3.1.7 Seasonal Changes in Albedo

The rate of reflection by the earth's surface of incident solar radiation is called albedo. The average albedo for the entire surface of the earth is 30%, and different types of surface carry different albedo values. For example, the surface of the ocean has an albedo of 8-10% when the sun is high; grassland has an albedo of 15-25%; and desert has an albedo ranging anywhere from 20% to 45%. The albedo of snow and ice also varies widely, depending on a number of factors. The albedo of fresh snow is extremely high at 75-90%, while that of wet snow pack is lower at 40-60%, and sea ice not covered by snow has an albedo of 30-45%. Considering the low albedo of open sea, green terrain and desert, it is clear that the regions covered by snow and ice are responsible for raising the earth's average albedo to 30%. Moreover, the albedo of snowy surfaces varies considerably according to the characteristics of the snow; as the following table indicates, this variation is in the range of several tens of percent.

　

Albedo of various types of snow (Radionov, 1997)

　

When we look at the difference in albedo between winter and summer at each latitude in both hemispheres, we see that no seasonal variation occurs at the lower latitudes or on the permanently frozen Antarctic ice sheet. The regions where significant variation in albedo occurs between summer and winter are areas of snow cover in the Northern Hemisphere where snow accumulates only in the winter and the fringes of the Antarctic continent where the ice cover expands during the winter. The albedo of the multi-year sea ice areas in the Arctic Ocean changes seasonally because the sea ice surface melts in the summer.

　

Latitudinal and seasonal variations of albedo (Ono, 1995)

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