The science of vortices

[Still Waters]

Vortices are spinning sections of fluid or gas that retain their shape by virtue of the energy embodied within them as they turn. They used to be common at parties and in pubs, in the form of smoke rings that could be admired before they melted away. Water flowing down a bath plug and tropical tornadoes act in the same way, on a different scale.

http://www.telegraph...f-vortices.html