Is there a method to calculate with sufficient accuracy Direct Normal Radiation and Diffuse Horizontal Radiation having only Global Horizontal Radiation and Total Sky Cover in tenth? Latitude and Longitude of the location are known so solar elevation angle and azimuth can be obtained. Other weather data such as Dry-bulb and Dew-point temperature are also available.
Of course, neglecting the atmosphere, direct solar radiation on a horizontal surface is proportional to the cosine of the Sun's zenith angle and there is no diffuse radiation. The existence of the atmosphere renders the problem much more difficult. Hopefully, the book "Optics of the Atmosphere" by Earl J. McCartney may be helpful. In general, for small zenith angles with a clear sky, direct solar radiation accounts for 80% to 90% of the light, and diffuse radiation for 10% to 20%. Of course, during dawn and dusk, with the Sun still below the horizon and with a clear sky (and always with a completely cloudy sky), 100% of the light is diffuse. At intermediate times with a clear sky, the values are also intermediate.
Thank you for the reply and for the book suggestion Jack Denur . My question has two main parts:
1-If it is possible to calculate direct normal radiation in an ideal condition of clear sky for the entire year. It means if there is a model that takes into account variables as day of the year (distance of the earth to the sun) and the atmospheric losses of radiation due to sun elevation angle (length of radiation beam through the atmosphere) and moisture.
2 - Once direct normal radiation is calculated for every hour of the year in the ideal condition of clear sky with sun, the second main part of the question if it is possible to use the total sky cover hourly data to derive the final hourly data for direct normal radiation.
Consequently using the cosine law it is possible to calculate the direct radiation on an horizontal surface and finally use the global horizontal radiation d(available) and the direct radiation on an horizontal surface to obtain the diffuse horizontal radiation.
The following article may be helpful. See the Abstract below. It investigates the effects of cloudiness on light specifically during solar eclipses, but perhaps the results may be helpful in general.
Cloudiness and Solar Radiation During the Longest Total Solar Eclipse of the 21st Century at Tianhuangping (Zhejiang), China
- Peñaloza-Murillo, Marcos A.;
- Pasachoff, Jay M.
Abstract
The lack of comprehensive solar radiation monitoring during the longest total solar eclipse of the 21st century at Tianhuangping (Zhejiang), China, on 22 July 2009, has led to this investigation in order to evaluate the cloudiness contribution in estimating the impact on global solar radiation throughout this phenomenon. In doing so, we applied a cloud cover empirical model to obtain the global solar radiation and, at the same time, we deduced a theoretical model to get the direct solar radiation in which both the occultation and obscuration functions of this eclipse are included. We took limb darkening and atmospheric transmission into account. Though the weather during our eclipse observations agreed with the forecasts for that day, clouds and some rain, we were nonetheless able to observe all phases of the eclipse from our observation site at Tianhuangping. This experience suggests that for coming eclipses a record of the in situ observation protocol of cloudiness is mandatory. Our results for comparing global solar radiation models indicate that our total solar eclipse radiation model is quite acceptable and representative of that which could have happened at that time.
Publication: Journal of Geophysical Research: Atmospheres, Volume 123, Issue 23, pp. 13,443-13,461Pub Date: December 2018DOI: 10.1029/2018JD029253
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