I didn't mean to imply that more D can be created in spring and fall. It's more difficult to get Vitamin D during the spring and fall which is precisely why it's important to spend more time outside during those times of year. In the summer it's very easy to get enough D. For school children a 20 minute recess outside in June would be enough. But it takes longer during spring and fall to get the same amount of D. What I propose is allowing children to have month long spring and fall breaks from school so that they may spend more time outside during those times of year when D is harder to make. And of course skin must be exposed. Temperatures in spring and fall in the middle of the day are often warm enough to permit short sleeve shirts and short pants.
Winter is more difficult, of course, though for me I've created a special sitting area shielded from the wind that allows me to sun some days in winter when the sky is clear.
10-4, thanks for the clarification.
My kids spend more time outside at school: breaks, lunch, after school activities.