<p>What's actually happening is scattering of light, both off of aerosol particles and nitrogen and oxygen molecules.  </p>

<p>For a review of quantitative models of light pollution as a function of distance from cities see <a href="http://articles.adsabs.harvard.edu//full/1991ASPC...17...56G/0000056.000.html" rel="noreferrer">Light Pollution Modeling</a> . Some consider height as well as distance.  </p>

<p>To get a rough estimate of what size of lights-out event would be needed to obtain a particular improvement in sky darkness, I would suggest looking at the light pollution map at <a href="http://darksitefinder.com/maps/world.html" rel="noreferrer">Dark Site Finder</a> or <a href="http://www.lightpollution.it/worldatlas/pages/fig1.htm" rel="noreferrer">The night sky in the World</a>. </p>

<blockquote>
  <p>Colours correspond to ratios between the artificial sky brightness and the natural sky brightness of: &lt;0.01 (black), 0.01-0.11 (dark-gray), 0.11-0.33 (blue), 0.33-1 (green), 1-3 (yellow), 3-9 (orange), 9-27 (red), >27 (white)</p>
</blockquote>

<p>Look at a coastal city such as Miami (which is extremely light-polluted) and see how the degree of light pollution drops off with distance from the city. Or look at a very isolated city such as Bismark, North Dakota.  I would estimate 20 to 40 kilometers away from a source you see a factor of 10 reduction in artificial brightness.  </p>

CC BY-SA 3.0
2016-10-04T13:06:18.140Z
Thanks for your answer! Is it better to just call it Raleigh scattering (via density fluctuations) than scattering off of individual nitrogen and oxygen molecules? Also, using a sky brightness map this way sort-of assumes that the roll-off in population density with distance from a city is fast enough that at 20 to 40 km from city center there is little contribution from local light sources.
uhoh
CC BY-SA 3.0
2016-10-04T13:23:50.180Z
Yes, Rayleigh scattering, like this http://www.colorado.edu/chemistry/volkamer/publications/group-pubs/thalman-2014-rscsNArO2air-accepted.pdf And yes I was trying to hand pick places where there were few light sources outside the main source, due to ocean or rural land, to compare to your situation where all the lights in a given region are turned off, and only those outside the region are contributing.  But the model review article includes models with consideration of distributed populations.
DavePhD
CC BY-SA 3.0
2016-10-04T13:31:02.340Z
Ah I see what you mean - very nice. I'll try to dig into the paper to see if I can extract some info about the importance of humidity, and an approximate ceiling. The Light Pollution Modeling paper is very helpful. Aerosols may be closer to the ground, but Raleigh scattering is stronger at lower density (and therefore higher altitudes) where there are larger fluctuations, so it's actually a really interesting problem!
uhoh
CC BY-SA 4.0
2021-04-23T02:58:42.800Z
I've just run across [Skyglow: a retrieval of the approximate radiant intensity function of ground-based light sources](https://academic.oup.com/mnras/article/439/4/3405/1150977) by accident.
uhoh
CC BY-SA 4.0
2021-05-17T00:18:44.273Z
@Earthworm yes while the paper I've cited just above your comment doesn't say so explicitly, many of the aerosol particles it describes will be in the size range where Mie scattering is a useful approximation. I'm [somewhat familiar](https://physics.stackexchange.com/q/458041/83380) with the mathematics of Mie scattering. This reminds me I can probably post an answer to that question now, unless you'd like to.
uhoh
CC BY-SA 4.0
2016-10-04T12:47:29.917Z
2018-07-30T10:32:17.147Z
2018-07-30T10:32:17.147Z
DavePhD
<p>The recent news reported by the <a href="http://www.nytimes.com/2016/09/30/world/europe/iceland-northern-lights-aurora.html" rel="nofollow noreferrer">New York Times</a>, the <a href="http://www.bbc.com/news/blogs-news-from-elsewhere-37506571" rel="nofollow noreferrer">BBC</a> and the <a href="http://www.npr.org/sections/thetwo-way/2016/09/29/495946913/reykjavik-turns-off-street-lights-to-turn-up-the-northern-lights" rel="nofollow noreferrer">NPR</a> web sites (for example) as well as in the <a href="http://icelandreview.com/news/2016/09/28/lights-northern-lights-show" rel="nofollow noreferrer">Iceland Review</a> and <a href="http://en.vedur.is/weather/forecasts/aurora/" rel="nofollow noreferrer">Iceland Magazine</a> sites, about the shutting-off of Reykjavik's streetlights in order to better enjoy the <a href="https://en.wikipedia.org/wiki/Aurora" rel="nofollow noreferrer">Aurora</a> (northern lights), got me thinking.</p>
<p>There are many life-long city dwellers who've never really seen a relatively dark sky or more than a handful of stars. Even people who could do so sometimes have never made the trip out of the city far enough for the experience.</p>
<p>So I was curious if this &quot;lights-out&quot; technique could ever be tried in a city or somewhere else besides in Iceland.</p>
<p>I'd like to estimate how big of a radius of &quot;lights-out&quot; would an observer need for the sky to be noticeably darker. But for that I'd have to know what is actually doing the scattering, and how far up does it extend.</p>
<p><strong>My question:</strong> Why (actually) is the night sky so bright in the city? How far up is that happening?</p>
<p>I'm also curious if it is generally stronger in some climates vs others.</p>

