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Is it just me, or does the sky seem darker when there are lots of low clouds?

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#1 Kevin Thurman

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Posted 17 October 2020 - 02:15 AM

One thing I've noticed on those partly cloudy nights where the sky is mostly covered by fluffy, fast, low clouds, but where the transparency is still great, is that the space between the clouds seems much darker than the sky usually is. Am I the only one who has had this thought? My immediate assumption is that the contrast between bright clouds and the darker sky just makes it look darker, but it's just an illusion of the mind. My more optimistic interpretation is that low clouds block some light pollution from reaching higher up and scattering into the upper atmosphere, giving a slight reduction in skyglow. Is there anything to this?



#2 Jim Waters

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Posted 17 October 2020 - 02:28 AM

Likely an optical illusion caused by contrast difference.  Never trust the brains visual system.


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#3 sg6

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Posted 17 October 2020 - 02:55 AM

It's you!  lol.gif  lol.gif  lol.gif

Here when low cloud the street lights sort of illuminate it and it bounces back down.

Some of the dark moonless nights can be quite bright when cloud is around.


Edited by sg6, 17 October 2020 - 02:55 AM.

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#4 packerman

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Posted 17 October 2020 - 02:58 AM

Think of your pupils and your brain always trying to compensate for light and dark to provide the greatest dynamic range at any given point in time based at where you're looking.  If there are no clouds then the sky will look brighter, if there are clouds it will look darker as the eyes will balance to the scenes range. There are actually a lot of optical illusions built on how the brain compensates around this.

 

A good example is here:

https://www.popularm...rast-explained/


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#5 TOMDEY

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Posted 17 October 2020 - 04:08 AM

Yeah... it's a convincing illusion. Under truly dark skies... the clouds look black (aka invisible) not brighter than the starry sky. I've often gotten fooled by that, because the only way you know a cloud is there is that the stars disappear!    Tom


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#6 TOMDEY

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Posted 17 October 2020 - 04:18 AM

Like this >>>    Tom

Attached Thumbnails

  • 69 starry sky with clouds 60.jpg

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#7 endlessky

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Posted 17 October 2020 - 05:35 AM

I also agree with the illusion of contrast given by the surrounding cloudy (grey) areas making the dark ones look darker.

 

To me, the sky always looks darker when there's no clouds.

 

On top of that, there are those exceptionally clear, transparent nights - for example after a thunderstorm - where the sky looks so dark and clear that you feel you can almost touch the stars.

 

No cloud induced contrast illusion can beat that, to me.


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#8 Migwan

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Posted 17 October 2020 - 06:52 AM

It can be optical due to light pollution.  Basically, your pupils are going to be somewhat constricted.  Hence the sky between the clouds may appear darker than it is.  

 

Then again if conditions are right, those clouds might just be blocking a lot of the light pollution from reaching the upper atmosphere.   So though your pupils may a bit constricted and less able to take advantage of it, the upper atmosphere may actually be a little darker. 

 

Lastly, in the fall and winter, cold dry air coming over the Great Lakes kick up lake effect.   Conditions on such nights, if not causing too much overcast, can be very transparent and great for observing in between the clouds.  And now you know why I prefer go to and setting circles over star hopping.  

 

jd


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#9 BrooksObs

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Posted 17 October 2020 - 10:17 AM

Most replies are correct that the darker skies are simply the result of the visual reaction to contrast with the bright clouds. However, if the clouds are very low and unbroken over some intense urban light source like a city, it can darken the skies, as seen from a distance, considerably. The darkest skies that I have ever experienced from my rural home 75 miles north of NYC have been on occasions when that city was socked in by sea fog while my skies were clear.  The same occurs in the San Francisco Bay area when fog covers that city.

 

BrooksObs


Edited by BrooksObs, 17 October 2020 - 10:18 AM.

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#10 John Fitzgerald

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Posted 17 October 2020 - 10:26 AM

Same as above. The only exception is, if you are on a mountain that is clear, and a large area below is socked in by fog or very low clouds, it can darken your local sky considerably.  Otherwise, clouds reflect any nearby light pollution back at the ground, making the entire sky appear brighter.


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#11 Kevin Thurman

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Posted 17 October 2020 - 03:36 PM

Most replies are correct that the darker skies are simply the result of the visual reaction to contrast with the bright clouds. However, if the clouds are very low and unbroken over some intense urban light source like a city, it can darken the skies, as seen from a distance, considerably. The darkest skies that I have ever experienced from my rural home 75 miles north of NYC have been on occasions when that city was socked in by sea fog while my skies were clear.  The same occurs in the San Francisco Bay area when fog covers that city.

