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 degrees N) in the winter months compared to UV levels in the central region of New Zealand's South Island (45 degrees 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.