끼다: to cloud over, or hang around
The fog in this sentence is hanging around, so it is misty.
Also, this is the grammar pattern (blunt form + it-da (sorry, no Keyboard on this computer))which indicates the current condition of something. So, as opposed to "It is getting foggy" this is more like "It is currently foggy". Another example would be the difference between "The door is opening / The door opens" and "The door is currently open".
The English translation says "misty" even though the Korean word for fog is being used. Mist and fog aren't the same in English. The translation should be "it is foggy".
To me, "it is misty" means that kind of fog that's drizzling down like lazy rain. Stuff outside will have a dewy layer and may develop droplets after a while. "It is foggy" just means you can't see past a certain distance because of "clouds" down at our level. Things aren't necessarily getting moist like if it were misty. Anyway, I'm wondering which of these the Korean phrase can refer to.