Fog rolling down the river is what kind of phrase?

Fog rolling down the river is a form of figurative language known as personification. Personification occurs when a non-human object is given a human characteristic or action. In this case, fog cannot literally "roll" down the river. I mean, just imagine a person rolling down a hill like a derp-head. Fog doesn't do that, but in a figurative way, it "rolls" down the river. I hope you can understand : ')

Its a form of figurative language.