Interesting, in Canada we're told that the angle made by the sun in winter in places at "high" latitudes (> 50) prevents us from getting the right kind of light/UV in the winter, and to compensate by taking daily supplements of vitamin D.
>>dorfsm+(OP)
You just need more sunlight. At tropical latitudes you get enough D in five minutes with only your face exposed. At northern latitudes it just takes longer, or you can expose more skin.
>>sliver+91
If the sun is very low in the sky, no UVB radiation will reach you. The body needs the UVB radiation in order to produce vitamin D. I think a general rule of thumb is that if your shadow is longer than you are, then there is no UVB.