Recent Advances in the Relations between Bright Sunshine Hours and Solar Irradiation

Modeling Solar Radiation at the Earth Surface, Viorel Badescu, Editör, Springer-Verlag , Bucuresti, ss.115-141, 2008

Chapter 5

Bulent G. Akinoglu

1 Introduction

It seems quite a realistic view to state that the data of bright sunshine hours are

the only long term, reliable and readily available measured information that can be

used to reach highly accurate estimates of solar irradiation values on the Earth surface.

Kimball at 1919 demonstrated for the first time the existence of the relation

between the average daily irradiation obtained by means of phyroheliometric and

photometric measurements and the duration of sunshine measured by a Marvin sunshine

recorder. He presented the relations graphically and included also the relation

between the solar irradiation and cloudiness. Using the data of several locations in

USA he came to a conclusion that: “In fact, the radiation-ratio and sunshine data

plot very nearly on the straight line connecting 100% sunshine and 0% sunshine

radiation intensities” (Kimball 1919).

Alternatively, one might exclaim that the history started at 1924, with a simple

empirical linear relation proposed by Angstr¨om (1924). Since then hundreds

of articles appeared in the literature from all over the world which made use of

this well-known Angstr¨om’s linear correlation in the same, similar and/or modified

manner. The correlation derived by Angstr¨om from measured data of Stockholm, in

its original form, was:

H = Hc (0.25+0.75n/N) (5.1)

where H and Hc are the total irradiation income on horizontal surface for a day and

for a perfectly clear sky, respectively while n/N is the time of sunshine expressed

as the fraction of greatest possible time of sunshine. One of the chief results that

Angstr¨om reached was “A clear conception of the radiation climate . . . cannot be

obtained without a detailed experience of the amount of energy furnished by the

diffuse radiation” (Angstr¨om 1924).

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Bulent G. Akinoglu

Middle East Technical University, Ankara, Turkey, e-mail: bulent@newton.physics.metu.edu.tr