Variation in United States Cloudiness and Sunshine, 1950–82

Abstract

The variation in cloudiness and percentage of possible sunshine for the contiguous United States and six regions thereof is examined, based on data from 101 National Weather Service stations for the years 1950–82. Over this 33-year interval, linear regression analysis indicates a 3.7% increase in United States cloudiness significant at the 1% level, but a nonsignificant (at the 5% level) 0.9% decrease in sunshine. This difference in trend magnitude and significance may be related to an increase in cirrus, perhaps partly aircraft-induced. Changes in United States cloudiness during this interval have been greatest in autumn (7.0%) and least in spring (1.7%), and for year-average values greatest in the Southwest (4.7%) and South-Central (4.4%) regions, and least in the North-Central (2.5%) and Northeast (2.8%) regions. There is a highly significant (at the 0.1% level) correlation of −0.92 between year-average United States cloudiness and sunshine, so that dust, smoke and haze have not had a large effect on sunshine duration. There have also been no anomalous changes of sunshine duration relative to cloudiness following the Agung, Mt. St. Helens and El Chichon eruptions. The positive correlations between cloudiness values (and sunshine values) for the most distant regions of the United States show that some of the interannual variations in these quantities are of large spatial extent. There has been an appreciable tendency (though not quite significant at the 5% level), particularly in the Southwest and the North- and South-Central regions, for cloudiness to be above average and sunshine below average in years of warm sea surface temperature (SST) in the equatorial eastern Pacific (El Niño). This tendency was pronounced during the unusually warm SST years of 1972 and 1982, when United States cloudiness was 4.0 and 4.7% above average, respectively, and sunshine 3.5 and 3.1% below average, respectively. There is no evidence for a consistent relationship between the quasi-biennial oscillation in the tropical stratosphere, and United States cloudiness or sunshine. There has, however, been a significant tendency (at the 5% level) for United States cloudiness to be above average and sunshine below average in years when Indian summer-monsoon rainfall is below average, and vice versa, with respective correlations of −0.42 and 0.46. There is no evidence for a relation between Northern Hemisphere surface temperature and United States cloudiness or sunshine in the same year, but there is evidence for a relation with cloudiness and sunshine one year later.