Abstract
Using data from sunshine recorders in the contiguous United States, the mean interannual cloud-cover variations at 103 locations are calculated. A typical location has a mean absolute year-to-year variation in cloud cover of 3.94%, which corresponds to a variation of about 14 overcast days per year and to about a 2.8% variation of the annual mean global insolation. As a consequence of these natural variations, it will take about 30 years of sampling to determine the annual mean cloud cover to an accuracy of 1% with 95% confidence. Regional year-to-year variations are smaller than those at individual locations; and, for the contiguous United States as a whole, it averages 1.2%. Theory suggests that on hemispheric and global scales the interannual variations in total cloud cover are very small, being of the order of a few tenths of a percent.
