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There definitely is 'atmospheric fog' at night.
There are three major components to atmosperic scattering:
a) raleigh scattering
b) mie scattering
c) regular old fog
a) Raleigh scattering is what makes the sky blue during the day. The air loves blue light, and steals some of the blue out of rays of light that pass through the air. The light that it steals is bounced around in the air, and distibuted more or less evenly.
If you are looking at a distant light source, two things happen. 1) some of the blue light gets stolen from that light source. This is called "outscattering" 2) Some of the blue light that is scattered throughout the air is 'given back' to the ray coming from the light source. This is called "inscattering". The key here is that outscattering is proportional - for every unit of distance a ray of light travels through the air, it will lose a certain percentage of blue light. Inscattering is not proportional - for every unit of distance a ray of light travels through the air, a fixed amount of blue light will be added back to the ray.
What this means is that bright light sources lose a net amount of blue - they lose more than they gain back from inscattering. This is why distant clouds appear a gentle yellow-orange. Dark light sources gain more blue than they lose. This is why distant, dark colored mountains become bluish. The further away it is, the more closely it's color matches the sky color.
At night, this is still in effect. Without the sun pumping the air full of blue light, the inscattering is basically null. So at night, distant sources of light will simply be reddened. Distant lightning at night, for example, often looks orange or pink. You can sometimes see this with city lights, too.
In photshop, this can be simulated by using an orange layer set to multipy mode, to simulate the outscattering, and a blue layer (the orange hue shfted by 180 degrees) set to screen mode to simulate the inscatter. The blue screen layer needs a depth mask. The orange multiply layer needs a special depth mask. This special depth mask would be the actual depth mask blended with the greyscale version of the image to simulate the effects of both depth and brightness. Season to taste.
b) Mie scattering is is the tendency of the air (with dust and water droplets in it) to scatter white-yellow light. It does not scatter the light evenly in all directions, so this effect is directional and is strongest near the sun. When the sun is low to the horizon on a hazy day, the sky near the sun is a bright white-yellow.
This wouldn't really be in effect at night, except slightly, around very bright light sources.
The effect is purely additive, so in photoshop, screen mode works just fine so simulate this.
c) Fog is just that. Sort of thick air. At night, this would darken distant lights.
A layer set to normal mode, whose opacity is set with a depth mask would simulate this in photshop.
Hope this helps.
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