Percent of Possible Sunshine and the Total Cloud Cover

Abstract

The total cloud cover is deduced from measurements of the percent of possible sunshine at 72 locations in the United States. This sunshine-derived total cloud cover is then compared to conventional ground-based observations of total cloud cover made by meteorological observers. A linear relationship between the two estimates is calculated, and the difference between the two estimates as a function of latitude is fitted with a least-squares linear equation. It is found that on the average the sunshine-derived values of total cloud cover are about 13% lower than the corresponding ground-based estimates of total cloud cover. The difference between the two estimates may be attributed to projection problems by the ground-based observer where sides of clouds are viewed and added to the estimate of total cloud cover or to the failure of sunshine recorders to detect thin cirrus clouds. Projection problems by the meteorological observers is probably the most likely cause because satellite and aircraft observations confirm the sunshine observations. The difference between the sunshine-derived and ground-based estimates of total cloud cover as a function of latitude also indicates that the ground-based observers are probably having difficulties with the total cloud cover estimates. It is concluded that a more standard definition of the meaning of ?cloud? and ?total cloud cover?, is needed for radiation budget and climate modelling studies.