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Estimating Cloud Brightness

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#1 Asbytec

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Posted 24 July 2013 - 07:55 AM

Forgive me if this has been beaten around in this forum, but I just moved to the suburbs (Bortle yellow/Green, I think) and am curious about sky conditions. I hope this is still good for planets, surely, and challenging double stars.

It's rainy season and very cloudy. But, I thought seeing the extent of lit clouds overhead could give some indication of what to expect on a clear night. Is there some relationship one can use?

By the way, found some fascinating sites related to light pollution, if you've not seen them. I especially like the Google at night link, I compared my new location to some previous sites. As it shows, and as suspected, my new site is a slight step down due to the proximity of a town nearby.

http://www.globeatnight.org/webapp/
http://ngdc.noaa.gov..._viirs_ntl.html
http://ngdc.noaa.gov...composites.html
http://www.lightpoll.../pages/fig1.htm
http://geology.com/a...ia-at-night-...

Thanks in advance.

#2 Gil V

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Posted 24 July 2013 - 10:42 AM

Interesting question. Until I started looking into this in more detail, I never realized that the cloids were supposed to be dark in a blue or grey zone!!

#3 GlennLeDrew

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Posted 24 July 2013 - 11:21 AM

There are complicating factors.

The illumination a cloud receives depends on the integrated light emitted by all sources below. If that light is not extending more or less uniformly to the horizon in all directions as 'seen' from the cloud base, the illumination will decrease with height. In other words, for the illumination of a cloud to remain constant with height, the 180 degree hemisphere occupied by the landscape below must be fairly evenly distributed with light sources.

In most cases the light distribution is patchy to at least some extent, resulting in 'hot spots' on the cloud base. The lower the cloud, the more discrete and well defined these bright patches.

#4 Asbytec

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Posted 26 July 2013 - 01:38 AM

I never realized that the clouds were supposed to be dark in a blue or grey zone!!


Interesting point. I can see clouds from a blue/grey zone, but they appear more white. Higher up, they may appear dark. I cannot recall.

The clouds I am seeing now appear orange-ish. But, even with complete overcast, clouds varying slightly in density and height, the appearance of bright clouds (hot spots) is rather spotty and concentrated toward the small city.

The zenith is somewhat lit, but I think it will be just fine when the clouds clear. After all, clouds reflect(?) more light than the atmosphere might scatter back toward earth. If those city photons keep heading outward, they should have no affect at the eyepiece. It's the scattered few that cause problems.

So, anyway, I think a low cloud layer, say 3,000 to 5,000' would tend to over estimate the amount of light pollution in both extent across the sky and intensity. Yes?

In most cases the light distribution is patchy to at least some extent, resulting in 'hot spots' on the cloud base. The lower the cloud, the more discrete and well defined these bright patches.

That's exactly what I see. A moderate hot spot toward the city and up a bit above the horizon. And a tiny bit overhead. Of course the uneven cloud base makes for some variability.

I just cannot estimate the altitude of the 'light dome' based on the extent of the lit cloud base. The clouds might give some indication, but it's hard to figure. Otherwise, gotta wait until November dry season to get a clear night look at it.

Thanks for replying...I'm impatient. :(

#5 Tony Flanders

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Posted 26 July 2013 - 05:27 AM

Interesting question. Until I started looking into this in more detail, I never realized that the cloids were supposed to be dark in a blue or grey zone!!


At a truly dark site, clouds are always darker than the background; they're visible only as places where the stars are missing.

In the gray zone, clouds overhead are always dark, but ones near the horizon over distant cities will appear bright. For instance, from northern Arizona, clouds over Phoenix (150 miles away) appear bright.

In the green and blue zones, clouds usually appear bright but may sometimes appear dark, depending where they are with respect to the dominant light source, how high they are, and their composition.

In the yellow zone and brighter, clouds always appear brighter than the starry background, often by a large margin.

#6 FirstSight

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Posted 26 July 2013 - 08:37 AM

I still recall a really strange experience observing in my suburban N. Raleigh driveway on a night where the transparency and lack of humidity made for conditions nearly as good as it gets in such an environment (except for the rare small handful of exceptional nights when a thin wisp of milky way becomes dimly visible directly overhead through e.g. Cygnus). Good enough that my observing was concentrated toward viewing spring galaxies, granted not anything challenging enough to require a true dark site for.

With stunning abruptness, a fast-moving layer of low stratus clouds moved in such that the sky went from being crystal-clear to completely overcast with glowing pinkish-grey clouds within about 30 seconds. The effect weirdly mimicked the accelerated arrival of dawn on a cloudy day; there was an unread newspaper lying just outside my garage that I hadn't even seen was there in the dark, and there was enough light to clearly read the headlines, and the colors and individual leaves of foliage were washed-out, but articulately visible. Then after about five minutes taking in this weirdly abrupt change, I was just about to start packing my gear in when the cloud bank moved away as rapidly as it had appeared, just like a dimmer switch slowly but perceptibly dialing down the light, and within less than a minute, conditions had returned to dark-as-it-ever-gets.

#7 ev2

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Posted 30 July 2013 - 03:04 PM

Interesting question. Until I started looking into this in more detail, I never realized that the cloids were supposed to be dark in a blue or grey zone!!


Indeed - this is something I just learned as well. At my home, I can literally read a book by light reflected from clouds if it's overcast. But here, the overcast is often fog which rides maybe 500ft above ground level, so it's particularly intense. :mad:






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