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Name: Victor Carrano
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Age: 30s
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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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