Temperature has nothing to do with D absorption. Supposeldy 20 minutes in the sun a day is sufficient, with enough skin exposed (face/arms iirc). Depending on melatonin I suppose.
D3 is the usual kind in supplements and also is added to milk; made from irradiated vegetable oil. D3 is not radioactive of course.
D is made on the skin when the sunlight hits the natural skin oil. So bathing or washing off the sun-exposed skin should not be done for a few hours after sun exposure in order for the skin to absorb the Vit D.
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