Why (actually) is the night sky so bright in the city? How far up is that happening?
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.