body heat?

While I was getting ready for tonight's Io eclipsing and occulting
Ganymede, I defocused jupiter out by about an inch at which point,
through the eyepiece I could see the image of my spider and heat
currents in the air (driveway?, houses?). I found that if I held my
hand out in the light path of my dob, I could see the currents coming
off my hand.
I was wondering if people have noticed distortion in the images due to
the heat coming off their legs directly below the tube opening? I
imagine it would have to be very good seeing to notice the effect, but
the next time I get great conditions I'm going to try sitting sidesaddle
and see if I can tell.
Perhaps this is a well-known effect?
Bill

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