Due to extensive damage to my face because of an accident, I am currently undergoing skin therapy (laser/IPL/medication), that makes my skin extremely sensitive to UV radiation. So I need to stay out of the sun, at the very least between 11:00 and 16:00, need to reapply sun screen every two hours, and generally be careful with exposure. Since that makes me hyper aware to the sun, perceived warmth, UV index and all that, it made me reasearch the subject.
I live in The Netherlands and the meteorology institute published real time UV measurements (as UV index) at https://www.rivm.nl/Onderwerpen/Z/Zonkracht However the UV index is defined as the amount energy that is radiated on the flat surface of the earth. As we humans generally have more vertical surfaces, especially sensitive surfaces like the face, would it not make more sense to not just measure in the horizontal plane? Especially during the evening and the morning the sun is at a low angle, so the (relative) energy on a vertical plane would be a lot higher than the above graph suggests.
I made a small program that parses this image, and recalculates the UV index for the vertical plain (divide the UV index by the tangent of (90 degrees minus current zenith angle). This would be the output (blue line is vertical UV index):