Great - can you provide a reference to this? As far as energy is concerned - the shorter the wavelength usually the more penetrating power. I'm not so sure there is a proportionate difference between UVA and UVB exposure as the sun agle changes. As the angle increases TOTAL UV exposure drops as the light has to go through more atmosphere but i'm interested in seeing your citation that UVB halts sooner than UVA. Thanks
“Sun exposure Most people meet their vitamin D needs through exposure to sunlight [5,31]. Ultraviolet (UV) B radiation with a wavelength of 290-315 nanometers penetrates uncovered skin and converts cutaneous 7-dehydrocholesterol to previtamin D3, which in turn becomes vitamin D3 [9,32,33]. Season, geographic latitude, time of day, cloud cover, smog, skin melanin content, and sunscreen are among the factors that affect UV radiation exposure and vitamin D synthesis [33]. The UV energy above 42 degrees north latitude (a line approximately between the northern border of California and Boston) is insufficient for cutaneous vitamin D synthesis from November through February [5]; in far northern latitudes, this reduced intensity lasts for up to 6 months. In the United States, latitudes below 34 degrees north (a line between Los Angeles and Columbia, South Carolina) allow for cutaneous production of vitamin D throughout the year [27].”
http://ods.od.nih.gov/factsheets/vitamind.asp