The Cloud Edge Effect

Jesse3959

Member
The simplest answer to what's happening here is that it's hidden behind an inferior mirage because the water is warming the air just above it.

The other reason I had suspected dissolved water to play a significant role in refraction was due to the cloud-edge effect - I've noticed when I'm standing outside on a partly cloudy day, often times just as a cloud edge nears the path between me and the sun, I will feel an intense increase in the warmth of the sun for just a brief moment before the cloud shades me. There seem to be 3 camps of people on that topic - those who say the cloud edge effect is not real, those who say it's refraction, and those who say it's just the additional lumens from the bright cloud up there nearby.

I'm leaning towards refraction because the effect is so sudden, so intense, and so and short lived: If it was just the brilliance of approaching clouds, the effect would gradually increase as the cloud approached with the sine of the angle or something, but that has not been my experience. This effect is also recorded on solar panel power systems.
 
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Mick West

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Staff member
There seem to be 3 camps of people on that topic - those who say the cloud edge effect is not real, those who say it's refraction, and those who say it's just the additional lumens from the bright cloud up there nearby.
Well, my immediate impression would be to go with "it's not real". But there do seem to be many solar power enthusiasts who say it is.

Clouds are not water vapor though, they are a fine mist of water droplets (or ice crystals in colder air). Any refraction isn't through "dissolved water" (water vapor in the air,) it would be from passing through water droplets (think rainbows).

But then most clouds are not refracting light in a way that could focus more sunlight, they scatter light. Why would scattered light increase solar power generation?

So I'd have to go with "just the additional lumens from the bright cloud up there nearby" as the only explanation that makes any sense. If you are getting direct sunlight, plus the sun is right next to a cloud, then the forward scattering would result in you getting more light than you would if there were no clouds in the sky.
 

Mick West

Administrator
Staff member
It's interesting looking at the attempts to explain this out there. For example:

Metabunk 2020-02-18 06-38-42.jpg
https://www.tmeic.com/industry-solution/reliability-solar

That very roughly describes what is happening, but has no real connection to the actual physics. The sun's rays are essentially parallel. not radiating out from a nearby point.

it's right in one regard though, it's full sun PLUS some extra light from clouds. Why is it called the "cloud edge" effect then? Well, when the sun is directly behind the edge of a cloud then the forward scattering brightly lights up the edge of the cloud "close" to the sun.

Metabunk 2020-02-18 06-50-48.jpg

This is a puzzle for Flat Earthers (and everyone). Why does the sun only light up the edges of the clouds it close to, if it's hitting all the clouds evenly? This might lead you to think the sun is relatively close behind the clouds, so the brighter clouds actually are physically near the sun!

Of course all the cloud as the same distance from the sun. The difference is the angle between the cloud and the camera (or you). Forward scattering only spreads out sunlight a relatively narrow angle - so it's only deviating for its parallel path a bit, so this gives the illusion that the clouds "near" the sun are actually NEAR the sun.

It also means that when clouds are near the sun, and the sun is unobstructed, you get more sunlight than if there was a clear blue sky.
 

Mick West

Administrator
Staff member
Some actual science:

https://www.semanticscholar.org/pap...gård/6879a8c5eb441c7aa0c44704b874e39f42a591e8
Metabunk 2020-02-18 07-06-11.jpg
Yay, I was right!
 

deirdre

Senior Member.
I will feel an intense increase in the warmth of the sun for just a brief moment before the cloud shades me.
intense? being female I used to lay out in the sun quite a bit, so was literally looking at the sky for hours and watching that clouds float over toward the sun (note: the very act of putting on sun screen guarantees the formation of big cumulous clouds that didn't exist before). and I never noticed an increase in warmth. and having super low body fat i'm craving those warmth rays on 'chilly' days.. there is an increase in light. it gets brighter really briefly.

Mick's quote above might be proving me wrong, but I don't understand what it is saying in regards to additional 'heat' or how much additional heat that is.
 

Mick West

Administrator
Staff member
Forward scattering is a function of wavelength and particle size. It occurs where then particle size is large, relative to the wavelength of the light.
Image result for forward scattering wavelength size
Since infrared radiation (heat) has a longer wavelength than visible light, it's going to scatter less, for the same size particles. So the effect, I think, will be less for heat. How much less? I'll leave that to the physicists :)
 

Mick West

Administrator
Staff member
This diagram shows the relationship between particle size, wavelength, and the type of scattering. Forward scattering is more towards the top left. So you can see the intersection of Visible and Cloud droplets is closer to the top left than Thermal IR and Cloud Droplets.
This is not an easy diagram to understand.



Metabunk 2020-02-18 08-42-23.jpg

Source: https://www.e-education.psu.edu/meteo300/node/785
 

Mick West

Administrator
Staff member
Here's a practical experiment I just devised.

Metabunk 2020-02-18 09-18-40.jpg

A sheet of white paper is illuminated by a bright lamp. It is evenly lit. Then a sheet of wax paper is introduced. The sheet of paper is partially shadowed, but the region next to the shadow is actually brighter than before. This is because it is getting the light directly from the lamp PLUS the scattered light from the paper.

Wax paper works well because it has forward scattering.

Here's the setup from above, showing the light position:


Metabunk 2020-02-18 09-24-10.jpg

Here's the wax paper, showing good forward scattering. Presumably due to wax particles of about the right size. Metabunk 2020-02-18 09-25-17.jpg

Here's the parchment paper, little, if any, forward scattering, which I'm guessing it because it's all fibers, not particles.

