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There is quite foggy day today here, I cannot see end of the street. This made me think about how much worse can it be.

So here is my question: how thick a fog can be? What's the smallest visibility distance possible?

By visibility distance I mean how far I can see a black object against a white background in the fog.

UPDATE:

There were some nitpicks, so let me clarify a bit:

  • I think of water/ice fog.
  • It's said that angular size matters, so let's say an object whose angular size as big as the full moon.
  • Whiteout doesn't count, assume the soil remains dark.
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Is your fog restricted to $ H_2O $ ? Just ask the Chinese how far they can see in their smog! Another nit: there's this thing called the NIIRS scale for visibility. How big is your black object, in angular space? –  Carl Witthoft Feb 7 at 16:58
    
@CarlWitthoft I think of the usual water fog. Does angular size matter? I think of usual landscape elements: houses, trees, pylons, etc. –  Calmarius Feb 7 at 17:03
    
Yes - rather obviously, the larger the item, the easier it is to see some sort of contrast even when badly blurred. –  Carl Witthoft Feb 7 at 17:53
    
I am sure it can be less tham 3 meters. –  Optionparty Feb 7 at 21:12
1  
Does a whiteout (snow) count? And you're mentioning water fog, but ice fog can be much denser. –  gerrit Feb 9 at 17:06

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