Abstract
Examined are annual, quasi-biennial and long-term variations in percentage of possible sunshine (S) within six regions of the contiguous United States, based on observations at 103 stations for the years 1950–1972. S averages 20% greater in summer than in winter, and 20–25% greater in the Southwest than in the Northeast. There is a tendency for S to be maximum near the time of quasi-biennial east wind maximum at 50 mb in the tropics, with the average difference in S at times of east and west wind maxima varying from 1.5% to 0.7% within the regions, and equal to 1.0% for the United States as a whole. The authors believe this difference is related to a quasi-biennial variation in eccentricity of the polar vortex. Within the United States during the 23-year period there has been a highly significant 8% decrease in S during autumn, and a somewhat compensatory 3% increase during spring. The pronounced long-term downward trend in S during autumn, with its implication of an upward trend in albedo, represents an interesting climatological phenomenon with impact on a local and perhaps hemispheric scale. In all six regions there has been a year-average decrease in S since 1964, with values varying from 2.9% in the Northeast to 0.2% in the Southwest, and amounting to a significant 1.3% for the United States as a whole. The possibility is considered that this decrease is related either to an overall increase in cloudiness, an increase in aircraft-induced cirrus cloudiness, or an increase in turbidity due to pollution.