Hi Jon, I am not sure I know which paper you referenced, but I agree with you that the 120 w/m^2 threshold is a little bit arbitrary. To my experience, in a sunny morning the radiation is less than those 120 w/m^2. Unfortunately, instruments that can really distinguish between the direct sun light and the diffusive radiation from the sky are very expensive. Most people only have a general radiation sensor, which cannot know whether the sun is shining or the sky is bright. Light is light. You cannot fill in the lack of information by calculation.
After considering this all I bought a sunshine duration sensor from Instromet <https://instromet.co.uk/sun-duration-sensor/>. This device is expensive, too, but still achievable to normal people. It uses a shadow bar. If the radiation differs between before and behind the bar, this is considered sunshine. A well-defined shadow is not necessary here. Karen n7uv...@gmail.com schrieb am Freitag, 3. Mai 2024 um 01:23:52 UTC+2: > Hi Karen - > > I went and read the paper that is referenced for the sunshine duration > method. It describes an empirical approach to estimating whether or not the > sun is "shining". To me, as you suggest, if I have a crisp, well-defined > shadow, then there is indeed a 6000 K point source in the sky. If I meausre > in a low humidity, low atmospheric turbidity environment, with low > atmospheric particulate count, that shadow should indeed be crisp and well > defined. If I measure in a location where the sun is clearly above the > horizon, but the shadow is less well defined due to those confounding > factors and others, I can still assert that it is sunny, yet it is a degree > less sunny than at the former location. > > If one looks at brewster76/ > > - util-archer <https://github.com/brewster76/util-archer/tree/master> > - /user <https://github.com/brewster76/util-archer/tree/master/user> > > /radiationhours.py > > > there is an arbitrary value (to be inserted by the user) on what > constitutes full sunlight. > > _____________________________________________ > Adds a new observation field to weewx: [sunshine_hours] > > If the radiation observed during an interval of time exceeds 120 W/m2, > then the interval is considered sunny, > and [sunshine_hours] is set the length of the time interval. > > When [sunshine_hours] is summed over a day, the result is the number of > hours during the day when radiation > exceeded 120 W/m2, or 'hours of sunshine'. > > The threshold of 120 W/m2 can be overwritten in weewx.conf: > > [RadiationDays] > min_sunshine = 120 > _______________________________________________ > > so, for Brewster76, 120 w/m^2 is the threshold. This is more or less > consistent with the original paper, which attempts to get the necessary > fudge factors on atmospheric clarity via long--term empirical observations > vs the theoretical value for the insolation at that minute at that > location. It by no means is a measure of the shadow, or just how crisp it > might be. > > Various places in the USA all claim to have the most sunshine. I always > wondered how they measured that, and what constituted sunshine. If indeed > it is via models like what is presented in the paper and the subsequent > code examples, it is just a marketing term. > > If there is a measuring device, I call it the umbrameter, that can > actually measure the depth of shadow cast, and quantitatively show that > indeed it is a sunny minute, that would be a cool instrument. I think a > quality video camera or other 2D sensor could inspect the image of the sun > and, through sufficient training, could say that "this image is within x% > of the expected image of the sun taken above the atmosphere, that would be > a useful instrument. > > Sunny days and sunshine duration appear to be a matter of opinion. > > Cheers - Jon N7UV > On Thursday, May 2, 2024 at 12:03:15 AM UTC-7 Karen K wrote: > >> n7uv...@gmail.com schrieb am Donnerstag, 2. Mai 2024 um 01:01:52 UTC+2: >> >> When I hear "sunshine duration", I'm not sure whether that means the >> duration that the scary fiery (I live in PHX %^) ball of plasma is at and >> above the horizon (which is already provided under the Celestial tab) >> [image: Screenshot 2024-05-01 155959.png] >> or something about the total energy delivered to a square meter of the >> ground over the period of a day. >> >> >> Sunshine duration means the time the sun is really seen in the sky and >> typically casts a shadow. The standard instrument to measure sunshine >> duration is the Campbell-Stokes sunshine recorder >> <https://en.wikipedia.org/wiki/Sunshine_recorder>. >> >> Sunshine duration is *not* total daylight time. >> >> If a cloud is situated before the sun, this is considered no sunshine. If >> clouds are elsewhere in the sky it does not matter. >> >> -- You received this message because you are subscribed to the Google Groups "weewx-user" group. To unsubscribe from this group and stop receiving emails from it, send an email to weewx-user+unsubscr...@googlegroups.com. To view this discussion on the web visit https://groups.google.com/d/msgid/weewx-user/9af1aee0-369c-4cc7-9ba9-6e57873b56ddn%40googlegroups.com.
