Researchers have examined a number of climatic variables in an attempt to understand the mechanisms that govern rainfall runoff. The paper “The Applicability of Morton’s and Penman’s Evapotranspiration Estimates in Rainfall-Runoff Modeling” (Water Resources Bulletin

[1991]: 611–620) reported on a study that examined the relationship between x cloud cover index and y sunshine index. The cloud cover index can have values between 0 and 1. The accompanying data are consistent with summary quantities in the article. The authors of the article used a cubic regression to describe the relationship between cloud cover and sunshine.

Cloud Cover Index (x) Sunshine Index (y)