Global and normal incidence radiation intensities measured at two sites located in the semi-arid Negev region of Israel have been utilized to develop a set of monthly empirical regression equations for determining the diffuse radiation that simulate the data shown graphically in the classic paper by Liu and Jordan. This equation expresses Kd, the ratio of the daily diffuse on a horizontal surface to the daily extraterrestrial radiation on a horizontal surface, as an exponential function of KD, the ratio of the daily beam on a horizontal surface to the daily extraterrestrial on a horizontal surface. The databases utilized in developing the equations consisted of measurements made in Beer Sheva and Sde Boker during the years 1990-1998 and 1990-1999, respectively. The monthly empirical regression equations were validated by using an independent set of data measured in Beer Sheva during the years 1983-1989. It was observed that a single set of monthly equations was capable of characterizing the Negev region of Israel. The statistical analysis of the results is presented and discussed in detail.