The Visible Albedo of Surfaces in Central Minnesota
1966
Bray, J. R. | Sanger, J. E. | Archer, A. L.
Incident and reflected visible radiation under clear skies was measured in summer 1960 over surfaces in central Minnesota. Measurements were made with Weston photometers, model 756, on the ground, from ladders, and from a Bell 47—G2 helicopter. Percent albedo over natural upland surfaces at 60—65° solar altitude decrease from 12.0 for inorganic sand to 6.1 for organic sand, to 5.3 for short grassland, and to 2.2 for forest; lowland surfaces decreased from 4.0 for water to 2.3 for forest. Albedo decreased with an increasing chlorophyll a + b content of vegetation, but the decrease was negligible above a chlorophyll content of around 1.0g/m². Urban surfaces were more reflective than agricultural surfaces, which had a higher mean albedo than natural vegetation. The mean albedo of an aerial transect from Bethel to St. Paul was 6.5% and was estimated to have increased around 15% the settlement of the area by European man. Percent albedo over a Quercus forest increase from 2.4 at 65—70° solar altitude of 11.6 at 8—10° and the decreased to 3.8 at 0° altitude. A similar albedo pattern was found for a pioneer (short) grassland which increased from 5.2 to 13.2 and then decreased to 4.6%. Changes in albedo with solar were attributed to changing proportions of diffuse and direct radiation and to Fresnel's Law. Light penetration into the Quercus forest decreased most rapidly in the lower tree canopy layer and decreased with increasing solar altitude. Albedo within the Quercus forest increased with increasing forest depth and with increasing solar altitude, reaching a maximum of 12.7% at a forestheight of 0—3 m and a solar altitude of >60°.
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