A Recent Increase in Sunshine Duration within the Contiguous United States

Abstract

The tendency for a long-term decrease in annual sunshine duration within the contiguous United States reversed abruptly in 1972, and in 1976 the sunshine duration was 2% above average. The reversal was particularly pronounced in autumn, with the 11% decrease between 1953 and 1972 replaced by a 7% increase thereafter. In the light of this recent increase, it is unlikely that the earlier decrease in sunshine duration was chiefly due to aircraft-induced cirrus cloudiness, and the close relation between the percentage change in sunshine duration and amount of solar radiation received at the earth's surface within the United States also makes it unlikely that an increase in atmospheric turbidity was mainly responsible. Rather, long-term (climatological) changes in area cloudiness are undoubtedly the basic cause of the observed changes in sunshine duration. It is emphasized that these long-term changes in sunshine duration or cloudiness, of as much as 10% and extending over 10 years or more, may be of significance when considering certain applications of solar energy.