Re: UV Index vs. angle of sun?
- From: Martin Brown <|||newspam|||@nezumi.demon.co.uk>
- Date: Mon, 04 Jun 2007 02:09:23 -0700
On May 30, 6:53 pm, hell...@xxxxxxxxxxxx wrote:
I recently bought an Oregon Scientific UV meter and made some
measurements which surprised me by showing a much higher UV index than
I expected when the sun was low in the sky. My experience had been
that, when the sun was lower than 45 degrees above the horizon, I
could spend considerably more time in the sun without burning. For
example, I was in Sweden many years ago near the summer soltice,
forgot sun screen (it was 1976 after all!), being in the sun for a
number of hours, and not burning)
You were probably just lucky - the air is fairly clean and clear up
there.
At first this surprised me because at 45 degrees, there is only 41%
more atmospheric path for the sun's rays to penetrate. But then I
realized the attenuation is exponential and needs to be measured in
dB. If the atmosphere attenuates UV by 20 dB at high solar noon, then
that extra 41% translates to 8.2 dB of extra attenuation, for an
equivalent SPF of 6.6. With the sun 30 degrees above the horizon there
would be 100% more atmospheric path since 1/sin(30 deg) = 2, and that
would add 20 dB, equivalent to an SPF of 100. Of course, if the basic
attenuation were 10 dB instead of 20, all the SPF's would be reduced
accordingly.
Based on my experience I guesstimated that the basic attenuation was
closer to 20 dB than to 10. But my UV meter seems to indicate an
effective SPF of only about 3 when the sun is 30 degrees above the
horizon, which would correspond to a basic attenuation of about 5 dB.
Is the UV meter right? Does anyone know the basic attenuation, as I am
calling it?
There are two components to the UV light hitting the ground. The
direct light from the sun which should be measurably attenuated by
passage through the extra depth of atmosphere if you use a collimator
to isolate it. The amount of direct UV measured should obey the
expected dependence on path length through the atmosphere.
But there is also secondary Rayleigh scattered light from the entire
visible blue sky (which also scatters UV very effectively). It is the
latter that is confusing your measurement of UV incident at ground
level.
You can see the influence of atmospheric scattered UV in the twilight
period making some plants and dayglo posters visibly fluoresce in the
mid evening gloom. The proportion of UV to total viisble light rises
for a while after sunset.
Regards,
Martin Brown
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