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After recently visiting the Dead Sea in Israel and not getting any skin burns, I was wondering about the reasons for that. Searching the literature, it seems that the UV radiation is indeed lower in the Dead Sea (ca. -400 m) compared to Beer Sheva (a nearby city at ca. +300 m): 1 2 3. Both UVA and UVB rays are lower, but UVB rays are attenuated the most.

These sources give mostly measurements, but hardly discuss the reasons for the lower radiation. I always thought that the ozone layer blocks most of the UV radiation, but here it seems that just a few hundred meters of atmosphere can greatly reduce the amount of incident UV.

What are the reasons for that? Is it simply because there are more air molecules that absorb the radiation?

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You are almost totally correct when you asked:

What are the reasons for that? Is it simply because there are more air molecules that absorb the radiation?

The Ozone Layer blocks most of UVB, but does not affect the amount UVA entering the atmosphere (as seen in the illustration below):

enter image description here

(Image source)

But there is a little more to it than that. From NASA's Earth Observatory's webpage What Determines How Much Ultraviolet Radiation Reaches the Earth’s Surface?, elevation's role in attenuating UVB (and by similar processes, UVA):

high elevations UV-B radiation travels through less atmosphere before it reaches the ground, and so it has fewer chances of encountering radiation-absorbing aerosols or chemical substances (such as ozone and sulfur dioxide) than it does at lower elevations.

The aerosols absorb and scatter incident UV radiation. So nearer to the surface, there is a greater concentration that the light must pass through - and as the altitude decreases, the UV radiation path is therefore increased, so the further 'down' in altitude, the more the air mass and an increased presence of aerosols absorb and scatter solar UV radiation.

In respect to the aerosols in area you have specified (the Dead Sea), according to the first link in your question, The Analysis of the Ultraviolet Radiation in the Dead Sea Basin, Israel (Kudish et al. 1997), they state

the air above the Dead Sea is characterized by a relatively high aerosol content due to the very high salt content of the Dead Sea

Their results indicate that

The relative attenuation in the ultraviolet range as a function of wavelength, i.e. site-specific spectral selectivity, decreases with increasing wavelength.

thus, indicating that the attenuation is also wavelength dependent.

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  • $\begingroup$ Could it be that the Dead Sea, being a lower elevation area rimmed by mountains on both sides, also has unusually large aerosol contents in the air? $\endgroup$ – Gimelist Nov 2 '14 at 5:17
  • $\begingroup$ Quite possibly, particularly mineral dust $\endgroup$ – user889 Nov 2 '14 at 5:18
  • $\begingroup$ @Michael I added a bit more to my answer with respect to your comment query $\endgroup$ – user889 Nov 2 '14 at 5:46
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    $\begingroup$ @Michael, yes, you are right, they have focused on one mechanism - that's why this is a good question. Site specific spectral selectivity is that specific wavelengths and ranges of wavelengths are attenuated by local aerosols and conditions. $\endgroup$ – user889 Nov 2 '14 at 5:57
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    $\begingroup$ This one of solar irradiance makes the point a little better I think. The point is that the absorption continues to occur where the atmosphere gets thicker (e.g. below sea level), which would make the differences between top of atmosphere and surface even greater upload.wikimedia.org/wikipedia/commons/thumb/e/e7/… $\endgroup$ – farrenthorpe Nov 7 '14 at 17:20

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