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| Date: | Wed, 17 Apr 2002 09:10:44 -0400 |
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Dear Dr. Dharam:
I saw already that Dr. Howard gave you a solution and program to do the conversion of sunshine hours to global solar radiation. I will only give you some additional facts about this problem.
Usual techniques rely first on the calculation of the fraction (measured sunshine hours / theoretical maximum duration of sunshine hours) for the day, decade or month in question, and second upon the use of one or another empirical relationship between this fraction and global solar radiation. You should be careful about the use of this last relationship to ensure you know the correct measuring units in which results are.
Here the meaning of measured sunshine hours is obvious (you'll be using a Campbell heliograph or something like it). Theoretical maximum duration of sunshine hours is also called simply daylength (an astronomical parameter equal to the number of hours between dawn and sunset, so it is different each day through the year). I can provide you with algorithms and a program for the astronomical calculations of daylenght in any part of the planet if you think it could be useful to you.
The empirical relationship between (n / N) and global solar radiation is quite another problem. In many, many places have been obtained (slightly different) empirical relationships of the type
global solar radiation = Ra*(a + b*(n / N)) , where a, b are empirical (regressional) numerical parameters and Ra is called extraterrestial solar radiation (another astronomical parameter, meaning global solar radiation without the influence of an atmosphere, for which calculation we could supply you with algorithms and program if you like)
one of the most used is FAO relationship
Rg = Ra*(0.25 + 0.50*(n / N)) , where global solar radiation (Rg) would be in the same units as Ra.
(these particular values of a and b are valid only in tropical climates)
We donīt use this relationship here because we obtained our own equation by a different path using what is called Rayleigh radiation.
Hoping this can help you,
Sincerely yours,
Roger
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