidence as the evidence of enough-during extensive ice cover in these three seasons. In the winter 2010/11 ice cover extended widely for some time, but did not last longer. At 2002/03 and a few other ice seasons the Baltic Sea was ice-covered in moderate extent, but the ice cover stayed long time. For 11 winters (32 % of the period) the relational weighted ice days number differs considerably (> 10 %) from the relational maximal ice extent. These winters yield one third of the studied ice seasons. Statistically every 6th winter is severe by the weighted ice days number whereas only statistically every 8th winter is severe by the maximal ice extent on the Baltic. Hence there are more intrinsically severe seasons than the maximal ice extent gives. The maximal ice extent fails to account with the ice cover durability. The weighted ice days number enables to describe the ice cover behavior more representatively. Using the weighted ice days number adds the temporal dimension to the ice season severity study.

The Gulf of Finland is one of the heaviest ship traffic areas in the world. The ice-covered period in the gulf lasts up to 140 days in severe winters. Common features are openings in ice (flaw leads) and ice ridges. The winter of 2002/2003 was exceptionally harsh: the entire Gulf of Finland was for a long time covered with thick ice, severe weather conditions caused much ice deformation and numerous ship incidents happened. We investigated the dependence of ice deformation rate on wind speed and direction, and variation in ice conditions in space and time. The numerical sea ice model HELMI was used to determine relations between wind conditions, ridged ice and ship damages. The occurrence frequency of leads in different regions was analysed by MODIS satellite imagery. The strongest wind blew from the NE, SW and NW in the winter of 2002/2003. The growth rate of deformed ice was more related to the wind direction than to speed. The ridging was most intensive when the wind blew from the E, SW and NW. Openings were formed almost everywhere during moderate or strong winds. Elongated leads, caused by northerly winds, were more common in the Finnish coastal region

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Sea ice ridges and other types of deformed ice are the main obstacles for the winter navigation. During the severe winter 2002/2003, about 60% of the ship hull damages, registered in the Baltic Sea, occurred in the Gulf of Finland. We have analysed ice deformation features, derived from the HELMI sea ice model in relation to two ship damages that occurred in the Gulf of Finland this winter. The damages happened close to the high growth rate area of deformed ice at the interface of different ice conditions with notable ice thickness gradients. It is concluded that the rate of ridged ice production is an indicator of the compression of the ice pack and a potential indicator of ice-induced danger to shipping.

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