At Barrow, the most northerly point in the U.S., radiation measurements were carried out. It was found that multiple reflection between the ground and stratus clouds, due to high surface albedo in winter, enhances the global radiation. These measurements are in agreement with model results. This, together with cloud amount, makes effective transmittance of the atmosphere highest in spring, resulting in the highest values of irradiation in May. For the same cloud amount, the effective transmittance of the atmosphere is always larger in winter, when there is a highly reflective surface, than in summer.