Thread

The UV index is not the warm sensation of sunlight on bare skin

99 points · 79 comments · evakhoury

nomel · 1790116546 · focus · HN ↗
I don't know about this. I can feel where there are holes in my sunblock (which only block UV) which lets me know where to apply it, and the sensation is not one of heat of the sun, it's different. I'm not sure about the mechanism, but the presence of the sunblock stops the "UV" type sensation. This from the moment I put it on to long after the water component of the sunblock has evaporated. And, the type of sunblock doesn't matter (the drier waterproof kind or various lotion varieties). And, I do notice a difference between the different SPF, along with some "bad" sunblock that I just kept applying but couldn't make the feeling go away (it had apparently separated, and everyone that used it was burned).
I think the "real" test would be to just shine a UV light in a dark room, to see if the source can be identified while blindfolded, or something.
1. crazygringo · 1790123344 · focus · HN ↗
  Thank you -- I'm exactly the same. You're right, it doesn't feel like heat, it feels like... something else, I have no words for it.
  It usually goes together with bright direct sunlight in the summer, so I think most people don't learn to notice or distinguish it.
  But I find that it's on particular overcast days in the summer, when there is no direct sunlight, that you can feel it intensely without the accompanying sunlight/heat. It reminds me to put on sunblock. But it's not all overcast days -- it's strongest when there's a light cloudy "haze". In contrast, when it's thick clouds approaching thunderstorm, you obviously don't feel it at all.
  So it's that kind of thing that's hard to judge before you go outside, but as soon as you do, you get a clear "oh, I definitely need to go back in and put in sunblock" or "oh, this is fine, no sunblock needed today".
  The best way I can explain it is as kind of an "oppressive" feeling, almost like a kind of pressure on your skin. Like, you can describe heat that way too -- oppressive or like a kind of pressure -- but in this case, you're feeling those things when there is no heat.
  And when you are at the beach on a hot sunny day, I find it kind of magical how the sunblock removes the feeling entirely. It's still sunny and bright and hot, but you don't feel the oppressive/pressure feeling together with it.
  1. phonon · 1790125237 · focus · HN ↗
    "no words for it"
    Actinic?
    <a href="https://www.merriam-webster.com/dictionary/actinic" rel="nofollow">https://www.merriam-webster.com/dictionary/actinic
  2. kgc · 1790133098 · focus · HN ↗
    Also get the pressure feeling, but via eyes.
  3. stogot · 1790136557 · focus · HN ↗
    It is a numb pressure for me
2. taneq · 1790124432 · focus · HN ↗
  I’ve always thought of sunblock as taking the ‘sting’ out of the sunlight. It’s a separate sensation that I can feel even on a cold day.
  This is in Australia where the hole in the ozone layer often results in 10x ‘normal’ UV levels, you can get noticeably burned in a few minutes.
  1. meatmanek · 1790125767 · focus · HN ↗
    > This is in Australia where the hole in the ozone layer often results in 10x ‘normal’ UV levels, you can get noticeably burned in a few minutes.
    By 10x you mean 10% more, and by ozone layer you mean the fact that the southern hemisphere's summer coincides with Earth's perihelion while the northern hemisphere's summer coincides with aphelion.
    <a href="https://www.abc.net.au/news/science/2025-02-04/sun-summer-uv-sunburn-skin-cancer-australia-ozone-layer/104870806" rel="nofollow">https://www.abc.net.au/news/science/2025-02-04/sun-summer-uv...
    esrauch · 1790128267 · focus · HN ↗
    I expect they just meant UV 10, which is 10x the energy of UV 1.
  2. jandrewrogers · 1790128796 · focus · HN ↗
    Australia’s UV is only modestly higher than other places at similar latitude. It is similar to equatorial regions and isn’t remotely close to 10x anywhere. You can experience the same UV index in the tropics.
    Some parts of South America have a UV index twice that of Sydney’s maximum. All of the actually extreme UV index locales are extreme altitude and near equatorial.
    piltdownman · 1790167857 · focus · HN ↗
    The Peruvian Andes has the most extreme UV index in the world due to being 10,000ft+ above sea level and on the most susceptible line of latitude. That being said, even with the atmospheric deficit, the max peak recorded by ground and local meteorological observations was 18. Can't seem to find a citation for 2x Sydneys max (14/15).
    Indeed looking at Oceania; Sydney, Perth, and Auckland all hit 14/15 regularly in the Summer months. Auckland in particular has ~40% more UV than comparable northern latitudes.
    Given the genetics and immigration is heavily skewed in favour of the British and Irish diaspora, it's about 3x the UV Index of what the UK and Ireland see. Dunno where 10x came from.
    jandrewrogers · 1790179744 · focus · HN ↗
    The UV index is >20 in the Andes of the northern Atacama. It is considerably stronger than the tropics thanks to the extreme elevation and ~0% humidity. The global record for UV is at Licancabur on the Chile/Bolivia border with an index of 26-43 (the exact numbers are apparently contested). We only know this because scientists study the unusual lake there. Some other places in the region that few people visit are almost certainly higher based on their geography.
    The highest UV index I can find for Sydney is 14, which is typical of many places in the tropics.
    I've spent time in the 20+ UV of the northern Atacama and it is a different beast. The indirect UV reflected by the soil is strong enough to burn you even if you are not exposed to direct sun. It requires precautions that I never needed in Australia or elsewhere in the tropics.
3. samusiam · 1790127275 · focus · HN ↗
  After you sure that isn't because your skin is moist where sunblock was recently applied, and therefore cooler?
  1. esseph · 1790131452 · focus · HN ↗
    Sunblock is supposed to reflect UVA/UVB, but doesn't do much for infrared or visible light, which is why your skin gets warmer.
    (Edited, fixed)
    jrochkind1 · 1790132295 · focus · HN ↗
    Sunblock is not supposed to reflect heat (infrared). It's supposed to reflect UV. Different spectrum. Kind of the whole thing the OP was about. I have no idea if sunblock also reflects infrared though, incidentally.
    <a href="https://care360.basf.com/emea/en/industries/personal-care/core-competencies/all-about-sun/sun-protection/how-do-they-test-a-sunscreen-s-protection-factor" rel="nofollow">https://care360.basf.com/emea/en/industries/personal-care/co...
    dreamcompiler · 1790175198 · focus · HN ↗
    Sunblock mostly absorbs UV (and turns it into heat). Some reflection occurs but it's mostly absorption.
    esseph · 1790181435 · focus · HN ↗
    After looking into this even more, there are two types:
    Mineral - Reflects
    Chemical - Absorbs
    dreamcompiler · 1790185349 · focus · HN ↗
    I thought so too but it's not the case. Even the mineral sunscreens mostly absorb.
    <a href="https://cen.acs.org/business/consumer-products/What-in-sunscreen-and-how-does-it-protect-your-skin-from-the-sun-rays/99/i27" rel="nofollow">https://cen.acs.org/business/consumer-products/What-in-sunsc...
    nomel · 1790194493 · focus · HN ↗
    Video with a UV camera is interesting: <a href="https://www.youtube.com/watch?v=E9tEEDpeJoo" rel="nofollow">https://www.youtube.com/watch?v=E9tEEDpeJoo
    Black means absorbed, of course.
    nomel · 1790276573 · focus · HN ↗
    And, omg, you can buy an active version on amazon, for this exact purpose: sunblock coverage check!
    <a href="https://www.amazon.com/s?k=uv+mirror+for+sunscreen+test" rel="nofollow">https://www.amazon.com/s?k=uv+mirror+for+sunscreen+test

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