Years with a large amount of snow in combination with a late spring, constitute a risk for flooding. Dirty snow reflects less of the solar radiation than white snow, the albedo of the snow is lowered. If ash is spread on south facing slopes the melt rate can be increased and runoff distributed over a longer time. For a horizontal snow cover, blackened with 0.03 kg ash/square meter, there was a 35% decrease in albedo compared to natural snow. The increase in melt rate, expressed in water column, was 70%. A snow cover with a slope of 30 degrees and blackened with 0.15 kg ash/square meter, had an increase of 105% in melt rate, compared to naturally sloped snow. The horizontal snow cover, with 0.03 kg ash/square meter, showed an increase in degree-day factor from 3.3 to 5.7 mm/degree Celsius,day. On cold days the degree-day method did not simulate snowmelt well for the blackened snow. The influence of solar radiation was very strong and snow melted regardless of the air temperature.