Hot Optics

[BrainstormUK]

Everyday I warm my mug up at work with hot water before making tea in it. I notice that as I look in that there are 'lines' slowly swirling around disappearing and new ones appearing... What is going on? You do need to try this to see what I'm talking about, but I'd really like to know what's happening - after all it's only water!

One possibility is that the lines are interfaces in temperature differences (and so density differences) which then give a difference in refractive index so light falling on the interface is reflected differently to other areas and so they show up as lines.

Any ideas most welcome
Cheers

I posted this elsewhere on this site but this looks like the place to be

[Fu-Kwun Hwang]

Could you add a picture or a video (even better) so that I can understand what it is?