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Statistical relationships between the hourly radiation intensity values of both ultraviolet-B (UVB) and ultraviolet-A (UVA), and of the corresponding global radiation at two sites in Israel, the Dead Sea and Beer Sheva, are presented using radiation data measured from January 1995 to January 1999. It was determined, based upon the square of the correlation coefficient (r 2 ), that in the case of the UVB, a quadratic equation provided a better fit of the database, whereas in the case of UVA, a linear equation was sufficient, i.e. no significant improvement in r 2 was obtained using a quadratic equation. Each monthly empirical equation was tested by utilizing measurements from a single month of independent observations. The accuracy of the individual equations was determined by calculating the mean bias error (MBE), the root mean square error (RSME) and the percent coefficient of variation (CV). The results of this statistical analysis showed that, at both sites, the UVA relationships were characterized by a CV of B10%, with the exception of a single month. In the case of the UVB correlations, the CV values exceeded 10% for more than 75% of the year, both at the Dead Sea and at Beer Sheva. CV values of \20% were observed for more than a third of the individual months at both the Dead Sea and Beer Sheva. Monthly and seasonal relationships from one site were then tested by applying them to the second site. In both cases, the relationships between both UVA and UVB and global radiation were found to be site-specific. On the basis of this statistical analysis, the UVA relationships were found to be far superior predictors than the corresponding UVB ones. In fact, the UVB relationships have limited applicability for predicting UVB from the corresponding global solar radiation intensity.

The broad-band normal incidence UVB beam radiation has been measured at Neve Zohar, Dead Sea basin, using a prototype tracking instrument composed of a Model 501A UV-Biometer mounted on an Eppley Solar Tracker Model St-1. The diffuse and beam fraction of the solar global UVB radiation have been determined using the concurrently measured solar global UVB radiation. The diffuse fraction was observed to exceed 80% throughout the year. The application of the results of these measurements to the possible revision of the photoclimatherapy protocol for psoriasis patients at the Dead Sea medical spas is now under investigation. The suggested revision would enable the sun-exposure treatment protocol to take advantage of the very high diffuse fraction by allowing the patient to receive the daily dose of UVB radiation without direct exposure to the sun, viz. receive the diffuse UVB radiation under a sunshade. This would require an increase in sun-exposure time intervals, as the UVB radiation intensity beneath a sunshade is less than that on an exposed surface.

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