Dirty snow melts faster than clean snow because the dirt in dirty snow is dark in color, absorbing solar radiation. This causes dirty snow to melt faster than clean snow, which reflects most solar radiation back into the atmosphere rather than absorbing it.
Clean snow is almost pure ice, which has a higher heat capacity than snow mixed with dirt. Dirt has a lower heat capacity than ice, meaning it requires more energy to heat the same volume of ice 1 degree than dirt. When dirt mixes with snow, it lowers the heat capacity of snow. If a significant area of snow is covered with several inches of dirt, the cold is insulated by the dirt from the heat of solar radiation, causing that area to melt slower. This is not the same as dirty snow created by snow mixed with dirt.
In the Himalayan mountains, increasing pollution from Asia has been causing grey snow, first noted in satellite imagery in the late 1990s. The grey snow melted faster and in greater quantity than in previous years when the snow was still white and unpolluted. This phenomenon raised concern for environmentalists considering the region's main water reservoirs, which are the more rapidly melting Himalayan glaciers. Environmentalists also voiced concerns regarding the impact of increasing glacier melts on the area's environment, including the risk of flash floods.