Samuel model

To compute the monthly-mean-daily-global radiation for any month, the monthly mean daily value of (S/Z) for that month is taken. With the annual average value of (S/Z) the annual-average-monthly-mean--daily-global radiation is comput

Solar energyphotoperiodicitedurationenergie solaireobservation meteorologiquedaylightduracion



Initial contribute: 2021-09-10


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Application-focused categoriesIntegrated-perspectiveGlobal scale

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Quoted from:Samuel T . Estimation of global radiation for Sri Lanka[J]. Solar Energy, 1991, 47(5):333–337.


The location of Sri Lanka in the Indian Ocean between 5°55 ' and 9050 ' north latitude has the advantage of the potential to develop solar energy to supplement the existing energy sources. The design and the estimation of performance of any system where solar energy is one of the inputs requires solar radiation data. Global radiation is one of the basic solar radiation measurements that is required to model some systems. Direct and diffuse components and the spectral distribution data can improve the modelling of some systems and are necessary to model some other systems.

Only global radiation measurements have been made in Sri Lanka in a few of the meteorological stations spread throughout the country. Therefore, as a first step, this paper estimates global radiation for as many meteorological stations as possible. There are several formulae that relate global radiation to other climatological parameters such as sunshine hours, relative humidity, maximum temperature, and average temperature. In this paper a generally accepted modified form of the formula first introduced by Angstrom is used. It has been extensively studied and widely used. Moreover, it relates global radiation to hours of sunshine that have been measured for several years in many of the meteorological stations in Sri Lanka. The annual average of the ratio of the hours of sunshine to the length of the day, i.e., annual average of (S/Z), is found to vary considerably and to lie in the range 0.42-0.66.  

However, only four of the meteorological stations have good data on global radiation. The constants a and b of the Angstr6m formula therefore cannot be determined for all the stations spread throughout the country and their variation studied. Fre're et al on the other hand have found, using data from many parts of the world, a general graphical representation for the variation of a and b with annual average (S/Z) lying in the range 0.28 to 0.75. This variation of a and b can be expressed as quadratic functions of the annual average (S/Z). Therefore these quadratic functions are modified and used to determine a and b values for stations in Sri Lanka.

Samuel proposed the ratio of global to extraterrestrial radiation as a function of the ratio of sunshine hours for third-order polynomials:

\( H/H_0=-0.14+2.52 S/S_0 −3.71(S/S_0) ^2 + 2.24(S/S_0) ^3 \)




T. D. M. A. SAMUEL (2021). Samuel model, Model Item, OpenGMS,


Initial contribute : 2021-09-10



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