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Solar/UV Info

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Cambium
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Solar/UV Info

Post by Cambium »

Would love to hear a discussion on the readings of the Solar and UV numbers just for knowledge.

On the WeatherSmartIP grabs it reads Solar Radiation(w/m2) & UV-Index
On Cumulus the monthly log reads UV Index, Solar Radiation, Max Solar radiation, Hours of sunshine
On Ecowitt it reads Solar fc and UVI

Would love to hear those who are familiar with the numbers and what it means, how its calculated, the differences in seasons, ect, ect
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Re: Solar/UV Info

Post by Cambium »

Today I thought wasnt going to show anything but apparently 1235fc was read even with a full cloudy day.

On the Cumulus log its show 363 under Max Solar Radiation (23rd column) for today

Not trying to fix anything, just trying to learn the differences, the info on solar, and how these softwares are reading it.

Thanks
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Re: Solar/UV Info

Post by mcrossley »

The usual weather measurement of solar irradiation is either "direct" or "global".

Direct just measures the direct irradiation from the sun, these sensors are what professionals use, they a very expensive.

Global sensors measure all the irradiation from the whole sky. They have diffusers that create what is called a cosine response to the height of the sun in the sky. These sensors are what amateur stations normally use.

Both direct and global sensors measure in watts per square metre.

The Ecowitt stations have neither of these, they have sensor that measures illumination in lux. There is no direct conversion between lux and w/m² as it depends on what wavelength the lux measurements are taken. That said the usual approximation is to divided lux by 126.7 to get w/m².
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Re: Solar/UV Info

Post by Gyvate »

it's transposed digits :o - the value is 126.7 8-) - and a nice compilation of the reasoning behind this can be found at https://help.ambientweather.net/help/wh ... tor-126-7/ (there are many other conversion related articles in the web)
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Re: Solar/UV Info

Post by mcrossley »

Fat fingers :groan:
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Re: Solar/UV Info

Post by Cambium »

Cool, great info!

Not directly related but National Weather Service stopped keeping track of sunshine hours. I guess they didnt want to pay someone to look at the sky all the time. Now they use these sensors and its just not accurate enough.
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Re: Solar/UV Info

Post by Gyvate »

If you really want to track your local sunshine, have a look at www.sunrecorder.net
I'm using the device for about one year now and am very happy with it.
(actually it's under maintenance - that's why it doesn't show on the map - will be reinstalled beginning of the new year).
It has an interface to CMX so your real local sun hours will be recorded (if you select the corresponding setting).
It has the quality of a real pyranometer but is significantly cheaper. 8-)
The only challenge for some people may be that the connected software package only runs under Windows.
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Re: Solar/UV Info

Post by AllyCat »

Hi,
Gyvate wrote: Mon 20 Dec 2021 10:05 pm ... and a nice compilation of the reasoning behind this can be found at https://help.ambientweather.net/help/wh ... tor-126-7/
Hmm, I'm not very impressed with that and IMHO their statement "Solar Radiation is a shorter wave length" is misleading and arguably incorrect. (Infra-) Red actually has a longer wavelength than Green/Blue/Violet.

The term "spectrum" means a range of frequencies (in the electromagnetic spectrum), which for sunlight can be divided into the Visible part (from Violet to Red) sandwiched between the Ultra-Violet and the Infra-Red (heat) bands. The energy (i.e. watts/square meter) reaching the Earth's surface is quite small in the UV band (a few percent); approximately half is in the Visible part of the spectrum and half in the IR region. Human eyes can detect the Visible part and our skin detects some of the IR. Not surprisingly, the eye is most sensitive to the middle (Green) part of the spectrum, but the "Lux" value is an average of all the Visible frequencies, often measured with a Light Dependent Resistor (LDR) such as Cadmium Sulphide in (old) camera Exposure Meters. But nowadays, light is normally measured with a Silicon Photodiode, which can be anything from the tiny cells of a camera sensor up to the PV panels on our roofs. However, the peak sensitivity of a silicon photodiode is usually just into the Infra-Red, which is convenient because its bandwidth is similar to the major part of all the energy received from the sun. Their sensitivity falls away towards the blue (and of course UV) end of the spectrum and past the "Near (to visible) Infra-Red" section, but these diodes are far cheaper and more convenient than a professional "pyrometer".

I haven't examined the modern Ecowitt stations (yet), but the original Fine Offset WH308x, etc. stations adopted a wireless protocol that carried a "Lux" value as a (3-byte) binary number with a LSB of a tenth of a Lux (approximately moonlight). But the lowest (non-zero) value (and resolution) reported was typically 12 Lux, so the lowest byte was almost irrelevant. However, the "Solar Pod" didn't appear to contain a LDR but a "normal" Photodiode, so I suspect that it actually was measuring (a reasonable representation of) the watts/m2 value, and it's the Lux value which was potentially "wrong" (i.e. by including IR content). Of course, we don't know what "conversion factor" FO used to create the Lux value, and things don't bode well that some of those stations used a totally wrong conversion factor from Lux to Watts/m2. Also they have the "Very High" and "Extreme" UVI band labels accidentally swapped on the Console Display, and their calibration was highly dubious anyway, as mine "measured" a UVI level of 11, behind a glass window in UK! They did make a reasonable attempt to reproduce the "Cosine Law" (with a diffusing dome), which is simply the theoretical proportion of light from a "slanted" source which reaches an (ideal) horizontal surface. Normally "low" angles (near to the horizontal) are a problem because of reflections off the surfaces of protective/covering materials such as glass, which thus don't reach the sensor itself.

