QUESTION:
How do you get the speed of the winds on Neptune without instruments on
the planet?

Answer from Sanjay Limaye on April 25, 1996:
Most of the knowledge about windspeeds on Neptune comes from measuring the
cloud motions.  It is assumed that the clouds generally move with the wind.
 This assumption has been found to be reasonably valid on the earth by
comparison between wind speed and direction measurements of small clouds and
by the drift of balloons. For the earth, the distinction has to be made
between storm systems and small clouds for determining the wind.  For
example, a hurricane as a whole does not move at the same speed and
direction as the clouds in the spiral bands for example.  There are other
means of inferring wind speed and direction without direct measurements with
instruments which depend on the known relationships between wind and the
pressure and/or thermal structure of the atmosphere, that allows us to
estimate the wind speed and often direction quite well.  On Venus for
example, if we know if the average temperature as a function of height at
all latitudes (longitudinal average), we can estimate quite well the east
west wind as a function of height and latitude, but cannot determine the
north-south component of the wind.

Another means of determining the wind speed and direction is to determine
the drift rate and direction of an atmospheric entry probe, such as Galileo
or the Huygens Probe to be launched into Titan's atmosphere in a few years.
 Pioneer Venus and the soviet Venera probes that entered the atmosphere of
Venus some years back allowed us to determine the wind as a function of
height as the probes descended to the surface in about one hour after entry.
 These probes come into equilibrium very quickly after they enter an
atmosphere, and thus their direction of motion and speed are good indicators
of the planet's wind field at the location of the probe.  The main
difficulty then is to be able to track the probes accurately enough to be
able to determine the drift, and this can be done either with Doppler radar
or using radio triangulation or radio intrerferometry, depending on the
resources available.