Earthdata Forum

Hello everyone,

I have downloaded the daily type data in SSE of Power Data Access Viewer, as per in the screenshot attached.

For all these 3 parameters (ALLSKY, CLRSKY and KT), how are they interrelated with each other?

I've tried multiplying the KT with CLRSKY to get ALLSKY but I got different value.

Your kind explanation is very much appreciated with thanks.

Thank you for your question. A Subject Matter Expert has been notified and will answer your question shortly. Please stand by!

Thanks for using POWER data. Allsky refers to the solar irradance at the surface average under clear and cloudy conditions. The classic clearness index KT is defined as the ratio of the allsky solar irradiance at the surface divided by the solar irradiance received at the top of the atmosphere. Thus, this KT is actually more a solar transmittance. Starting on Jan 1, the clear-sky flux is the solar irradiance estimate when cloudiness is less than 1% over grid box area that includes your location of interest. In 2007 and before the clear-sky flux is determined by computing the fluxes for an atmosphere without clouds. Currently, the satellite information is collected, averaged and processed over at 1 degree x 1 degree area and is remapped using 1/2 x 1/2 degree using replication (ie., the four 1/2 x 1/2 grid boxes within the 1 x 1 box have the same value). Please see the "Methodology" (https://power.larc.nasa.gov/docs/methodology/) for more information but feel free to respond with additional questions.

Hi Dr. Paul Stackhouse,

Thanks for your prompt reply.

Are the insolation clearness index and cloud index the same thing? I have referred to some books/journals and encountered these two terms, quite confusing to me actually.

Regarding the ALLSKY parameter, is it the same as Global Horizontal Irradiation (GHI)? Means I can straight away use it for solar PV system calculation or I need to factor in the cloud cover?

Many thanks for your kind explanation.

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Unfortunately, I am not familiar with the "cloud index" that you refer to. Our clearness index is:

KT = all-sky solar irradiance at the surface (or GHI) / Total Solar Irradiance (or extraterrestrial irradiance)

All-sky quantities mean that all scattering and absorption by gases, aerosols and clouds are considered in the estimate. And yes, our all-sky solar irradiance is another name for "global horizontal irradiance" or GHI. The TSI considers the solar constant of 1361 W m^-2 (as measurements by the NASA SORCE Mission) modified by all astronomical and solar zenith angle conditions. I would try to find a reference with the exact definition of the "cloud index" that you refer to as having been used in other literature that would help us figure out whether our KT is the same as the "cloud index" that you've also heard about. Does this help clarify?

Hi Dr. Stackhouse,

I should preface my question by saying I am not a scientist, by any means. I work for an electric cooperative, and we installed a "Solar Demonstration Lab." We use the Lab to educate our members about solar power. We share kWh production, maintenance issues we've had, etc. The Lab has operated since late 2015. I suspect the modules are degrading faster than their warranties allow (up to 3% in year one and only 0.7% over the following 24 years). Would the attached chart be a fair way to illustrate our decrease in production has had little to do with weather and more to do with our modules degrading faster than the manufacturers' warranties allow? I apologize in advance, if I have not included the data you need to answer my question.

Thank you,

Joey Magnini

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Dear Mr. Magnini,

I apologize for the delay. Your question will be difficult to answer because I don't know how responsive your system is to changing solar irradiance values. The clearness index decrease over the time span you show is about 6%. Yet, the output of the system has decreased 15%. Perhaps, more telling are the years 2018 to 2020 where the KT was stable, but the output from your system continued to decrease. Perhaps, if you assessed the monthly variation of your system output vs the solar irradiance you would be establish the sensitivity of your system electrical output to the solar irradiance variability. Then you can judge whether the output is decreasing in time relative to solar irradiance changes. I should also write that we aren't solar engineers and mostly familiar with how to generate the surface solar irradiance estimates used to derive the KT index. I hope that helps and good luck diagnosing your system performance.

Paul Stackhouse for the POWER Team