Steam

Steam says something is hot. It comes from a source, so the frame carries a reason for the vapour: a grate, a vent, a kettle, a body in cold air.

It also puts a vertical into the picture. Most atmosphere spreads sideways, and a rising column crosses the frame the other way, which is why one vent can hold a whole composition together.

It rises fast near the source, slows as it cools, then spreads and disappears. That shape is the difference between steam and every other atmosphere here, because the column is narrow at the bottom and wide at the top.

It reads against a dark background with a light close to it. A street lamp beside a vent, a window behind a kitchen, a headlight through a grate. Steam far from any source goes invisible.

Cold air makes it last. In a warm scene the vapour vanishes within a metre, so a shot that needs a tall column needs a cold night in the prompt as well.

Use it on a night street, in a kitchen or a workshop, and in any scene where the temperature is part of the story.

Skip it when the air is supposed to be still and even. A column of steam is a moving local event and it pulls the eye to wherever it starts.

Smoke rolls and spreads in any direction from a fire. Steam goes up from a heat source and then thins out. If it has a rising column, it is steam.

Fog hangs at a level and comes from nowhere in particular. Say rising, or the model gives you fog.

Martin Scorsese shoots the New York streets of Taxi Driver (1976) with steam lifting out of the manhole covers, lit by whatever sign or lamp is nearest, so the road reads as hot and awake at night.

The vapour hangs instead of climbing. Say it rises, name the source it rises from, and say it spreads and thins as it cools.

The second failure is steam with no light near it. Put a lamp, a window or a headlight beside the source, or the column will not read against the background.