CC BY-SA 3.0
2016-09-30T14:29:58.840Z
You might want to summarize your question a bit and prioritize what your main points are that need answering.
arkaia
CC BY-SA 3.0
2016-09-30T16:09:17.427Z
@aretxabaleta done!
uhoh
CC BY-SA 3.0
2016-09-30T19:45:18.957Z
Its not so much the sky is bright, but it is the effect light pollution has on the eye. When lights are present (such as streetlights) the eye contracts to reduce light coming in, making it harder to see small features in the nighttime sky. Reducing light pollution allows the eye to see more details in the sky.
BarocliniCplusplus
CC BY-SA 3.0
2016-09-30T23:49:06.560Z
@BarocliniCplusplus that's an interesting point. Does that suggests that by simply finding or making a walled area (e.g. a building's rooftop) one could wait 10 minutes for vision to adjust and then see a dark sky, and thousands of stars and the milky way? It's really that simple?
uhoh
CC BY-SA 4.0
2018-07-30T10:15:57.953Z
@uhoh No, it isn't.
gerrit
CC BY-SA 4.0
2018-07-30T10:50:31.373Z
@gerrit unless the walls are really, really, really tall ;-)
uhoh
CC BY-SA 4.0
2018-07-30T13:12:12.183Z
@uhoh Then you could see only a really, really, really small section of the sky :) Actually, [astronomers can do remarkable things nowadays](https://www.rmg.co.uk/discover/behind-the-scenes/blog/first-light).
gerrit
CC BY-SA 4.0
2018-07-30T13:34:12.223Z
@gerrit Interesting! [How will “modern equipment” allow the Royal Observatory to now avoid some of the effects of light pollution at Greenwich?](https://astronomy.stackexchange.com/q/27161/7982)
uhoh
<meteorology><atmosphere><humidity><water-vapour>
Why (actually) is the night sky so bright in the city? How far up is that happening?
uhoh
# Why (actually) is the night sky so bright in the city? How far up is that happening?

(Asked by: username_0 on 2016-09-30)

The recent news reported by the [New York Times](http://www.nytimes.com/2016/09/30/world/europe/iceland-northern-lights-aurora.html), the [BBC](http://www.bbc.com/news/blogs-news-from-elsewhere-37506571) and the [NPR](http://www.npr.org/sections/thetwo-way/2016/09/29/495946913/reykjavik-turns-off-street-lights-to-turn-up-the-northern-lights) web sites (for example) as well as in the [Iceland Review](http://icelandreview.com/news/2016/09/28/lights-northern-lights-show) and [Iceland Magazine](http://en.vedur.is/weather/forecasts/aurora/) sites, about the shutting-off of Reykjavik's streetlights in order to better enjoy the [Aurora](https://en.wikipedia.org/wiki/Aurora) (northern lights), got me thinking.

There are many life-long city dwellers who've never really seen a relatively dark sky or more than a handful of stars. Even people who could do so sometimes have never made the trip out of the city far enough for the experience.

So I was curious if this "lights-out" technique could ever be tried in a city or somewhere else besides in Iceland.

I'd like to estimate how big of a radius of "lights-out" would an observer need for the sky to be noticeably darker. But for that I'd have to know what is actually doing the scattering, and how far up does it extend.

**My question:** Why (actually) is the night sky so bright in the city? How far up is that happening?

I'm also curious if it is generally stronger in some climates vs others.

