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Pale skin, Vitamin D & Scotland.

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SterileNeutrino
1272338.  Sun Jan 28, 2018 3:38 pm Reply with quote

Except during solar eclipses, half the Earth is sunlit and half in shadow all the time. Averaged over the seasons, high latitudes and low latitudes both receive 12 hours of daylight and 12 hours of night. So there is ample UV to ensure adequate levels of vitamin D at all latitudes. The question is: how much skin is exposed for how long? In cold countries, we cover up and stay in doors a lot. It is this - clothing and shelter - that restrict our vitamin D levels, not a lack of available UV. Pale skin is an evolutionary adaptation to staying out of the sun, not the weakness of the sun itself: Scotland is not as dark as people make out.

 
crissdee
1272345.  Sun Jan 28, 2018 5:17 pm Reply with quote

I have heard that there are deep valleys in Scotland (and possibly other places as well) that receive no (direct) sun at all at certain times of the year.

 
Baryonyx
1272383.  Mon Jan 29, 2018 5:48 am Reply with quote

Also you sleep some of the time so if you're at an extreme latitude you'll waste some sunlit time sleeping (in the summer). Not a problem on the equator.

 
'yorz
1272385.  Mon Jan 29, 2018 5:55 am Reply with quote



The Norwegian village that finally saw sunlight - thanks to mirrors (2013!)

 
dr.bob
1272407.  Mon Jan 29, 2018 9:34 am Reply with quote

SterileNeutrino wrote:
Except during solar eclipses, half the Earth is sunlit and half in shadow all the time. Averaged over the seasons, high latitudes and low latitudes both receive 12 hours of daylight and 12 hours of night. So there is ample UV to ensure adequate levels of vitamin D at all latitudes.


This seems to ignore the angle that the light is entering the atmosphere.

At the equator, the sunlight will come almost straight down for most of the year, and will therefore have to pass through much less atmosphere, resulting in less attenuation of the UV light. By contrast, at higher latitudes, the sun enters the atmosphere at an angle, causing it to pass through much more atmosphere before it reaches the ground, reducing the amount of light generally, and UV light in particular, that makes it that far.

So there absolutely is a variation in the amount of UV dependent on latitude despite the average length of daylight being roughly equal.

 
SterileNeutrino
1272533.  Tue Jan 30, 2018 7:55 am Reply with quote

That's right. I didn't include these second-order effects to keep my first post simple. Absorption by the longer path through the atmosphere is certainly greater at higher latitudes. However, this is somewhat counteracted by the upright posture of humans while outdoors. A person standing at the equator only exposes the top of the head and shoulders directly; in Scotland we get the winter sun full in the face: it would be all over if we didn't wear clothing - Brrrrr!

I spend 8am-10am outside all year round, 57 deg. north, and have a permanent sun tan on the face even at Xmas. I have very pale skin and can get sunburn on my receding hairline, if I don't wear a hat, in an hour, even in winter. Humans don't need a full 8 hours of sun to make vitamin D: if we did the evolution of very black skin would have been counter productive.

Finally, UV must be subject to Rayleigh scattering across the sky, just as blue light does to make the colour it is. If this were not the case, Day-Glo fabric (which converts invisible UV into visible light) would only work in direct sunshine, which would be pointless. I've seen Day-Glo ski-wear light up on the slopes in the darkest, most shadowy of conditions: that's what it is for. So living on the north side of a hill (as we do) does not prevent exposure to UV from across the sky in daylight.

I stand by my original point: it's covering up our skin with clothes, and sheltering indoors, due to the cold that limits our vitamin D production at high latitudes, not the absence of sufficient UV.

BTW I believe vitamin D is stored in the body's fat. So ancient humans are likely to have put on weight in summer, stored vitamin D in the fat, and then released the vitamin D as they used up the fat stores in winter. These days, we keep our weight up all year round.

 
Baryonyx
1272534.  Tue Jan 30, 2018 8:12 am Reply with quote

There must be some other factor given that UV indices vary around the world and at different times

1. UV index for the Hebrides in 2015, per day: http://www.hebwx.co.uk/wxuvdetail.php?year=2015

This measure doesn't rely on humans covering up.

Are we also perhaps subject to reflected light, thus negating the equator/upright-position problem? I don't know, maybe this is negligible.

 
Alfred E Neuman
1272553.  Tue Jan 30, 2018 10:55 am Reply with quote

SterileNeutrino wrote:
I spend 8am-10am outside all year round, 57 deg. north, and have a permanent sun tan on the face even at Xmas. I have very pale skin and can get sunburn on my receding hairline, if I don't wear a hat, in an hour, even in winter.


Are you saying that you burn as quickly at 57 as you would at the equator?

And just to the record, even within the tropics where the sun is directly overhead at a couple of points a year, its only at noon, not all day. You definitely get a sunburn all around in the tropics, not just on top of your head.

 

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