Some results of helicopter measurements of the albedo of different land surfaces

Abstract

Two series of albedo measurements are reported in which observations were made from a helicopter hovering above the surface of eight different types of land cover. One series, above a mixed hardwood forest in East Tennessee, consisted of 20 measurements repeated at three different stages of vegetative development. The second series, in Israel, included five types of mountain vegetation and two examples of coastal plain land cover. Five replicate observations were made over each type and the measurements were repeated on six occasions. The results showed mean values of albedo to range from a minimum of 0·11, for the hardwood forest, to a maximum of 0·42 for bare sand dunes. The size of the albedo was inversely related to the height of the vegetation canopy, presumably because of the greater opportunities for the reabsorption of reflected radiation within deep canopies. Other results also support the view that the spatial arrangement of the canopy was more important that the optical properties of individual elements in determining the overall reflectivity. Albedo differences between replicate plots of the same vegetation type or between replicate sites within one plot were considerable. The average coefficient of variation was 18 per cent and the variability ranged from surface to surface and season to season. By contrast, the seasonal variation in albedo values was much smaller.