A clear sky atmospheric model valid for the mediterranean zone

Abstract The ASHRAE clear sky model in the version proposed by G.L. Powell has been applied to a set of clear days from two coastal locations of Yugoslavia. The results have provided further elements to confirm the existence of a clear sky atmospheric model valid for the whole Mediterranean zone. This model is characterized by values of the atmospheric extinction coefficient higher than the corresponding ASHRAE ones with upper limits defined by the regression curve derived by using data of Palermo.

The reference frequency distributions of daily relative sunshine from different locations

Abstract In some applications the relative sunshine distribution curves are very usefull. The present work tends to provide the above frequency distributions for the Italian area, using the daily data of eleven stations. A preliminary compatibility test is applied in order to verify if a dependence on location exist and it is deduced that the area referring to the various locations is statistically compatible with a few exceptions. Therefore, it is possible to give generalized frequency distributions curves which can be considered reference curves for all Italian locations. The empirical distributions are also compared with the calculated ones by the method of Bendt et al. and a good agreem…

Monthly reference distribution of daily relative sunshine values

Abstract In this paper it is shown that for a given location the monthly distributions of the daily relative sunshine depend only, within statistical errors, on their monthly averages and are independent of the month of the year. Furthermore it is put in evidence that a reference distribution of the daily relative sunshine values for every month of the year can be found by the knowledge of the average deduced from data of some years.

Diffuse solar radiation statistics for Italy

Abstract In this paper diffuse radiation is correlated with usually measured or more easy computable quantities in order to have a larger number of data available for the various applications of solar energy. Here are used two types of correlation: (a) diffuse radiation as a function of global and extraterrestrial radiation; (b) diffuse radiation as a function of ralative sunshine and extraterrestrial radiation. On the basis of the above correlations, empirical equations have been proposed and their coefficients have been calculated by fitting the experimental data of three Italian stations. The degree of accuracy of the fit has been estimated by a coefficient introduced and called by us “r…

The ASHRAE clear sky model. An evaluation in the Mediterranean Zone

Abstract The ASHRAE clear sky model in the version proposed by G. L. Powell is here applied to a yearly set of data for Palermo with small scale time intervals. The model is shown to yield computed values generally higher than the measured ones. New values for the atmospheric extinction coefficient are derived and proposed for the application to the Mediterranean atmosphere. A successful verification is performed using hourly data of Genoa, a location belonging to the same meteoclimatic area.

An analysis of direct solar transmittance and atmospheric turbidity in the Mediterranean climatic belt

Abstract The empirical model developed by King and Buckius for the direct transmission in clear sky is applied to hourly data of two Yugoslav locations exhibiting typical meteoclimatic characteristics common to the whole Mediterranean belt. The occurrence of “very clear”, “clear” and “very hazy” hours, after the usually accepted definitions, is determined and discussed. The degree of accuracy of the previsional model is investigated performing the comparison with the corresponding values obtained directly by energetic procedure. A general overestimate of the transmittance is experienced with notable deviations in meteorological conditions affected by the presence of dense clouds which behav…

An atmospheric model for computing direct and diffuse solar radiation

Abstract This paper proposes an atmospheric model, which extends the computation of the direct radiation given by Cole's model to the cloudy sky and shows a method to calculate the diffuse radiation. Therefore the monthly average values of the global radiation incident on a horizontal surface at Palermo are computed by this method. These values are compared with the experimental data provided by the Istituto di Idraulica Agraria, Universita di Palermo and generally exhibit a mean deviation not more than 10 per cent. The deviations become lower than 8 per cent taking into account the effect of the underlying surface albedo. In order to better verify the validity of the proposed method, it sh…

A more reliable relation between Angström and Linke atmospheric turbidity parameters

Abstract This work analyses the correlation between Angstrom and Linke atmospheric turbidity parameters (β and T ) which are commonly used in studies about the atmosphere's behaviour since they efficaciously monitor a point turbidity state and model the aerosol attenuation of solar radiation. Starting from the usual linear relation β = a + bT , a more reliable one is here derived by hourly data of three coastal locations with typical Mediterranean meteoclimatic characteristics, for values of the experimental ratios of diffuse and global solar radiation ⩽0.3. The expression found notably differs from the starting one as shown by the time variable.

An atmospheric model applied to some actinometric stations of Sicily

Abstract In this paper the application of an atmospheric model proposed by S. Barbaro et al. in a previous work for computing the direct and diffuse solar radiation is extended to some actinometric stations in Sicily, namely Gela, Messina, Pantelleria, Trapani and Ustica. The comparison of the monthly average values of the global radiation computed by this method with the experimental data shows a good agreement, that confirms the encouraging results obtained for Palermo in the previous work. This further test of the validity of the proposed model allows this one may be used with good reliability to predict the radiation on locations lacking in actinometric instruments. Finally, the influen…

Global solar radiation in Italy

Correlation between relative sunshine and state of the sky

Abstract It is possible to construct a sunshine map referring to a large zone, lacking in a widespread network of recording stations, correlating the relative sunshine with the “state of the sky”, defined by the monthly mean number of the clear, mixed and overcast days, whose data are more largely available. In the correlation here proposed climatological and meteorological factors are introduced in order to express the helioclimatic characteristics of every location. The application to Italy, using the data of the period 1973–1977 provided by the 31 stations of ITAV, has suggested the classification of these stations in seven “helioclimatic groups” characterized by different values of the …