Have found the relationship of sunlight and vitamin D
Vitamin D is not produced in human skin when the UV-B intensity is less than a given threshold. The duration of cloudless conditions and sunlight are the critical factors. The further north, the less effective is the sunlight.
In Tromsø, the sun is so low after 5th October that no cutaneous vitamin D is produced in humans after that date. In Oslo the date is 1 November.
The weather as well as location play important roles. Clouds attenuate the UV-B rays and reduces vitamin D production in human skin considerably. Extreme conditions of persisting clouds in northern Norway this autumn cause concerns about the vitamin D status of the northern-Norwegian population.
As a result of the research work, senior scientist Ola Engelsen at NILU has devised an Internet calculator in which the user can compute the duration of cutanous vitamin D as a function of time, location and atmospheric conditions.