Why (actually) is the night sky so bright in the city? How far up is that happening? The recent news reported by the New York Times, the BBC and the NPR web sites (for example) as well as in the Iceland Review and Iceland Magazine sites, about the shutting-off of Reykjavik's streetlights in order to better enjoy the 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 &quot;lights-out&quot; 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 &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.
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.
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 . 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 or The night sky in the World. 


  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)


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.