On Rayleigh Scattering and UV Light Absorption

[mk9898]

1. I've read that UV light cannot "penetrate" the atmosphere as easily during winter. But what does this exactly mean? My hypothesis is that due to rayleigh scattering, since light with smaller wave lengths scatter more, the UV light doesn't end up reaching the surface of the Earth (i.e. gets reflected into space). But, this hypothesis isn't water tight, because blue light is still visible in winter. Yes, UV light has a much smaller wavelength than blue or violet light and therefore could scatter much more in winter, but since the sky is still blue in winter, I am not convinced that just due to the angle of the sun in winter, there is less UV light.
   
   
2. Another thing, even if the UV light is more scattered, wouldn't that just mean, that there is a higher chance the scattering points eventually go back into space i.e. we are still getting UV light but at a less intensity as say in summer?
   
   
3. With this logic, the amount of blue light that we see in the sky would also have a less intensity in the winter, right?
4. In general, UV light scatters more than say blue light. Since the sky is in general always blue during the day, why would we need direct sunlight to obtain UV light i.e. vitamin D when UV scatters so much? Since we see blue sky, we should also, if our eyes were able to see UV light, see a lot of UV light in the sky. Since also the Sun appears orange throughout the day, "most" blue light is not hitting us directly from the sun but rather via rayleigh scaterring. So that should be valid for UV light as well, right?

 

[anorlunda]

I think you're getting hung up on more/less, all/none, direct and oblique.

Blue light reaching our eyes may have entered the atmosphere in a direct line to your eye, or at an angle tangent to the atmosphere, or other angles. It may have multiple scatterings, and multiple chances of absorbsion, before reaching your eye. Light that doesn't scatter, moves on a more-or-less straight line from the source to your eye.

That makes your reasoning oversimplified.

How about just asking questions without giving your hypothesis first?

 

[mk9898]

How about just asking questions without giving your hypothesis first?

Jeez are you guys never happy? First you want people to give their opinion of what is right and now someone like you doesn't want us to do so?

But what you said before your question is pretty obvious and doesn't help.

 

[Chestermiller]

Scattering is not the only effect present. There is also absorption by oxygen in the Hertzberg continuum and Schumann Runge bands, and ozone down to about 300 nm. These are the dominant effects for UV (and near UV) radiation. All this is related to why ozone depletion is such a big issue. See the reference:

Miller, C., Meakin, P., Franks, R.G.E., and Jesson, J.P., The Fluorocarbon-Ozone Theory – V. One Dimensional Modeling of the Atmosphere: The Base Case, Atmospheric Environment, 12, 2481-2500 (1978)

 

[Drakkith]

In general, UV light scatters more than say blue light. Since the sky is in general always blue during the day, why would we need direct sunlight to obtain UV light i.e. vitamin D when UV scatters so much?

However much UV may be scattered, there is still far more that isn't, and it is predominantly this unscattered UV light that gives you a sunburn and that your body uses to make vitamin D.

Since also the Sun appears orange throughout the day, "most" blue light is not hitting us directly from the sun but rather via rayleigh scaterring.

The Sun does not appear orange throughout the day. Except in the early morning and late evening, the Sun appears white, regardless of the time of year.