Hi Joe!
The Office of Dietary Supplements at the National Institute of Health state that 5-30 minutes of sun exposure without sunscreen for a minimum of two days a week should be adequate to maintain Vitamin D3 stores in the liver. This completely lines up with the teachings of Dr. John Christopher, who insited that his patients sunbathed unclothed daily, starting with 2 minutes and working up to... if I recall, an hour. Sunbathing should be done either before 10 AM or after 3 PM, to avoid the most intense sunlight, especially in summer. You don't want to burn.
Maybe some Northern snowbird can tell you what the rules are for playing in the snow, because being a Southern girl, I don't know. :)
Wings
...in Vitamin D production.
Here's a little chart I found:
How our bodies convert sunlight into Vitaminj D- image by www.healthyfellow.com
So you can see that the liver converts sunlight into usable Vitamin D and stores it there for future use. The kidneys balance calcium. As Uny mentioned, in order for Vitamin D to be assimilated properly, our livers and kidneys must be in good shape. So it all works together.
Oh- you *can* get Vitamin D3 from, in particular, fish oil and salmon. Please do your research. Schulze, Christopher, and so many other great natural healers are/have been adament vegetarians, and as Uny listed, there are effective plant sources for dietary Vitamin D.
Sometimes I just stand amazed at how beautifully designed our bodies are...
Wings