Everyday Physics : Swirls in the rising smoke
Why does cigarette smoke rise smoothly at first, but then form swirls after some distance?
Here is a gorgeous, detailed image of blue and white smoke ascending.
And the reason why the smoke rises smoothly at first and then starts making swirls:
The hot gas that carries the smoke particles initially rises slowly.
However, its net buoyancy, due to its higher temperature than surrounding gas, causes it to accelerate upward.
When the speed reaches a critical value, the flow becomes turbulent and breaks up into eddies. The critical speed is typically obtained after a rise of up to a few centimeters.
