When you’re drawing or painting a stream of smoke rising in still air, here are a couple of things to keep in mind.
The column of smoke begins as a fairly uninterrupted column of less dense gas. As it rises, the column begins to interact with the surrounding air by forming a curling vortex.
Each vortex usually spirals outward from the central line and breaks up as it gets higher. The most unexpected shapes happen higher up.
It helps to think of the smoke column as a three dimensional semi-transparent plastic bag, with some parts seen edge-on and therefore more opaque.
Two of these images are by Irene Muller. They all come from the image collection “Dark Roasted Blend.”