The worst sunburn I ever received was on a beach in Wales on a dull, misty day. I have holidayed many times on Greek islands in the height of summer but have never experienced sunburn like it. What could be the cause?
鈥� I doubt that your questioner used sunscreen on the Welsh beach, through being lulled into a false sense of security. After all, the sun wasn鈥檛 visible and the low temperature would have meant there was less of a sense of burning. On the Greek islands however, I am guessing that they would have applied sunscreen liberally, and subsequently retreated to a suitable area of shade when too hot.
Unfortunately for the skin, the ultraviolet (UV) radiation that led to the sunburn passed through the clouds. And the mist would have acted as a diffuser to ensure an even sunburn.
Advertisement
To make matters worse, the sand and sea would have acted as mirrors, reflecting some of this radiation back up 鈥� possibly onto skin unused to receiving much sunlight. This was certainly my experience on a visit to Jungfraujoch in the Swiss Alps: after less than an hour outside, I had sunburn above my boots where sunlight had reflected off the snow and passed under the hem of my trouser legs.
UV radiation comes in three bands. UVA is the lowest frequency, with UVC being the highest and most dangerous, but fortunately it is blocked by ozone. The one that causes sunburn is UVB light.
In 2004, Australian researchers reported an increase of 40 per cent in the intensity of UVB radiation under broken clouds. Although counter-intuitive, this is just one of several studies suggesting that clouds enhance UV. Although the mechanism isn鈥檛 clear, the effect appears to be maximised when high-altitude cirrus clouds refract UV and this light is reflected by low-altitude cumulus clouds. Hazy conditions accentuate the outcome.
鈥淚n 2004, researchers reported a 40 per cent increase in the intensity of UVB under broken clouds鈥�
UV-induced damage to DNA triggers the production of melanin, a photoprotective pigment that acts as a natural sunscreen to reduce further damage. Burning happens through high levels of exposure.
Exposure to UVB depends on several factors: the time of year (it is higher in summer); the time of day (with a peak at solar noon, when the sun is highest in the sky); altitude (UV levels increase by about 10 per cent for every 1000-metre rise in altitude); the amount of time spent outside; and latitude.
All other things being equal, one would expect greater exposure in Greece, which has an ultraviolet index of three compared with two for Wales. Perhaps surprisingly, sunlight during the summer solstice is only about 20 per cent more intense in Greece than in Wales. But this can be trumped by cloud enhancement of UV or, if sunscreen is not applied in chillier climes, by the mistaken belief that sunburn is only a hazard on a sunny day.
Mike Follows, Sutton Coldfield, West Midlands, UK
We pay 拢25 for every answer published in 快猫短视频. To answer this question 鈥� or ask a new one 鈥� visit newscientist.com/lastword. Terms and conditions apply.
This article appeared in print under the headline 鈥淗azy blaze鈥