AI1ract--The use of more readily available meteorological data in radiation models has always been a limiting factor in many studies. The current paper presents a model originally based on cloud amount to calculate the monthly average daily insolation on horizontal and inclined surfaces___ The cloud amount is replamd by fractional pemible sunshine in accordance with the findings of other research workers. Existing sunshine and radiation records for 10locations are used to estimate the prevailing atmospheric transmittance associated with this type of model. Effects of muldple rellection between ground-surface and atmosphere are also incorporated.
The available data are used for correlation purposes employing Root Mean Square Error (RMSE} as a measure of the model performance. This yielded very satisfactory results, especially for long averaging period simulation. On the average, discrepancies between computed and observed monthly average daily total radiation of lest than 5e/~ and corresponding RMSEs ranging between 2.1 and 3.6~ based on an a~;eraging period of I year are considered to be within the accuracy associated with instrumental measuztments.
Additional teat of the model validity results in calculated ratios of diffuse to total radiation on a horizontal . surface that are in reasomble agreement with well-established methods. However, in view of the low ranking accuracy of the measuringequipment and the rather short time records used in the analysis, further verification of the model will still be required as more reliable data become available.