Metabunk 2020-02-18 09-25-45.jpg
 

Jesse3959

Member
Preface: I wrote this before you guys posted your comments, so if it seems like I didn't read the whole thread, that is why.
~~

All very interesting! I'll certainly pay more attention when summer comes around and we get some sun!

When I referred to water vapor I assumed there was an invisible region around the cloud where the dissolved water was at a higher concentration than in the cloud itself.

As to the light doing forward scattering, is that partly diffraction? (i.e. as in a diffraction grating) and partly reflecting at glancing angles off the sides of the droplets? All transmissive modes (i.e. light going into the drops) would basically focus to a point a drop's diameter away and spread at an extremely wide angle.

In general, a cloud scatters kind of in all directions evenly but if it's thin there's definitely a silver lining, so I'm wondering if it's diffraction and external surface reflection. But then again it depends on the situation, I've also seen where there's an even thin cloud and you can clearly see the disc of the sun through it, but there's no visible forward scattering.

I'm definitely going to have to pay more attention. Come to think of it, I've really only noticed forward scattering at cloud edges but not when there's a similarly thin but large even cloud layer.

I've actually seen pine trees on a distant hill lit up in what looks like the forward scattering - the trees were on a hill and the sun was behind the tree and every needle on that tree was brilliantly blazing bright. I figured it might be diffraction around the edges and reflection off the nearly ray-parallel surfaces.

As to solar power, my old boss had (and still has) a grid tie solar system - and he used to show me the power graphs it produced and he'd show me how the power would peak drastically high on partly cloudy days.

But before that, as a youngster, I always noticed that sudden hot surge when I was outside on a sunny day but I never understood it, then when my old boss told me about the cloud edge effect, then I started looking up when I felt the surge of heat and noticed it did correlate with the edge of a cloud.

Here's a couple pictures of tree-glow I took in 2014: (As I recall there were bugs swarming around the trees and that's what the white specs were. But at that time my camera did not take videos, just stills.) (It could also be that the trees were covered in heavy dew but I don't have any way of knowing.) (The two pictures bear the exif date and times of 2014-09-26 12:32:05 and 2014-09-19 17:57:59 - but it's likely my camera clock was off a day or two and some hours since I never set it. But it's safe to assume the photos were taken about 6 hours later in the day for one of them.)
deleteme1.jpg
deleteme2.jpg
 

Jesse3959

Member
intense? ... used to lay out in the sun ... I never noticed an increase in warmth.

Mick quoted that the the highest measured was over 1800 watts per square meter. By comparison, the "Standard sun" on earth is considered to be about 1000 watts per square meter.

So it can peak to nearly twice the normal energy level. (1.8 times to be specific.)

When I notice it most is when I'm already too hot and working in the sun. When we're already too hot we're much more sensitive to heat.
 

Mick West

Administrator
Staff member
Here it is in the sun. I stretched it over some card to get a clean incident angle and edge:

Metabunk 2020-02-18 09-56-03.jpg

You can just make out the edge effect there, but it's more obvious what is going on if I stack slices from three cases. Parchement paper (no forward scattering). Wax paper (forward scatter) and nothing.
Metabunk 2020-02-18 10-03-43.jpg
Note on the left edge, the Wax paper is the same brightness as the "nothing" setup, and get brighter as you get closer to the shadow. But the Parchement paper is brighter overall.

Increasing the contrast makes it clearer. There's a very slight brightening with parchment, but it's gradual across the sheet. The wax paper is brightest by far right next to the edge.
Metabunk 2020-02-18 10-05-16.jpg
 

deirdre

Senior Member.
the highest measured was over 1800 watts per square meter. By comparison, the "Standard sun" on earth is considered to be about 1000 watts per square meter.
I don't think the quote is saying I should feel almost twice as hot.

edit add: note: im only talking about the heat on my skin, not trying to cool an overheated/working-out body off... so we are really comparing different things. but technically if the air is hotter you should feel cooler as you'd be evaporating faster, unless it is humid out. :)
(but i digress, sorry)
 
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Mick West

Administrator
Staff member
Here's the wax paper, showing good forward scattering. Presumably due to wax particles of about the right size.
So I stuck it under the microscope. Wax paper looks like this:
Metabunk 2020-02-18 11-19-07.jpg

Parchment paper looks like this:
Metabunk 2020-02-18 11-15-44.jpg

Lots of discreet small particles in the wax paper, which is what you need for forward scattering. They are about the right size too, around 5-40µm.
 

Jesse3959

Member
Very interesting. I wonder what part of the forward scattering is from the wood fibers being optically coupled in the wax, and what part is from the wax molecules itself, since wax is translucent when not melted.

I did a quick test with some perfectly clear mineral oil soaked into paper and it makes it look and act rather like wax paper, and I warmed up some perfectly clear plastic and melted a thin layer of candle wax onto it, and it too looks and acts rather like wax paper as well.

However I'm not sure if they are acting more like regular scatter in all directions or if they fit the distribution typical of forward scattering. They do both pass a faint original beam of light from my flashlight though.
 

Mick West

Administrator
Staff member
However I'm not sure if they are acting more like regular scatter in all directions or if they fit the distribution typical of forward scattering. They do both pass a faint original beam of light from my flashlight though.
To judge the forward scattering it's best to use a small bare bulb or LED. I remove the cap from my flashlight
Metabunk 2020-02-18 22-29-19.jpg

Which gives a bright point light.

Metabunk 2020-02-18 22-29-52.jpg

Then you need to be sufficiently far away that you can tell the falloff isn't simply the distance from the light.

Wax Paper (forward scattering)
Metabunk 2020-02-18 22-34-11.jpg

Parchment paper (mostly random scattering)

Metabunk 2020-02-18 22-34-31.jpg
 
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