Solar Radiation, Illuminance, and UV Index, is there any difference?


#21

the docs of the sensor tells us that it measures both uva and uvb. it returns values on a scale from 0 to 11. It has a graph in figure 3 with the uv response. It has a linear scale. This looks close to the blue line in the second figure of the wikipedia article you referred to (note that that graph is on a log scale, so the values on the tail to the right are more then 10 times lower then the maximum, which matches the graph in the docs of the sensor).
So I think the intention is to return a value that IS related to sunburn and the UV index you see in other weather reports.


#22

Thank you for your reply. My intervention to this thread is mainly because, in practice, I’ve not been able to correlate the WF UV figures with the official ones. I hope the following figs will give you an idea of my concern.
2019-01-24_103120
Although it’s not very clear, the deep violet line at the bottom shows the measured UV in WF, showing a UV index peak of 5.3 at 11:37 and generally following the solar light curve (orange) including the peaks and valleys.


This shows the official measured UV covering the same period with an index peak of 3.9. You may wonder why the curve is relatively smooth, compared with WF; this is because light cloud, visible in the WD graph, is transparent to UVB, less so to UVA and even less to visible. The intermittency of visible light in my graph is reflected in the UV curve of WF, whereas there is relatively little intermittency in the official UV one, related to visible light.

I’ll continue to study this and I’ll try to obtain a better UV/solar graphs to explain my concerns better.


#23

That’s pretty interesting. I didn’t know that uvb passes through clouds. With what instrument is the official uv measured in your graph?


#24

Yes, it passes through light clouds, which is why you can still sunburn with a hazy sun. It depends on the size of the water droplets.

The original instrument was the British Dobson spectrophotometer ( https://en.wikipedia.org/wiki/Dobson_ozone_spectrophotometer ), which had two sensors, one in near UVA and the other in UVB and did a comparison between them (The first one was installed in Davos, Switzerland in the 1920s). Although still much used, it has been largely replaced by the Canadian Brewer spectrophotometer which is much more sophisticated ( http://kippzonen-brewer.com/about-brewer/brewer/ ) and measures more variables. The local one is a Brewer, I think installed at the Institute of Meteorology of the University of Cyprus.


#25

But I thought it was uva that passes through clouds due to its longer wavelength.
If you have fluffy clouds uvb can increase because it bounces off the clouds.
You are probably right that it depends on droplet size. I didn’t look into it.


#26

To be honest, I don’t have much of a clue, weather-wise. My expertise (23+ years working with UNEP) is more in the effects of stratospheric ozone depletion at the earth’s surface and, above all, the emissions of ozone-depleting chemicals. Secondarily from this experience, I picked up some semi-scientific knowledge concerning measurements of the ozone layer and consequently UV radiation. That was in a former life over 20 years ago and, since then, I’ve forgotten more than I ever learnt!

https://uk-air.defra.gov.uk/research/ozone-uv/the-uv-index

Practically, yesterday was overcast and no comparison was possible. Today, it is mostly sunny with variable intensity and again I’m seeing the UV curve from WF closely following the solar in shape. Incidentally, the peak level (UV and solar) today is lower than 2 days ago, which I think is due to Sahara sand blowing across the island. I guess I should wait for a day with wall-to-wall sunshine to get a better idea.