The column of smoke began as a fairly unbroken column of less dense gas. As it rises, the column begins to interact with the surrounding air, forming a curling vortex. 
Each spiral vortex generally outwardly from the center line and down as it becomes higher. The most unexpected forms occur earlier. 
It helps to think of the smoke column as a semi-transparent plastic bag in three dimensions, with some parts views on edge and therefore more opaque. -------
Two of these images are by Irene Muller. They all come from the collection of images "Dark Roasted Blend."
