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Assuming simple conditions e.g. sunny, bright day (no clouds), sun is directly overhead, what becomes the effective UV index for someone who is outdoors under solid shade (e.g. sitting under a concrete roof on the patio outside, not under a tree/umbrella that leaks light through)?

I'm not looking for an exact answer since I would assume that it depends on many complicated things such as the type of ground/surface in the surrounding area and how far does the concrete extend beyond that person, etc..

Only reflected UV rays would hit you from the sides in that case, but an estimated how much?

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  • $\begingroup$ There is a scattering by the atmosphere and off the ground and surroundings, but without making measurements or making some kind of model it really is hard to make any estimates. It also depends on the weather. On a cloudy overcast day you still get about 30% of the UV going through the clouds. On a bright day with a roof in the shade if there is enough light to read by, it is still hard to tell how much light is reading you, but could only be a few percent, and the UV amount is even smaller. $\endgroup$
    – UVphoton
    Jul 24, 2022 at 0:47
  • $\begingroup$ I noticed that the UV index according to some websites remains almost the same actually during cloudy days. This contradicts what you say that only 30% of UV passes $\endgroup$
    – WalksB
    Jul 24, 2022 at 18:03
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    $\begingroup$ Clouds are complicated. The UV index is calculated using a radiative transfer model that includes ozone and aerosols and also clouds. The national weather service in their model kind of divides it into categories of cloud cover it is close to 100% up to a certain point then drops to 60% and if completely overcast goes down to around 30 percent. Of course that is without a roof. $\endgroup$
    – UVphoton
    Jul 24, 2022 at 18:55

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The graph below shows measurements of UV as a function of cloud cover. So having the roof over your head almost all the sunlight is blocked, except what is scattered in from the sides. The other factor is how much UV reaches the ground depending on cloud cover. In low light still able to read a book etc. The amount of light reaching the person under the cover is actually really small, partly becasue the eye is really good, and open up its pupil etc. If you did a model you would probably find only a small percentage of sunlight would be hitting the person, and that would also depend on cloud cover.

UV index forecastability issues,  long et al. SPIE

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  • $\begingroup$ I'm hesitant to say that this answers the question because I'm assuming that the device they used to make these measurements was pointing upwards towards the sky, whereas in my case all the rays would come from the sides. $\endgroup$
    – WalksB
    Sep 9, 2022 at 21:33

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