<p>I found one study from 2001 {1}:</p>
<blockquote>
<p>It  was  also  determined  that  for  a  fulltime  outdoor  worker,  <strong>the  additional  UVA  exposure  could  approach  approximately  that  of  one  third  of  a  full  winter’s  day</strong>.  For  indoor  workers  with  an  outside  lunch  break  of  noon  to  1  pm,  the  additional  UVA  exposure  was  on  average  6.9  kJ m<span class="math-container">$^{-2}$</span> over three cloud enhanced days. To the authors’ knowledge this is the <strong>first paper to present some evidence of cloud enhanced UVA human exposure</strong>.</p>
</blockquote>
<p>The paper doesn't explain the mechanism causing the UV-A increase.</p>
<hr />
<p>References:</p>
<ul>
<li>{1} Sabburg, J. and Parisi, Alfio and Wong, J. C. F. (2001). Effect of cloud on UVA and exposure to humans. Photochemistry and Photobiology, 74 (3), 412-416. ISSN 0031-8655. [<a href="https://scholar.google.com/scholar?hl=en&amp;as_sdt=0%2C48&amp;q=Sabburg%2C+J.+and+Parisi%2C+Alfio+and+Wong%2C+J.+C.+F.+%282001%29.+Effect+of+cloud+on+UVA+and+exposure+to+humans.+Photochemistry+and+Photobiology%2C+74+%283%29%2C+412-416.+ISSN+0031-8655.+A&amp;btnG=" rel="nofollow noreferrer">GScholar</a>] [<a href="http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.563.2942&amp;rep=rep1&amp;type=pdf" rel="nofollow noreferrer">PDF</a>]</li>
</ul>

CC BY-SA 4.0
2021-09-22T23:31:23.713Z
2021-09-23T07:26:42.290Z
2021-09-23T07:26:42.290Z
Franck Dernoncourt
<p>The <a href="https://www.americanscientist.org/article/sunshine-on-a-cloudy-day" rel="nofollow noreferrer">American Scientist article</a> nicely explains that clouds sometimes increase the level of UV radiations received on the earth surface. However, all the papers they cite focused on UV-B radiations. Can clouds increase the level of UV-A radiations received on the earth surface? If so, by how much, and in which condition(s)?</p>
<hr />
<p>More details from the American Scientist article:</p>
<blockquote>
<ul>
<li>According to Sabburg, &quot;In our latest research [soon to appear in the Journal of Atmospheric Research], we use new equipment and refine our methodology, and the <strong>highest UVI [an index of skin reddening] enhancement we found was 25 percent</strong>.&quot; But those values are with respect to  expected clear-sky UV. Compared with the level of attenuation usually seen when clouds are present, such measurements can actually be 50 to 75 percent higher than predicted, says Sabburg.</li>
<li>Just how common is cloud enhancement? The various studies have found that between <strong>1.4 and 8 percent of all measurements show cloud enhancement compared with clear-sky values, depending on geographic location, but as many as 25 percent of those made on partly cloudy days may show it. Most often the enhancement lasts for 10 minutes</strong> or less—not a concern for the sun worshiper—but it has been known to persist for an hour.</li>
</ul>
</blockquote>

<clouds><sun><uv-light>
Can clouds increase the level of UV-A radiations received on the earth surface? If so, by how much, and in which condition(s)?
Franck Dernoncourt
# Can clouds increase the level of UV-A radiations received on the earth surface? If so, by how much, and in which condition(s)?

(Asked by: username_0 on 2021-09-22)

The [American Scientist article](https://www.americanscientist.org/article/sunshine-on-a-cloudy-day) nicely explains that clouds sometimes increase the level of UV radiations received on the earth surface. However, all the papers they cite focused on UV-B radiations. Can clouds increase the level of UV-A radiations received on the earth surface? If so, by how much, and in which condition(s)?

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More details from the American Scientist article:

> * According to Sabburg, "In our latest research [soon to appear in the Journal of Atmospheric Research], we use new equipment and refine our methodology, and the **highest UVI [an index of skin reddening] enhancement we found was 25 percent**." But those values are with respect to expected clear-sky UV. Compared with the level of attenuation usually seen when clouds are present, such measurements can actually be 50 to 75 percent higher than predicted, says Sabburg.
> * Just how common is cloud enhancement? The various studies have found that between **1.4 and 8 percent of all measurements show cloud enhancement compared with clear-sky values, depending on geographic location, but as many as 25 percent of those made on partly cloudy days may show it. Most often the enhancement lasts for 10 minutes** or less—not a concern for the sun worshiper—but it has been known to persist for an hour.

**Tags:** clouds, sun, uv-light


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## 1 answers

### Answer by username_0 on 2021-09-22. Score: 2

I found one study from 2001 {1}:

> It was also determined that for a fulltime outdoor worker, **the additional UVA exposure could approach approximately that of one third of a full winter’s day**. For indoor workers with an outside lunch break of noon to 1 pm, the additional UVA exposure was on average 6.9 kJ m$^{-2}$ over three cloud enhanced days. To the authors’ knowledge this is the **first paper to present some evidence of cloud enhanced UVA human exposure**.

The paper doesn't explain the mechanism causing the UV-A increase.

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References:

* {1} Sabburg, J. and Parisi, Alfio and Wong, J. C. F. (2001). Effect of cloud on UVA and exposure to humans. Photochemistry and Photobiology, 74 (3), 412-416. ISSN 0031-8655. [[GScholar](https://scholar.google.com/scholar?hl=en&as_sdt=0%2C48&q=Sabburg%2C+J.+and+Parisi%2C+Alfio+and+Wong%2C+J.+C.+F.+%282001%29.+Effect+of+cloud+on+UVA+and+exposure+to+humans.+Photochemistry+and+Photobiology%2C+74+%283%29%2C+412-416.+ISSN+0031-8655.+A&btnG=)] [[PDF](http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.563.2942&rep=rep1&type=pdf)]


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