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UV Radiation Variations
Name: Victor Carrano
Status: Other
Age: 30s
Location: N/A
Country: N/A
Date: N/A
Question:
How does ultraviolet intensity vary throughout the
day/year? What would a plot of uv intensity vs. the time of day look
like? I have never seen one. How does it fall off before and after noon?
Then, what about a plot of uv vs. time of year?
Thanks.
Replies:
The big question is: what reason would there be for any of these
plots to differ from similar plots for visible sunlight, except for a
constant factor, the ratio of UV light flux to visible light flux from
the Sun?
I can think of a few possibilities. It may be at sunset that the UV
flux is a lower fraction of the total Sun flux because the atmosphere
scatters UV light more than visible. When the Sun is on the horizon
its light passes through more air (because it is traveling slantwise
through the atmosphere). More of the blue light is scattered, hence
the Sun appears red. Probably more of the UV light is scattered, too.
Near the South Pole there is an ozone hole each spring which lets in
more UV light than normally reaches the ground, so that is another
factor.
As for the generic plot of light intensity vs. time during the day,
it probably looks like a cosine curve centered at noon, just because
that would obtain from the simplest variation in intensity with the
time-dependent tilt of a flat surface with respect to the Earth-to-Sun
vector. During the year, the variation is more complicated because of
the apparent motion of the Sun up and down the sky with the seasons.
You could probably calculate it with a couple hours of
trigonometrizing.
Grayce
Neglecting the effect of seasonal variations in ozone levels, UV intensity
will parallel light intensity. In cloudy weather, the UV intensity is
reduced a little less that the visible light intensity, because it
penetrates the clouds a little better.
Light intensity will be greatest at true noon, when the sun's rays are most
direct. Likewise, it will be higher in the summer than in the winter, for
the same reason. Cloud cover will have the expected effect.
Richard Barrans
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Update: June 2012
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