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The American Scientist article 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)?


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.
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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.


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] [PDF]
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