Variations in the relationship between total daily solar radiation and sunshine duration, as affected by changes in atmospheric turbidity, and displayed by the parameters a and b of the linear equation Q = Qo (a + b n/N) are studied as a function of time. Daily data are used in the analysis. It is found that a and b are inversely related and vary in a curve that can be described as parabolic over the period of the year. It is found that a' = 0.50187 - 0.0103760 x + 0.00012084 x2, and b' = 0.35526 + 0.0162589 x - 0.000199034 x2, where a' and b' are the least squares estimators of the parameters a and b, and x is any 5-day period.