**Tags:** meteorology, atmosphere, humidity, water-vapour

> **Comments:**
> **username_1**: You might want to summarize your question a bit and prioritize what your main points are that need answering.  
> **username_0**: @aretxabaleta done!  
> **username_2**: Its not so much the sky is bright, but it is the effect light pollution has on the eye. When lights are present (such as streetlights) the eye contracts to reduce light coming in, making it harder to see small features in the nighttime sky. Reducing light pollution allows the eye to see more details in the sky.  
> **username_0**: @username_2 that's an interesting point. Does that suggests that by simply finding or making a walled area (e.g. a building's rooftop) one could wait 10 minutes for vision to adjust and then see a dark sky, and thousands of stars and the milky way? It's really that simple?  
> **username_3**: @username_0 No, it isn't.  
> **username_0**: @username_3 unless the walls are really, really, really tall ;-)  
> **username_3**: @username_0 Then you could see only a really, really, really small section of the sky :) Actually, [astronomers can do remarkable things nowadays](https://www.rmg.co.uk/discover/behind-the-scenes/blog/first-light).  
> **username_0**: @username_3 Interesting! [How will “modern equipment” allow the Royal Observatory to now avoid some of the effects of light pollution at Greenwich?](https://astronomy.stackexchange.com/q/27161/7982)  

---

## 1 answers

### Answer by username_4 on 2016-10-04. Score: 15 | Accepted answer

What's actually happening is scattering of light, both off of aerosol particles and nitrogen and oxygen molecules.

For a review of quantitative models of light pollution as a function of distance from cities see [Light Pollution Modeling](http://articles.adsabs.harvard.edu//full/1991ASPC...17...56G/0000056.000.html) . Some consider height as well as distance.

To get a rough estimate of what size of lights-out event would be needed to obtain a particular improvement in sky darkness, I would suggest looking at the light pollution map at [Dark Site Finder](http://darksitefinder.com/maps/world.html) or [The night sky in the World](http://www.lightpollution.it/worldatlas/pages/fig1.htm).

> Colours correspond to ratios between the artificial sky brightness and the natural sky brightness of: <0.01 (black), 0.01-0.11 (dark-gray), 0.11-0.33 (blue), 0.33-1 (green), 1-3 (yellow), 3-9 (orange), 9-27 (red), >27 (white)

Look at a coastal city such as Miami (which is extremely light-polluted) and see how the degree of light pollution drops off with distance from the city. Or look at a very isolated city such as Bismark, North Dakota. I would estimate 20 to 40 kilometers away from a source you see a factor of 10 reduction in artificial brightness.

> **Comments:**
> **username_0**: Thanks for your answer! Is it better to just call it Raleigh scattering (via density fluctuations) than scattering off of individual nitrogen and oxygen molecules? Also, using a sky brightness map this way sort-of assumes that the roll-off in population density with distance from a city is fast enough that at 20 to 40 km from city center there is little contribution from local light sources.  
> **username_4**: Yes, Rayleigh scattering, like this http://www.colorado.edu/chemistry/volkamer/publications/group-pubs/thalman-2014-rscsNArO2air-accepted.pdf And yes I was trying to hand pick places where there were few light sources outside the main source, due to ocean or rural land, to compare to your situation where all the lights in a given region are turned off, and only those outside the region are contributing.  But the model review article includes models with consideration of distributed populations.  
> **username_0**: Ah I see what you mean - very nice. I'll try to dig into the paper to see if I can extract some info about the importance of humidity, and an approximate ceiling. The Light Pollution Modeling paper is very helpful. Aerosols may be closer to the ground, but Raleigh scattering is stronger at lower density (and therefore higher altitudes) where there are larger fluctuations, so it's actually a really interesting problem!  
> **username_0**: I've just run across [Skyglow: a retrieval of the approximate radiant intensity function of ground-based light sources](https://academic.oup.com/mnras/article/439/4/3405/1150977) by accident.  
> **username_0**: @Earthworm yes while the paper I've cited just above your comment doesn't say so explicitly, many of the aerosol particles it describes will be in the size range where Mie scattering is a useful approximation. I'm [somewhat familiar](https://physics.stackexchange.com/q/458041/83380) with the mathematics of Mie scattering. This reminds me I can probably post an answer to that question now, unless you'd like to.  

---

