Being able to tan in winter in some areas far away from the equator doesn’t mean we’re getting vitamin D. They’re not directly related. Tanning rays (UVA) more easily pass through the atmosphere. UVA rays can also pass through glass, but not UVB.

High altitude decreases the atmosphere (ozone, etc), which could allow vitamin D producing rays (UVB) to get through in winter far from the equator, if high enough and not too far north or south.

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Overview UV and vitamin D

• UVB decreases faster than UVA when far from equator or winter season

[…]

UVB and UVA are very different

UVB UVA
Wavelength 280-315 315-400?
Tan Nope Tan
Vitamin D Vitamin D nope
Sun Screen blocked by sunscreen varies
Tanning salon varies widely
with tubes used		 5X more intense
than sun
Thru glass
eye and window		 blocked by glass UVA passed by some glass
may destroy some vitamin D
Birds can see nope Many birds can see UVA
Reptile can see Yes reptile Yes reptile
Wrinkle Yes ??
Bleach furniture ?? Yes
Skin Cancer controversy controversy

UV transmission by standard window glass
No UVB (Vitamin D) gets thru, but most UVA (Tan) gets thru
uv window glass.jpg

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