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Why You Can't Get Sunburn Near Sunrise And Sunset

[For people finding this page by searching for things like "how late can you get sunburn", the short answer is that in the continental US, it’s virtually impossible to get a sunburn before 10 AM or after 4 PM, except in subtropical places like Florida.]

When the weather starts getting nice, I start thinking about the sun and UV rays and how to avoid sunburn. I always try to plan my outdoor activities (biking, tubing, etc) to avoid the worst part of the day in terms of sunlight, which is when the sun is directly overhead -- around 1 PM in North America. Sun exposure is crucial for proper health, but only if it’s time-limited so that it doesn’t give you a sunburn.

You’ve probably noticed that, while you can get a tan/sunburn in half an hour or so around midday, it’s pretty much impossible to get any tan -- let alone sunburn -- within an hour or two of sunrise and sunset. When I got my iPhone 4 last June, I stood in line at the Lehigh Valley Apple store from about 4:30 PM to 7 PM. It was a hot, sunny day; the Apple employees were giving out bottled water, and the line was in direct sunlight the whole time. Despite this, I didn’t get a bit of tan.

The reason is because sunlight has to travel through the Earth’s atmosphere before it reaches us on the surface, and the atmosphere blocks some of the UV light. When the sun is directly overhead, its light has to travel through a certain amount of atmosphere, but when the sun is at more of an angle (further from midday), the amount of atmosphere that sunlight must travel through is greater.

But how much greater? I wanted to figure this out, and a little Googling didn’t turn up anything. So I thought about how to model it, which is pretty simple: you just need to draw a big circle with a thin band surrounding it, then compare the thickness of the band at different angles. So that’s what I did:

The Earth’s radius is about 4000 miles, and the atmosphere (depending on how you want to define it) is, say, 80 miles thick. The precise thickness doesn’t matter for our purposes here, though; all that matters is the ratio between the smallest thickness (the midday atmosphere) and the greatest thickness (the sunrise/sunset atmosphere).

By drawing two exact circles that are concentric, and using 1 pixel to represent 8 miles, it’s easy to "calculate" the sunrise/sunset thickness: you just need to count/measure the pixels. There are 100 of them, compared to 10 pixels for the midday thickness, so the UV radiation must pass through ten times as much atmosphere at sunrise/sunset as it does at midday. That explains why virtually none of that UV light passes through around sunrise and sunset: the atmosphere is far too thick for the UV to penetrate.

Update: here’s a related graph from NASA (via Wikipedia) showing how much of each UV fraction is blocked by the atmosphere (mainly the ozone layer):

It shows that the atmosphere blocks all UVC, and most UVB, but only a small amount of UVA.

(show full-size image viewer)

Posted by Anthony on Jun 3, 2011 | 14 replies
Filed under: Health, Images, Outdoors, Science

Comments:

01. Dec 27, 2011 at 02:49am by Ryan:

Hi, about your post on UV rays and how very little of those rays are reaching us during sunrise and sunset, is this accounting for all types of UV rays? Theres a type of meditation i do called sungazing and enjoy doing for its many physical,mental and spiritual benefits but fear that getting prolonged sun exposure even during sunrise and sunset can still damage and prematurely age my skin. I was curious to know if UVA rays were still being projected during sunrise/sunset as these rays are what i read is what causes premature aging. Many weather websites i go to say there is 0 UV during these hours but others say there are UVA rays whenever the sun is out. Do you think the atmosphere is far too thick during these times to be worrying about about sun damage of any kind? or do you think aging to the skin can still occur. Thanks alot.

02. Dec 27, 2011 at 04:43am by Anthony:

My understanding of UVA is that, unlike UVB, most of it does reach the surface of the Earth, but it generally doesn’t cause any harm to us, as excessive UVB can. I personally don’t worry about UVA at all, but then I don’t worry that much about any kind of sun damage, because I slowly ramp up my exposure during the spring, and because I eat a diet that tends to protect from sun damage.

03. Dec 27, 2011 at 11:39am by Anthony:

Just to clarify, for the context of this post: I don’t believe the atmosphere blocks any significant amount of UVA, so I’d guess that even at sunrise/sunset, even at 10x the atmospheric cover, most of the UVA is still coming through and reaching the surface of the Earth.

04. May 17, 2013 at 07:23pm by J:

Are UVA rays as strong at sunrise and sunset as they are at midday?

05. May 18, 2013 at 12:20pm by Anthony:

The strength of the rays is a function of how much is blocked by the atmosphere. As I said in my previous reply, I don’t think Earth’s atmosphere blocks any significant amount of UVA. That would mean UVA rays are present during all daylight hours and their strength is the same the whole time.

06. Sep 18, 2013 at 09:01pm by Dorena Rode:

Hi,

Thanks for your information. I like the way you deduced that. I have something to add for people that might stumble onto this page that are learning about UV and sungazing.

