  In this paper, Jupiter's Great Red Spot (GRS) is used to determine properties
of the Jovian atmosphere that cannot otherwise be found. These properties
include the potential vorticity of the GRS and its neighboring jet streams, the
shear imposed on the GRS by the jet streams, and the vertical entropy gradient
(i.e., Rossby deformation radius). The cloud cover of the GRS, which is often
used to define the GRS's area and aspect ratio, is found to differ
significantly from the region of the GRS's potential vorticity anomaly. The
westward-going jet stream to the north of the GRS and the eastward-going jet
stream to its south are each found to have a large potential vorticity
``jump''. The jumps have opposite sign and as a consequence of their
interaction with the GRS, the shear imposed on the GRS is reduced. The
east-west to north-south aspect ratio of the GRS's potential vorticity anomaly
depends on the ratio of the imposed shear to the strength of the anomaly. The
aspect ratio is found to be $\approx$2:1, but without the opposing jumps it
would be much greater. The GRS's high-speed collar and quiescent interior
require that the potential vorticity in the interior be approximately half that
in the collar. No other persistent geophysical vortex has a significant minimum
of potential vorticity in its interior and laboratory vortices with such a
minimum are unstable.
