ABSTRACT
The surface of a glacier is composed of snow, firn, ice, water and debris (e.g. rock, pebbles, dust, soot) with a highly variable fraction of each component from glacier to glacier, which significantly impacts the optical properties of the glacier (Chapter 2). In Figure 8.1 a mid-August picture of a typical small Alpine valley glacier, Oberaar Glacier in Switzerland, (46.53◦ N, 8.2◦ E) is shown. The glacier’s area is 5.2 km2 and the length is 4.7 km, and it is the source of the river Aare. Oberaar Glacier flows from west to east, stretches from about 2300 to 3300 m a.s.l. and is bounded by relatively steep valley walls. While the transient snow line is easily visible in the image separating the darker bare ice from the bright snow, the glacier boundary at the left side of the image is difficult to determine due to the heavy debris cover. However, human recognition is able to trace the perimeter of the glacier by the change in slope angle along the contact with the Little Ice Age lateral moraine. The debris cover to the right side is not as compact, and some bare ice is visible through it. The fractions of snow, firn, ice, water and debris of a glacier are important for measuring the spectral properties of a glacier since several different surface types may be located within one image pixel when viewing a glacier from space. With a focus on temperate glaciers as they appear at the end of the ablation season and neglecting liquid water, debris cover and other pollution, the glacier surface is composed of ice and snow. As glacier ice originates from metamorphosed snow, the optical properties of a glacier surface are largely determined by those of snow. In this chapter, only the optical properties of snow are discussed; for a thorough overview in other parts of the electromagnetic spectrum see Chapter 5 and Rees (2005). The spectral characteristics of the debris cover strongly depends on the lithology of the surrounding rock walls and varies from place to place.