I have been trying to figure out, like Ryan, if sungazing is safe. The practice is actually looking directly into the sun for a few minutes one hour after sunrise and before sunset. Sounds kind of crazy, but it is a traditional practice. In any case, it is hard for a layperson to find good data on UV exposure at those times. This article was somewhat useful:

http://journals.tums.ac.ir/upload_files/pdf/16649.pdf

Unfortunately, they only have one data point that is an hour after sunrise. For a 30 minute exposure at an hour after sunrise it would be about 1 to 2 J/cm2. Well this is kind of meaningless too. So I looked up occupational exposure limits: http://www.arpansa.gov.au/pubs/rps/rps12_dft03.pdf According to them a 1 J/cm2 level breaks down to 2W/m2 and that level is okay for about a minute.

However, this data point was when the sun was at 11% above horizon - which is about an hour after sunrise. I can’t tell exactly, but their research suggest they weren’t getting any UV readings earlier than this point. So UV may be even less right at sunrise and until about 30 minutes after when the sun is still below 6% incline. I’m not sure the sensitivity of the instruments used but there data suggests that the radiation goes to zero.

07. Sep 19, 2013 at 12:58am by Anthony:

Interesting. I think we all stare at a beautiful sunrise or sunset for a few seconds every now and then, but that’s right when the sun is just barely visible. To do that an hour after sunrise, or an hour before sunset, would be much harder to do, and harder on the eyes. I’m not sure whether it’d be dangerous, but I won’t be trying it anytime soon.

But I do believe that in order to have good vision and healthy eyes, it’s important to spend time outdoors in the midday sun without sunglasses as often as possible. That was a natural part of human life for all of human history until recently. Just as we’ve over-corrected for sunburn and skin cancer by saying that all sun-skin exposure is bad, we’ve also been led to believe that simply being outside without sunglasses is dangerous for your eyes. And that’s just about the most absurd thing I can imagine.

I’ve seen the negative effects of over-shielding my eyes in my own life. I used to wear sunglasses every time I went outside during the daytime, and eventually I started to notice that I couldn’t stand being outside on a sunny day without them -- it was just too painful on my eyes. That didn’t seem right to me, and I figured that I must’ve made my eyes overly sensitive by always wearing sunglasses. I decided to stop wearing sunglasses (slowly weaning myself off them) except when absolutely necessary -- mainly while driving with lots of glare -- and sure enough, over a few months my eyes readjusted, and the daylight was no longer painful.

08. Apr 8, 2015 at 11:15am by nishan:

are we likely 2 get little bit of tanning during sunset

09. Apr 10, 2015 at 01:15am by Anthony:

No. The sunlight is far too weak at sunset to give you a tan.

10. Apr 10, 2015 at 06:54pm by Robert:

Sorry, your assessment may be correct up north, but closer to the equator your theory does not hold up. I used to live in NJ, and never got a sunburn after 4 pm. I now live in Sarasota FL and I get a sunburn up to 7pm pm. My kids soccer practice starts at 6pm, every time I forget sunscreen, my face gets a sunburn. The rest of the day I have no exposure to the sun as I work in an office. Hence no other influencing factors.

Regards
Robert

11. Apr 10, 2015 at 08:07pm by Anthony:

Yes, in the subtropics the sun is stronger for more of the day. I’ve updated the first paragraph to reflect this.

12. May 14, 2015 at 07:14pm by Bailey:

Okay great concept and I don’t mean to throw it all away but I live in the U.S. (Not Florida) and I’m in the high school band. And it starts at 7am and ends at 11am and by the end of the month I have very bad second degree sunburn. I applied sunscreen with zinc every hour and by the second week I could literally feel the sun burning my skin at 8am. And that 1 hour from 10-11 wouldn’t give me a burn like that, so I’m sorry but your wrong and if you have any advice for people like me then that would be great

13. May 14, 2015 at 11:31pm by Anthony:

Actually, yes, the sun’s rays are much stronger at noon than at sunrise. So the hour from 10-11 is worse than the hour from 9-10, etc. It’s entirely likely that being outside from 10-11 every day would give you a severe sunburn. And once you’ve got a sunburn, then further sun exposure even during a weak sun period, such as 8 AM, will be painful.

Regardless of all that, if you’re getting a sunburn, that means you’re in the sun too long! There are only 2 solutions: get out of the sun, or cover up. Sunscreen is NOT a solution; it’s only a band-aid. It only blocks some of the sun’s rays, so you’re still getting damage to your cells. And it encourages you to stay out in the sun longer than you otherwise could, which increases the amount of damage that your body receives.

Sunburn is a warning sign from your body, telling you that you’re spending too much time in the sun. Putting on sunscreen and staying out anyway is just foolish. It would be like when your car’s check-engine light comes on, if you "solved" the problem by unplugging that light, instead of getting the engine checked out. It’s a bad idea.

14. Aug 27, 2015 at 05:54pm by Robert Dawson:

I have been told that there are two types of ultraviolet rays. One type will not cause a tan before (say)8 o’clock in the morning or after (say) 4 o’clock in the afternoon.
It is that other type of ultraviolet ray that should give you cause for concern. It can pass through glass when you think that being inside will protect the skin from sun damage. It might not cause a tan at dawn or dusk but it will promote (I think) wrinkles and actinic keratosis.
Beware!

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