Why is it so hard to gain enough Vitamin D by solar exposure in the European winter

UV exposure, which is the main source for a sufficient level of vitamin D in the human body, is found to be up to a factor of 7 lower in Northern Germany (52° N) in the winter months compared to UV levels in the central region of New Zealand's South Island (45° S). When corrected for the influence of solar zenith angle, the vitamin D-weighted exposure is still a factor of 2 higher in the southern hemisphere at the corresponding latitude. The major part of the difference can be attributed to differences in cloudiness, and a minor part to total ozone and aerosols. Data from several stations in Europe show a high variability due to cloudiness differences between the stations and between different years, but they also show that the differences are not restricted to individual sites and may characterize a northern versus southern hemisphere contrast. Wintertime erythemally-weighted irradiance is also found to be much higher in New Zealand than in Europe. Whereas on a monthly average clouds weaken the UV irradiation by up to 25 % for most locations in New Zealand, the reduction is usually up to 50 % in central Europe in winter.