It has to do with UVB rays, rather than UVA rays. UVB rays only show up where I live from mid April to around October. Other months of the year, the Sun is to

It has to do with UVB rays, rather than UVA rays. UVB rays only show up where I live from mid April to around October. Other months of the year, the Sun is too low in the sky and those rays bounce off of the atmosphere. The closer you are to the equator, the more yearly coverage you get of UVB rays.