Normal fog is, well, just normal fog
But it can appear strange and the sounds may sound different, making you think something is
unnatural, while all is simply normal fog. ;D
The eerie strangeness can drive you made at night, if you think you are seeing things just outside your vision reach.
And perhaps something is using this fog cover to hunt?
And
The Mist also appear like just a normal fog, but then all hell can break loose
It starts as normal fog, then something unnatural happens. Or nothing.
Mist usually appear as normal fog, of course, but also appear as dust, snow or sand storm. Or whatever obscures vision and cloud the senses ...
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I sometimes add normal fog to my atmospheric description, just to make the players uneasy. Is it normal or supernatural?
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A good friend once posted a short story where she stopped in a cemetery during a fog night. It was great, how she depicted the thing she thought she saw, and the fears she had. Altough she knew it was all imaginary, it was powerful and scary. Alas, I do not find a copy of it.
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To answer the last question, no, I don't think it is possible to differenciate at first sight. But vistani can surely differentiate the two?
Joël