Formula
Shade = T + (RH - 40) / 15
- 0.7 × Wind
Sun = Shade + 8 × Solar
+ 4 × Reflect
(Sun = Shade when Solar is 0)
Solar = UV / ClearSkyUV
× (1 - (Cloud / 100)^0.7)
Beam = Direct / Shortwave
Cloud = 100
× (1 - Beam / 0.85)
Cloud comes from how much of the sunlight arrives as a direct
beam, so thin high cloud with the sun out still counts as sun,
and an hour whose sunlight reads 0 counts as full cloud. It
falls back to the forecast's cloud cover when a sunlight
reading is missing. Solar is 0 at night, and the reflect term
applies only while Solar is above 0. Numbers shown are the
defaults.