If all wavelengths (i.e. "colours") passed through the Earth's atmosphere in the same proportions, then none of this would matter (much), but equally there would be no need to make measurements for more than one wavelength (or band/spectrum). This opens a particular can of worms in the UV section of the spectrum, which is conventionally sub-divided into UVA, UVB and UVC regions. Basically, UVA normally does pass through the atmosphere (but typically not through glass) and is responsible for skin tanning, UVB partially passes through the atmosphere and is responsible for skin damage (e.g. cancers) and UVC does not (fortunately) pass through the atmosphere. The (official) "UV Index" has a specific definition which is very heavily weighted (up to 100x higher sensitivity) in the UVB (dangerous) region of the spectrum, but dedicated UVB sensors are rare and/or expensive. AFAIK, Davis are the only (meteorological station) manufacturers who make the measurement "correctly"; i.e. they use a wideband (UVA+UVB) sensor covered by an (expensive) optical filter tailored to the UVI specification (i.e. attenuating the UVA). Most other manufacturers just "infer" (guess) their "UVI" value from other measurements, typically from a UVA (occasionally UVA+UVB) sensor. One "UVI" sensor actually contains diodes which measure the Visible and the Infra-Red values and then calculates a UVI value from them! The VEML6075 sensor does appear to make a "genuine" UVB measurement (it has multiple "channels" including one for UVB+UVA+Vis+IR) but it's application/data sheet implies that for their "Calibration Reference Standard" they use a commercial Davis station!

Cheers, Alan.
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Re: Solar/UV Info

Post by HansR »

@AllyCat: thanks, nice.
Copy to the wiki combined with the previouss responses as background info before it sinks into oblivion?
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Re: Solar/UV Info

Post by Gyvate »

@AllyCat:
you made a nice and long statement which however is also debatable ... (but I don't want to do that here, because that was not my point.)
There are enough discussions and publications available by qualified scientists - nothing to add from my end

I was simply writing that you can find the reasoning behind why FOSHK are using the 126.7 factor/quotient is described at that link.
There was no claim that this would say to what extent this is "right", correct, exact etc.
The idea is that it produces a reasonably (context !) "exact" result. Not more, not less.
And the link refers only to solar radiation and not to the UVI topic which you elaborated lengthily.
UVI is a different cattle of fish (and the UVI readings produced by most of the semi-professional weather stations usually need calibration - otherwise they produce astronomical and unrealistic figures).

Otherwise, if you want to be really exact in both areas - which goes probably far beyond hobby meteorology - you need different instruments and approaches which are used in professional meteorology , aeronautics etc. and definitely not in semi-professional (hobbyist) weather stations. And a Davis Weather station doesn't come close to these instruments either.
Honni soit qui mal y pense. 8-)
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Re: Solar/UV Info

Post by AllyCat »

Hi,

@HansR: Thanks for your comment. I don't believe that I currently have Editing rights, but let's wait to see if there is any other feedback on my comments. :)

@Gyvate: Apologies, my comments were primarily intended as a reply to the OP who did ask specifically about UV. Maybe the ambientweather text has "lost something in translation", but the dictionary definition of "spectrum" is a range (of frequencies) and Lux is a "weighted average" of the whole Visible Band. So I thought that referring to a "spectrum at 550nm", and to adding on the "missing" (longer wavelength) IR Band as "Solar Radiation is a shorter wave length" was worthy of comment.

Cheers, Alan.
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Re: Solar/UV Info

Post by HansR »

AllyCat wrote: Tue 21 Dec 2021 11:11 pm I don't believe that I currently have Editing rights
Just a matter of asking ;)
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Re: Solar/UV Info

Post by Cambium »

If one was to look at sunshine data Cumulus reads from the station, what is a rule of thumb or best way to find sunny days?

Is it better to look at Sunshine Hours or Solar Data?
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Re: Solar/UV Info

Post by mcrossley »

I'd use hours of sunshine, but it depends how you are defining a "sunny day"?
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Re: Solar/UV Info

Post by Cambium »

mcrossley wrote: Fri 20 May 2022 4:09 pm I'd use hours of sunshine, but it depends how you are defining a "sunny day"?
True.. I guess using 8hrs+ for a full sunny day looks like only 9 out of past 30 days.

Sunrise 5:30am
Sunset 7:09pm
(13.5hrs)

Had a 7 day stretch of very little sun May 2-8.
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