The McCall Glacier is located in the Brooks Range, north Alaska. Most of its slopes have a northerly exposure with an inclination between 5°and 15°. The reduction in direct solar radiation owing to this northerly exposure is small (1.7%) in summer (ablation period, solar declination 20°), as the reduction in radiation received on a north slope during the noon hours is mostly compensated by the increase of energy during the “night” hours, as the sun does not set at that latitude in summer. With the shortening of the solar path, the decrease in direct solar radiation as compared with a horizontal surface becomes more important. At the equinox the loss is 24.8%, and at a solar declination of —10° (20 October or 24 February) even higher with 32.6%.
A further reduction in solar radiation is caused by the steep mountains, which surround the McCall Glacier. The duration of sunshine is reduced during the ablation period by nearly 40%, which, however, represents an energy loss of only 13.4%, as the screening effect of the mountains is most important with low solar angles, i.e. at times when the total energy received at the surface is small. The screening effect of the mountains becomes more severe with lower sun angles and shorter paths of the sun. During the equinox a loss in duration of 67.6%, and in energy of 55.7% is observed. For a solar declination of —10°, there is hardly any direct sunshine on the glacier at all. There is then a loss in duration of 93.6%, resulting in a loss of energy of 87.8%.
Together, these two components reduce the direct solar radiation by about 15% in the ablation period, 67% at the equinox and more than 90% at a solar declination of —10°.