Troll's broth

If the temperature is reasonably high during the day, and it gets significantly cooler when the sun sets, the difference in temperature between the still warm bogs and the rapidly cooling air creates a mist that drifts lazily around beneath the trees.

This is called "troll woman's broth" or something similar in most Scandinavian languages. And is a sure sign that it's time to dig up that beanie, those flashlights and all that woolen underwear again.