Although the UV index has been greatly beneficial for reducing high risk sun exposure since 1994 when it was first introduced by the Canadian Meteorological Service, there are some very important caveats to the measurement.
1. UV exposure to the eyes and vertical surfaces of skin, such as the face, can actually be more severe when the Sun is lower (though UVB:UVA ratio can also different). (E.g. the end of a summer’s day, or winter afternoons on a ski trail) This is partly a consequence of the fact that the measurement equipment upon which the index is based is a flat horizontal surface. There is no compensation for angles.
2. UV intensity can nearly double with reflection from snow or other bright surfaces like water, sand, or concrete.
Note the combined potential of a quadrupling of intensity, when sun is 30 degrees from horizontal.
Effectively, you can be exposed to a UV index of 8 on a sunny afternoon on a snowy day even if the measured index is 2!
Edited from original post on facebook April of 2020