Letters to the Editor 321
18. 1. Bennett, Correlation of daily insolation with daily total sky cover, opaque sky cover and percentage of possible sunshine. Solar Energy 12, 391-393 (1969).
19. J. L. Threlkeld and R. C. Jordan, Direct solar radia- tion available on clear days. ASHRAE Trans. 64, 45- 68 (1958).
20. T. K. Won, The simulation of hourly global radiation from hourly reported meteorological parameters--
Canadian prairie area. Paper E40-M, Proc. Third SESCI Conference, Edmonton, Alia (1977).
21. C. Gueymard, Utilisation des donn6es m6t6orolo- giques horaires pour le calcul du rayonnement solaire sur des surfaces inclin6es. Applications/i la simulation thermique des b~timents solaires passifs (with English abstract). Ph. D. thesis, University of Montreal (1983).
Reply to "Comments on POTSOL: Model to predict extraterrestrial and clear sky solar radiation and ground level solar radiation prediction model including
cloud cover effects"
Dear Sir: We welcome the comments and questions of Christian
Gueymard concerning the recently published papers in Solar Energy[l, 2]. Although the models propose limited changes to the original algorithm, the use of our versions has been widely accepted.
The following are specific responses to selected com- ments by Christian Gueymard. In response to comment (1), eqn (8) of  was developed by Imamura, et al. as follows:
ld = C I ( c r ) / C N 2,
Itcr) = CNlp e x p [ - ~ sec (0o)].
la = C[CNIv e x p [ - , sec (Oo)]/CN 2
Id = (C/CN)lp e x p [ - x sec (0o)].
In response to comment (2), we agree that a more detailed study should be implemented to improve on the work of Stephenson, but such a study was beyond the scope of these papers. In response to comment (4), the "calculated values" were part of a data tape obtained from the Na- tional Climatic Center. Table 1 of  was included simply to show that the results for the site using our model agreed with the precalculated values.
In response to comment (7), justification in using av- erage values for eqn (11) in  is supported by Imamura, et al., Appendix A, p. 18: "Test runs were made using both seasonal P, Q and R's and the average P, Q and R's. In terms of yearly integrated solar irradiance, the differ- ence between the results was insignificant,"
In response to comment (8), an interesting fact about
the Salisbury data is the low number of observations in- dicating 10% covered skies and 90% covered skies. The authors believe that the low number of observations in classes next to the two extremes of the data range is due to observational bias. An interview with the observers at Salisbury airport, where the cloud cover data were col- lected, reveals that observers may tend to enter 90% cov- ered skies as totally overcasted (100% covered), since from a pilot's viewpoint a cloud cover of 9 or 10 is basically the same. Similarly, the observers may enter 10% covered skies as totally clear. For a more detailed analysis, see Yaramanoglu, et al. .
In response to comment (9), the model is fully capable of providing hourly outputs. Daily totals were reported simply because that data base was available for both Sal- isbury and Ely.
Department o f Agricultural Engineering University o f Maryland RUSSELL BRINSFIELD Queenstown, MD 21658 U.S.A.
Department of Agricultural Engineering University o f Maryland MELIH YARAMANOGLU College Park, MD 20740 FREDRICK WHEATON U.S.A.
1. R. Brinsfield, et al., POTSOL: Model to predict ex- traterrestrial and clear sky solar radiation. Solar En- ergy 33, 485-492 (1984).
2. R. Brinsfield, et al., Ground level solar radiation pre- diction model including cloud cover effects. Solar En- ergy 33, 493-499 (1984).
3. M.S. Imamura, et al., Definition Study for photovoltaic residential prototype system. U,S. Department of Commerce Report No. N77-13533 (1976).
4. M. Yaramanoglu, et al., Estimation of solar radiation using stochastically generated cloud cover data. Energy Agriculture 4, 227-242 (1985).