Sunlight has long been recognized as a major provider of vitamin D for humans; radiation in the UVB (290–315 nm) portion of the solar spectrum photolyzes 7-dehydrocholesterol in the skin to previtamin D₃, which, in turn, is converted by a thermal process to vitamin D₃. Latitude and season affect both the quantity and quality of solar radiation reaching the earth’s surface, especially in the UVB region of the spectrum, but little is known about how these influence the ability of sunlight to synthesize vitamin D₃ in skin. A model has been developed to evaluate the effect of seasonal and latitudinal changes on the potential of sunlight to initiate cutaneous production of vitamin D₃. Human skin or [3α-³H]7-dehydrocholesterol exposed to sunlight on cloudless days in Boston (42.2°N) from November through February produced no previtamin D₃. In Edmonton (52°N) this ineffective winter period extended from October through March. Further south (34°N and 18°N), sunlight effectively photoconverted 7-dehydrocholesterol to previtamin D₃ in the middle of winter. These results quantify the dramatic influence of changes in solar UVB radiation on cutaneous vitamin D₃ synthesis and indicate the latitudinal increase in the length of the “vitamin D winter” during which dietary supplementation of the vitamin may be advisable.