 

BrooksObs

I experienced this at a darker site once. It was maybe a bortle 3ish place but there was a single glow from a town across a lake. One of the nights we were there there were clouds over that half of the lake and the glow was completely gone. Not just directly, but it didn't extend into the sky as much as it had before. I didn't notice much of a practical difference at the eyepiece or in my cameras, though, so I thought it was a similar illusion.



#12 Chucke

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Posted 17 October 2020 - 03:47 PM

Nice simulation, Tom.  That is how clouds look from here.



#13 John Fitzgerald

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Posted 17 October 2020 - 04:08 PM

I have read that Mt. Wilson Observatory sometimes experiences a much darker than normal sky, when the city below is clouded in, and the mountain sticks up above.


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#14 Tony Flanders

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Posted 17 October 2020 - 05:28 PM

My immediate assumption is that the contrast between bright clouds and the darker sky just makes it look darker ...


Funny, right up until I read this sentence, I was assuming you were talking about dark clouds making the sky even darker.

At my country home, high clouds are almost always brighter than the sky background, because they pick up light from the nearby cities. But low clouds are invariably darker than the sky background.
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#15 Jon Isaacs

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Posted 17 October 2020 - 06:45 PM

Most replies are correct that the darker skies are simply the result of the visual reaction to contrast with the bright clouds. However, if the clouds are very low and unbroken over some intense urban light source like a city, it can darken the skies, as seen from a distance, considerably. The darkest skies that I have ever experienced from my rural home 75 miles north of NYC have been on occasions when that city was socked in by sea fog while my skies were clear.  The same occurs in the San Francisco Bay area when fog covers that city.

 

BrooksObs

 

This also happens in the mountains east of San Diego. The vast majority of the population lives relatively close to the ocean so when the marine layer rolls in at night, it blocks the urban light pollution.

 

Jon


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#16 TOMDEY

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Posted 18 October 2020 - 08:49 AM

Nice simulation, Tom.  That is how clouds look from here.

Thanks! Yes, Arizona has lotsa nice dark places. My sim catches the flavour, although in actual practice those abrupt cartoonish edges are rarely so distinct. I've often been at the eyepiece and wondering why the scope is either underperforming, just plain disappointing or the target is completely gone! So I look up at the sky and notice a whole chunk of sky has inexplicably gone blank... as if the stars just disappeared. Then (once again) I realize... "Oh!... a cloud must be there."

 

Of course, it's nice when the sky is that dark, so that clouds can sneak up like that --- ~Stealth Clouds~    Tom



#17 Voyager 3

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Posted 18 October 2020 - 11:54 AM

If there is clouds ... I can see the reflected orangish glow from the city due north and the sky changes into 2nd sunset but the sun has set and I hate the sky looking like this mad.gif .



#18 KI5CAW

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Posted 18 October 2020 - 01:11 PM

I live east of Albuquerque with a tall mountain range between us. About a third  of the western sky is blotted out by Albuquerque's uncontrolled light pollution. One night, low clouds covered the city but not my location ( a rare event). The  entire sky was much darker and we gained a whole magnitude on naked eye stars; including the western sky! But that has happened only once in twenty yearsfrown.gif



#19 stargazer193857

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Posted 18 October 2020 - 09:15 PM

One thing I've noticed on those partly cloudy nights where the sky is mostly covered by fluffy, fast, low clouds, but where the transparency is still great, is that the space between the clouds seems much darker than the sky usually is. Am I the only one who has had this thought? My immediate assumption is that the contrast between bright clouds and the darker sky just makes it look darker, but it's just an illusion of the mind. My more optimistic interpretation is that low clouds block some light pollution from reaching higher up and scattering into the upper atmosphere, giving a slight reduction in skyglow. Is there anything to this?


Sounds very believable, since light from the ground is being blocked from reaching that area. But if it cuts out 2/3 of the light, that is just 1 rung of the Bortle scale.
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#20 FirstSight

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Posted 21 October 2020 - 04:57 PM

I recall one night several years ago observing in my north Raleigh, NC driveway under what passes as decent-for-the-suburbs dark-ish skies (good enough to see some brighter galaxies) when a very low bank of clouds (<1000 ft above ground) abruptly moved in. The effect was like someone turning up a dimmer switch as the clouds rapidly covered the sky, and the light very quickly transitioned to more the level of dusk on a cloudy day - all shadows disappeared.  I could articulately make out individual leaves in the shrubbery in my front yard, and I could have read larger, bolded print.  Before the clouds rolled in, transparency was very good - which meant that scattering from overall suburban light pollution was rather minimal, until the low clouds rolled in and began to reflect and scatter all that light in the cloud.

 

And then, after about 10 min, while I was packing stuff back in for the night (I'd given up on observing) - the cloud bank departed as swiftly as it had rolled in, and the "dimmer switch" effect ran in reverse.


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