Dear Dr. Dharam and Roger:

I have a thought about Roger's comments. Any relationship of the form: Rg =
Ra*(a + b*(n / N)) , where Rg is global solar radiation, Ra is
extraterrestrial solar radiation , and a, b are numerical parameters, has
an assumption about the ratio of clear day global solar radiation and the
extraterrestrial solar radiation (Ra). For instance, FAO relationship Rg =
Ra*(0.25 + 0.50*(n / N)) assumes that Rg/Ra for a clear day is 0.75. From
my experience, I know that Rg/Ra for a clear day varies from one day to
another and from on location to another. That leads me to belive that using
clear day global solar radiation instead of extraterrestrial solar
radiation in the above equation would give more accurate estimates of
global solar radiation using n / N. More than two years ago, I and Joe
Ritchie developed a procedure to find clear day global solar radiation at
any place in the world. I have not heard from Joe about it for so long, but
as far as I know, this work has not been published yet but I think it is
obtainable!

Regards,

Ayman
Ayman Suleiman, PhD
Research Associate
Civil and Environmental Engineering Dept.
7B DANA Engineering Building
Bucknell University
Lewisburg, PA 17837
Voice: (570) 577-1692
Fax   : (570) 577-3415