β Scribed by JACOVIDES, C. P.
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Pyrheliometric measurements of direct solar radiation performed at Athens Observatory (NOA) from 1954 to 1992, were used to test the applicability of several models which compute the Linke's turbidity factor, T L . The models used in this analysis belong to two categories: the direct, or the so-called pyrheliometric, formula and the indirect, or parameterized, method.
The statistical indicators, mean bias (MBE) and root-mean-square (RMSE) errors in combination with the t statistic are proposed for the evaluation and comparison of T L as produced by various models. The direct method is presented using three different parameterized algorithms for d R (l), proposed by Louche et al., Grenier et al. and Kasten; whereas the indirect schemes were Dogniaux's and Valko's parameterized models. Based on the comparison against the early pyrheliometric method, Grenier et al.'s model performed the best with respect to R 2 , MBE, and RMSE, whereas adjusted Louche et al.'s, Louche et al.'s, Valko's, and Dogniaux's II and I models follow in that order. It is significant that Valko's model performed the best with respect to the t statistic. Dogniaux's scheme was found to depend strongly on the aerosol's microphysics loading in the atmosphere, whereas Valko's scheme did not show such a dependence. Finally, the probable dependence on the water vapour content of the relationship linking T L to A Λngstrom's turbidity coefficient b, is discussed.
π SIMILAR VOLUMES
Using a formula for critical double scattering, which we recently obtained, the depolarization factor for CO? wns alculated. The values are in good ageement with recent experimental results by Tnppeniers et al. who found an anomaly close to the criticalpoint.