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Solar radiation measurement

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Jáchym:
OK, I appologize in advance as it might be stupid question but...

I personally don't have a weather station which would measure solar radiation so I cannot answer this myself.

A friend of mine was suggesting to me that you can actually determine the duration of sunlight based on the measurements from stations such as Davis VP2 or any other station that measures solar radiation. According to his view, in order to calculate the duration of sun shine you should simply look at the intervals at which the solar radiation measurements are not 0.

However, my understanding of how these stations work was that they actually measure the actual "amount" of light rather than sun shine so it only tells you when it is day/night, but it is not possible to determine the sun shine duration (to be precise, it is of course possible, but using a different device and not the solar radiation sensors that come as part of many weather stations).

As I said, I don't have a station measuring this myself so I would like to hear your opinions and if it is the case that I am correct, then if you know of any option how to measure the actual duration of sunlight in non-professional environment and ideally for some reasonable price.

Bushman:
What EXACTLY is he trying to measure?  Clear sky?  Direct sunlight - as opposed to diffused like you would get with thin cloud or fog?  http://www.pveducation.org/pvcdrom/properties-of-sunlight/measurement-of-solar-radiation

broadstairs:
Well measuring sunshine hours, which I believe is what your fried means, is not straight forward. Weather Display does it reasonably well. The Davis stations have a global solar radiation sensor which means that readings can go from 0watts/m² to over 800w/m² (possibly up to 1000w/m² because of cloud reflections etc) and that covers all lighting conditions from night through to bright sunshine. In order to calculate whether or not it is sunny you need to calculate the expected max solar radiation for your time of day, latitude/longitude,  an allowance for the effect of the atmosphere (attenuation) and compare that with the Davis reading and calculate a % value, then set a threshold above which it is considered sunny (usually around 80%), you then count the minutes for where it is above threshold which gives sunshine duration.

Often quoted is a value of I believe 100w/m² for sunny conditions but that only applies to a solar sensor which measures direct radiation and is one which tracks the sun across the sky.

So it is not trivial as you can imagine.

Stuart

Jáchym:
Thanks for clarification, basically this is exactly what I thought... that either you would have to have a device which detects direct sun shine reaching it or you have to estimate it from the expected maximum.

Either way, the point is - it is not as easy as he thinks - i.e. the solar radiation sensors do not show 0 when it is overcast.

thanks

mcrossley:
What Stuart said, plus some of the weather programs (I know Cumulus and WD do it) will calculate the current theoretical value for you, and apply your desired percentage threshold to provide an indication of 'sunshine'. Is it as good as a dedicated sunshine sensor?  No, but I have found it reasonably accurate, good enough to give a fair indication of the sunshine hours.

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