You probably do not need "the". If you use "the" then you are referring to a specific instance of "fog", "mist", etc. For example, "Our town gets a lot of snow, but the snow this morning was light". I am using "snow" as a general term at first, and then talking specifically about "the snow" that fell in the morning today. In your sentence it would depend on the rest of the sentence. You say "The mist", but more than one "mist" could be "in place x" throughout the year. "The mist in place x is usually cold". -Refers to specifically mist in place x. "Mist in place x is usually cold". -Refers to any mist in place x. Ultimately, these are the same because of "usually". Now let's change it a bit. "The mist in place x is cold." - A specific mist in place x is currently cold. Tomorrow, a new mist in the same place, or even the same mist in the same place may be warm. "Mist in place x is cold." - All mist in place x is always cold.