Well said and exactly on point.
Sorry not to have replied earlier; 3 days of ‘man-flu’! Also sorry about the typo in the URL ref – my fault.
Please have a look at https://www.canada.ca/en/health-canada/services/sun-safety/what-is-ultraviolet-radiation.html and in particular paras 3 and 4 of the factors.
FYI, professional calculation of the UV Index is done by separate and independent measurements of UVA and UVB (and other things) at different wavelengths, using the modern Brewer Mk III spectrophotometer. http://kippzonen-brewer.com/uv/measure-uv-radiation/
I don’t know the algorithm used to determine the index.
Less sophisticated ‘solid state’ instruments without moving parts use a CCD or photodiode array as the detector, which measures the complete spectral range simultaneously. However, these instruments are usually intended for measurement in laboratories with artificial light sources. They do not normally have the stability, sensitivity or stray light rejection for measuring UVB and UVE outdoors from sunlight. (my bolding)
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That’s great information but rather irrelevant here as the sensor being usec is a low-grade nonscientific device.
The UV value is a best-guess number and only a generic guide.
I agree absolutely, but a generic guide may cause mishaps, especially in low latitudes. A gentle warning may be indicated, which is the point I implied from the start of this discussion.
And that’s where individual responsibility comes in. I don’t know where you live but in Colorado we pay particular attention to UV from the national weather service. Living at a high altitude with little cloud cover has taught us to be cautious.
Any reading (UV, temperature, humidity, etc) should be treated only as a guide from ANY consumer grade weather system. This is not something unique to the WF system.
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I’m not sure I agree. It might be true that they are less accurate, and calibration might be an issue, but that doesn’t degrade them to just a guide. UV, temperature etc can be measured accurate enough for any real purpose. Nobody needs temperatures to 0.01 degree. Perhaps the biggest problem is that they measure local conditions, which means that for example the wind speed might be influenced by the big trees around the house (even the house is an issue). So the measured value might be hard to compare to other stations, but it is the local wind speed and in itself can be measured with enough precision and accuracy, even with consumer grade.
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I’m missing any point there. EVERYTHING that is a measurement is done with the precision and accuracy of the thing doing the measurements, at the place and time the measurements are done. That’s kinda obvious.
You might also want to reply to the post you’re disagreeing with, so we know which conversation you’re replying to
But FWIW, I definitely hear a whole lot of seemingly excuses from a lot of the usual suspects in the forums regarding not expecting much from the values WF is reporting to their customers for this gear. It gets tiring to hear. It really needs to stop.
(and it’s notable that it is never WF employees making the seeming excuses)
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sorry, I thought I was answering to @hankster 's post.
That is exactly my point. Just saying it is consumer grade and therefore should only be treated as a guide doesn’t do justice to many of consumer grade sensors. And I for one would like the weatherflow system to be pretty accurate, even thought it uses a few novel sensors that you don’t usually see an consumer grade weather stations.
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Sure, but no-one (hopefully) says “Well, my WF is currently showing a UV index of 4.5, so I’m not going to put on my sunscreen this morning”, any more than they’d say “The NWS UV index is only…” While it’s easy to measure these things with consumer grade sensors, and easy to log the data, it’s not clear if the UV readings are actually useful information.
On the other tentacle, if you have enough datapoints and you know the provenance of them, then maybe you can do some data-mining to show the effects of global climate change or something…
I have a button: If you torture the data enough, it will confess. 8*)
I’f I’m hanging out around the house, I typically won’t apply sunscreen, if I’m going to the beach I invariably will. No sensor, not matter it’s quality, will change that particular behavior…
if you are not interested in UV values, just ignore them. But you can probably hold the same reasoning you did for UV for temperature, or pressure or …
The thread has wandered slightly from the original subject but I cannot fault the temperature, barometer, humidity, visible solar intensity etc., which are all reasonably accurate because the sensors are actually measuring physical quantities. As a single example, as I write, my two “conventional” weather stations are measuring 18.7° C and 18.9° C and, as luck would have it, WF is measuring 18.8° C! Can’t do better than that! If I had a fully professional weather station with a Stevenson screen, I would not mind betting that the temperature inside it would be 18.5° C ± 0.5° C (note the average is slightly lower than my measured values, as there would be fewer radiation errors).
Unlike these physical parameters, the UV index is like sticking your thumb in the air to find the wind direction; it may give you an indication, but nothing better than that.
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what I meant is that you argue that your behavior doesn’t depend on the exact value of the uv-index. I’m saying that is probably the same for many other parameters. You are most likely not to change your behavior if the pressure is 1021 or 1030, nor are you going to change you behavior a lot between a temperature of 11 and 13.
With regard to uv index. as long as the value reported is more or less the correct value there is nothing to complain about.
If you are going to apply sunscreen or not depend on many other factors (including skin type). Any of the other mentioned values be it thickness of ozon layer, uvb) that doesn’t include those other factors is probably as useful as the uv-index. One might even argue that uv-index is more useful as it includes at least some sensitivity curve for sunburn (even though this sensitivity greatly varies per person)
Regarding my UV experiments.
I have a sun burning problem. When I am flying my paraglider here in Australia I am exposed to extreme burning. I decided to buy a helmet with a visor they stated as blocking 100% UV. I flew with the visor and without my other protections and was burnt.
Now I wish to test my visor and I thought I could use a Sky. The Sky showed only slight difference to UV. So then I tried my skydiving helmet visor and my sunglasses, and a hoya Skylight (1B) filter. None of these made much of a difference to the Sky UV reading.
I dont think that I can use the Sky to prove UV blocking. I will try to find an optometrist with a UVA and UVB measuring device to test my visors.
cheers Ian 
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Ian, the skylight filter isn’t a UV filter, see explanation in this video.
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Wow @eric,
I didnt realise, so I have done another test this time using a UV filter and will also provide the numbers:
This is the UV reading from the Sky on my roof without any lens over it showing a fairly straight line during the test period:
This is the setup for approximately 3 minutes from just prior to 12:55 and removed prior to 12:59:
This is dip from the Sky covered by the UV filter:
So from this evidence I dont think I can use the Sky UV reading to test how much UV is passing through my lenses.
cheers Ian
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Think I need to dig again in photo UV filters, not sure they are really meant to ‘block’ all but a part that affects colour shift I think. Will look that tonight when I come home
And be carefull under those UV readings …
Might I help out with some details?
Here optical and physiological aspects in cooperate:
Sadly, Hoya doesn’t provides full spectral curves, but it is obvious, it strongly depends on the type of filter one uses.
Additionally, the UVI calculation method and its spectral behavior (see below, taken from Wikipedia) have to be considered.
By Hankwang - Own work, CC BY-SA 3.0, Link
It represents the UV(A) (400 - 315 nm) and UV(B) (315 - 280 nm) part of the spectrum.
And finally, the sensor and the detailed setup within the sky might play a role. But, as it presents a UVI-value, we can consider most of the “magic” has already occurred.
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While this would appear comfortable in winter imagine waiting on launch in the sun at +40 deg with a down jacket on ready for the cold at 10,000ft where the UV is even stronger. And much of my flight I am facing the sun. Hence why I am trying to use a visor that blocks UV. (And balancing electrolyte fluid intake etc)
cheers Ian 
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