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When I was young I read one book in which is written that you get more tan on the morning than on evening even light angle from Sun to Earth is the same. Don't remember exact reason, I think because ultraviolet is more absorbed on the evening because air is more humid or something like this. Is it true?

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  • $\begingroup$ Less particles in the air in the morning than in the evening could be a reason. But this should be a pretty small effect. Do you have any source for this claim with some data? $\endgroup$ – Alexander Jul 27 '13 at 20:26
  • $\begingroup$ No, I don't have any source. I read this book about 30 years ago and as I remember there was no explanation in this book, but it was like obvious. I asked one physicist about this and he explained it this way. I thought it true, but later got some doubts. $\endgroup$ – Alexan Jul 27 '13 at 21:07
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This sounds false to me if it's proposed as a general statement rather than something about weather patterns that operate on a 24-hour cycle. Disregarding weather, the intensity of sunlight depends on (1) the angle at which the solar radiation intercepts the landscape and the surface of your skin, and (2) the thickness of the atmosphere traversed by a ray of sunlight, which causes absorption. Both of these factors depend only on the height of the sun in the sky. The claim would obviously be silly, for example, if applied to the arctic -- you wouldn't be able to get a tan in October, when the sun was setting, but you would be able to get one in April, when it was rising??

And if it is a statement about weather patterns, then it's certainly not something that can hold true in all places. E.g., on a lot of the California coast a common summer weather pattern is for there to be fog in the morning, which burns off later in the day -- this would be the opposite of what was claimed.

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  • $\begingroup$ okay, lets think that tan on the morning and evening is almost the same, we don't have any data, conflicting with it. I'm just thinking, maybe ask the same question on skeptics? $\endgroup$ – Alexan Jul 28 '13 at 15:06
  • $\begingroup$ found interesting site: uvawareness.com/uv-info/uv-strength.php: UV radiation is generally strongest in the hours around solar noon. For safety sake, you can take this to mean between the hours of 10am and 4pm. Solar noon is not the same as noon on a clock (12pm). Solar noon can vary by more than an hour before or after 12pm depending on location and on time of year. Even across states or provinces within the same time zone solar noon can vary a lot. $\endgroup$ – Alexan Jul 28 '13 at 15:26
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The only explanation I can think of for such an observation would be simply that it is generally hotter at the same Sun angle after noon than before noon, due to the thermal inertia of the atmosphere and surface. (This ignores other effects like differences in cloud cover or marine layer between the morning and the afternoon.) The result is that you won't want to stay out as long in the afternoon as you would in the morning. Assuming that you respond to your discomfort, you will spend less time at the same Sun angles in the afternoon than in the morning.

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  • $\begingroup$ no, it's not about time, spending on the sun, it's about that you get less ultraviolet for the same time and same conditions, only difference morning or evening. $\endgroup$ – Alexan Jul 27 '13 at 21:13
  • $\begingroup$ Except that's not true. $\endgroup$ – Mark Adler Jul 28 '13 at 1:36
  • $\begingroup$ Could photoproduction of ozone result in (slightly) more ozone in the evening - providing more protection? $\endgroup$ – uhoh Sep 26 '16 at 0:54
  • $\begingroup$ Yes, but those effects are only a few percent. The effects of tanning can't be measured that accurately. $\endgroup$ – Mark Adler Sep 26 '16 at 1:15

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