The context of the mist that went up is important. It's part of the narrative of the week of creation when the fountains of the great deep had to be sealed after the deep had drained through them into the interior of the Earth.
Mist or Steam?
A Mist Went Up
Contextually the mist that went up was a phenomenon which happened during the week of creation, specifically on the third day. However, in conservative Christian doctrine this condition persisted until the time of Noah.
The First Heaven
Part of the space between the firmament and Earth is the area in contact with the Earth that we call the atmosphere, or sky. This is where the birds fly. It's referred to as the open firmament.