[AllenG (OP)]

I was reading about the hexagon circling Saturn's north pole (which may be the oddest weather feature on any planet in the solar system) and it came to me that I have no idea how wind speed is determined on a gaseous planet.  On the Earth, wind speed is measured relative to the solid ground. What is the relative marker used on a planet that has no solid surface?

For that matter how is the rotational period for the gas giants determined, given that the upper atmosphere (which is the part visible to us) may outpace, or even travel in the opposite direction of the core of the planet?

[AllenG (OP)]

I've asked this question on several sites.
I've gotten no real answer for how wind speed is determined, and for rotation every answer has been different.
They range from timing the storms as they pass in and out of view, to watching the planet wobble (seemingly in the same manner one uses to discover extra-solar planets), to timing the variations in the magnetosphere of the planet (which in the case of Saturn has changed significantly from what Voyager observed to what Cassini is observing now).

No one seems to know for sure.

[AllenG (OP)]

Doing another Google search (btw, this thread now comes up first in said search) I found This Space.com article about Saturn's rotational period being shorter than previously thought--which in turn changes the estimates on Saturn's wind speeds.

It seems I have asked a rather complex question. 

[LeeE]

It's tricky to say what constitutes a surface on the gas giants.  Assuming the outer layers are radar transparent, Doppler radar would give you the speed of any radar reflecting lower layers.