Madonna facelift and vitamin D
First: Apologies for the typos and repetition in my last offering. A tidied version is here.
This was going to be just an update to a post from back in 2020 that summarised in one pretty picture how the UVI, representing skin-damaging radiation, varies over the length of New Zealand as a function of time-of-day and day-of-year.
But it grew (just a little), and now follows on nicely from my vitamin D post from a couple of weeks back ….
The original ‘Madonna’ plot was created for a 1996 article in NIWA’s in-house magazine by my colleague, Greg Bodeker. That was just prior to international agreement on the colours associated with various UVI levels. His strikingly colourful 3-D plot was based on a calculation for clear skies using a typical year of daily ozone values. Since our overhead ozone hasn’t changed much over the intervening period, it applies equally well today (though the day-to-day differences in UVI due to day-to-day differences in ozone will be randomly different).
Another Lauder colleague, Ben Liley, has just modified the plot to comply with the standard colours associated with each UVI level. Here it is, in all its glory.
Very pretty, eh? I’m not sure that it looks any better than the previous version, but at least it now complies with the agreed international UVI colour standard.
The take-home messages are:
1. The UVI is strongly peaked near ‘midday’ (which in New Zealand is around 1:30 pm during the summer ‘daylight-savings’ period, and about 12:30 pm in winter).
2. There’s a huge seasonal variation in the UVI, with winter being only 10 percent of summer in the south (and around 25 percent of summer in the north).
3. The UVI is higher in the north, especially in winter when it’s at least twice that in the south (so proximity to the Antarctic ozone hole is clearly not a factor).
Now the extra bit ….
The reason we updated the plot was to support health professionals who’re in the process of updating advice in their Companion Statement on Vitamin D and Sun Exposure in Pregnancy and Infancy in New Zealand. Vitamin D is essential for good health, including infants. Its production from UV radiation in sunlight is roughly proportional to the UVI, so the seasonal and diurnal dependence for its production is quite similar to that shown above. But, because of their slightly different wavelength-dependencies, the proportionality isn’t quite exact, especially at times when the UVI is small. In particular, the radiation that leads to vitamin D production then has a stronger dependence on sun elevation angle, which in turn leads to steeper diurnal changes and a larger summer/winter contrast.
The effect for southern New Zealand is illustrated by the 2-panel plot below. The results shown are based on measurements made at Lauder, Central Otago, but they would apply equally for other sites at similar latitudes (~45 S), like Queenstown or Oamaru. That huge summer/winter contrast in UV would be even larger in the extreme south of the country.
The details of the plot are a bit complicated, so I’ll summarise the two main points.
The plots show that, compared with sun-burning radiation, the production of vitamin-D from sunlight is:
(1) confined to about half the number of hours around the midday period, and
(2) its summer-winter contrast is larger by about a factor of two.
This means that while the sunburning UV at noon in mid-winter is 10 percent of that in summer, the ability to make vitamin-D from winter sunlight is only 5 percent of that in summer. When seasonal differences in clothing and outdoor exposure habits are factored in, I hope you can appreciate why it’s so difficult to produce enough vitamin D in winter at mid to high latitudes. In practice, the amounts produced in exposed skin will typically be only around 1 or 2 percent of those in summer. So, while it takes less than 5 minutes of sun exposure per day to maintain vitamin D in summer, it would take more like 5 hours in winter, even if that midday UV level was sustained throughout that long period (which, of course, it isn’t). The dashed curves in the upper plot below show that in winter, UV levels have essentially dropped to zero within 4 hours from noon.
That’s why our vitamin D levels fall over the winter months, and why people living poleward of 40 degrees latitude should consider taking winter supplements. I do.
(I could alternatively boost my vitamin D by taking a winter holiday in the Pacific Islands or Queensland Australia; but the carbon footprint of that option isn’t nearly as friendly for climate change).