Sunlight under the clouds

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Sunlight under the clouds
« on: May 15, 2018, 10:03:45 PM »
Hello,

Why is it sometimes possible to see sunlight under clouds? If the sun rotates above the clouds, or if it is directly in the firmament, how is this possible?

Have a nice day.


Re: Sunlight under the clouds
« Reply #1 on: May 16, 2018, 12:44:56 AM »
I could tell you why no one is answering, but the answer will probably be moved somewhere else. That seems to be a bit of a problem and may be why people arent posting that much anymore.

Re: Sunlight under the clouds
« Reply #2 on: May 16, 2018, 03:24:53 AM »
I found the explanation :)

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rabinoz

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Re: Sunlight under the clouds
« Reply #3 on: May 16, 2018, 05:25:47 AM »
I found the explanation :)
I've seen it before and his errors start right from 0:16 where he says, "Taking advantage of the fact that the atmosphere is full of moisture and that it is acting like a lens."
Some of the troubles with this explanation are:
  • The atmosphere is not "full of moisture". Even at 100% humidity at 20°C 1 m3 of air has a mass of about 1.225 kg and contains only 17.3 gm of water.
    So it is certainly not "full of moisture".

  • That small amount of water is in the form of water vapour which has a lower refractive index than even air, so humid air refracts light less than dry air.
    For example, at "normal pressure" and 20°C, the refractive index of dry air is about 1.000271 and that of 100% humid air is about 1.000270, virtually no change!

  • Sunsets and the suns rays under clouds occur under dry or humid conditions.
Water might refract light much more than air, but water vapour does not.

Sure, Rob Skiba gets refraction because he inserts a plastic lens in the sun's path.
But in reality, there is nothing but air to refract the light and that will bend the light from the sun by roughly 0.5° in the wrong direction!

They do say, "A little knowledge is a dangerous thing".

Yet we certainly do get this sort of thing before sunrise and after sunset:

Clouds lit on the underside before sunrise
       
Sun's rays shining up on clouds before sunrise
       
Clouds lit from underneath near Toogoolawah at Sunset
And upward slanting shadows on the underside of clouds with the sun lower than the peak of Mount Rainier:
     
Maybe the sun really does rise from behind the horizon and set behind the horizon.





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Son of Orospu

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Re: Sunlight under the clouds
« Reply #4 on: May 16, 2018, 05:46:16 AM »
It is a common misconception that light only ever travels in straight lines. 

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SphericalEarther

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Re: Sunlight under the clouds
« Reply #5 on: May 16, 2018, 07:04:08 AM »
It is a common misconception that light only ever travels in straight lines.
It is quite well known that refraction causes light to bend, but other than that light does travel in straight lines.

Atmospheric refraction which I believe you are referring to is usually very easy to spot as it provides a blurry vision, usually just above the horizon over large bodies of water, but it does not account for the suns light coming from below the clouds as you would suggest.

Re: Sunlight under the clouds
« Reply #6 on: May 16, 2018, 07:45:28 AM »
Also gravitational lensing causes light to bend. And refraction is more a case of light “breaking”, hence the term.
Be gentle

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SphericalEarther

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Re: Sunlight under the clouds
« Reply #7 on: May 16, 2018, 08:13:51 AM »
Also gravitational lensing causes light to bend. And refraction is more a case of light “breaking”, hence the term.
Yeah, true, but gravitatinal lensing isn't really something we can clearly see, and isn't something we need to calculate for here in our small solar system.

Re: Sunlight under the clouds
« Reply #8 on: May 16, 2018, 10:38:35 AM »
Also gravitational lensing causes light to bend. And refraction is more a case of light “breaking”, hence the term.
Yeah, true, but gravitatinal lensing isn't really something we can clearly see, and isn't something we need to calculate for here in our small solar system.
It has been observed with amateur telescopes, albeit very faintly. If I’m not mistaken there are some amateur guides to observe “Einstein’s Cross” and check it out yourself. Just saying this on the top of my head, I’m on my phone and don’t feel like googling for references :s
Be gentle

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JackBlack

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Re: Sunlight under the clouds
« Reply #9 on: May 16, 2018, 01:56:48 PM »
I found the explanation :)
So the same dishonest fresnel lens BS?
Earth's atmosphere is not a fresnel lens.
You may as well have said light magically bends upwards.

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SphericalEarther

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Re: Sunlight under the clouds
« Reply #10 on: May 16, 2018, 11:34:18 PM »
I found the explanation :)
So the same dishonest fresnel lens BS?
Earth's atmosphere is not a fresnel lens.
You may as well have said light magically bends upwards.
The closest thing I got to a true FE model, was to use refraction through the atmosphere to bend light more than it does.
It could conceptually explain objects behind the horizon, the local sun/moon disappearing, the curve at higher altitudes and more.
But then again it had some big flaws, like the fact that light would bend the opposite way as the density of the atmosphere got denser towards the surface.

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rabinoz

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Re: Sunlight under the clouds
« Reply #11 on: May 17, 2018, 12:35:45 AM »
It is a common misconception that light only ever travels in straight lines.
Thanks for pointing that out, but its not "a common misconception" to those who make precise astronomical observations.
It has been well known to astronomers since Tycho Brahe made careful observations to improve the accuracy of his local azimuth/altitude coordinates, especially of Mars.

Since you seem so interested, here are some modern tables of the refraction of light due to the atmosphere for various elevation angles:

Refraction deviation angles for an observer at sea level, M. E. THOMAS AND R. I. JOSEPH
From: Refraction deviation angles for an observer at sea level
The paper might be worth reading for those interested in views of the sun near the horizon.

So sure, we know that the near the horizon the atmosphere typically bends downwards by roughly 0.5°.