I am not a trained scientist in UV index and I guess this is basic training for the scientists and possibly more for their forum…
I had been wondering if the Tempest UV index value might require adjusting to allow for the angle of the sun.
Because if the sun is low on the horizon the horizontal sensor on top of the Tempest will measure less UV index than if exactly the same amount of UV radiation was coming from directly overhead.
To imagine what I am referring to then consider that when I am calculating solar energy entering solar panels I need to apply some formulas to adjust for the angle of the panels compared to the angle of the sun.
So I looked around trying to find information about measuring the UV index more appropriately by including vertical and angled radiation.
In my research it appears that the UV index is measured on a horizontal surface.
reference page 21, Annex C of this Australian guide. https://www.arpansa.gov.au/sites/default/files/uviguide.pdf
“It is a measure of the UV radiation that is relevant to and defined for a horizontal surface.”
Which means the Tempest horizontal sensor can potentially measure the UV index correctly.
BUT in my opinion it means that the UV index does not correctly measure the amount of skin burning UV from the sun.
It means that when the sun is at 30 degrees producing a reading of 5 UV index it will burn more than another time of the day or year when the sun is at 60 degrees recording the same 5 UV index.
The UV radiation will generally be highest at midday in summer but:
I believe that the UV burning radiation earlier and later in the day or times of year or locations on the planet when the sun angle is not directly overhead will be greater than the UV index indicates.
cheers Ian 
perhaps it should be compared to burning rate for a horizontal piece of skin?