**6. Conclusions**

The surface albedo before the onset of melt shows clear but spatially varying trends. The clearest trends are observed in the boreal forest zone, and can be as large as 9 absolute albedo percentage units over the 34-year-long study period. This is about 10–15% of the albedo of clean snow cover with no vegetation protruding above the snow surface. At grid cell level, both the albedo prior to melt onset and the start date of the melt season are responsive to changes in air temperature, wind speed and precipitation amount, with air temperature being the most significant driving factor. At most latitudes the mean albedo before the onset of melt has decreased over the 34 years.

The timing of the melt season shows strong rates of change in localized areas. These changes are better explained by climatic factors than land-use changes. Areas with consistent homogenous changes are larger in Eurasia than in North America. In the Central Siberian Plain the melt season takes place earlier than before. In the Canadian Rocky Mountains and the area around the borders of Russia, China and Mongolia, the melt season starts earlier, ends later, and lasts longer than before.

All in all, both the timing of the melt season and the albedo prior to the melt season changed in large areas between 1982 and 2015. In most areas, both the start and end dates of the melt season have advanced, and the albedo prior to melt onset has decreased, indicating a darkening of winter snow surfaces.

**Author Contributions:** Data curation, K.A., T.M., E.J., A.R. and P.L.; Formal analysis, K.A.; Funding acquisition, T.M.; Investigation, K.A.; Methodology, T.M.; Project administration, K.A.; Supervision, T.M.; Writing—original draft, K.A.

**Funding:** This research was financially supported by the CM SAF project funded by The European Organisation for the Exploitation of Meteorological Satellites (EUMETSAT).

**Acknowledgments:** The authors would like to thank the CM SAF team, Vesa Laine and Kaj Andersson for their co-operation in producing the CLARA-A2 SAL data record, and Outi Meinander for supporting discussions. EUMETSAT supported financially the generation of the CLARA-A2 SAL, which is available at www.cmsaf.eu.

**Conflicts of Interest:** The authors declare no conflict of interest.
