How does a whirlpool form?
is the Wiki entry about whirlpools.
"A whirlpool is a body of rotating water produced by opposing currents or a current running into an obstacle Small whirlpools form when a bath or a sink is draining. More powerful ones in seas or oceans may be termed maelstroms. Vortex is the proper term for a whirlpool that has a downdraft."
The entry goes on, of course, to talk about whirlpools in oceans and rivers; but here, before us, was such a tiny, but perfect one!
We watched it, riveted, for quite a while! Here is the video:
It also reminded me of the tornadoes I have watched in Missouri....