  The atmospheric properties above three sites on the Internal Antarctic
Plateau are investigated for astronomical applications calculating the monthly
median of the analysis-data from ECMWF (European Centre for Medium-Range
Weather Forecasts) for an entire year (2005) thus covering all seasons.
Radiosoundings extended on a yearly time scale from Dome C and the South Pole
are used to verify the reliability of the analyses in the free atmosphere and
to study the wind speed in the first 100 m as the analysis-data are not
optimized for this altitude-range. The wind speed in the free atmosphere is
obtained from the ECMWF analyses from all three sites. It appears that the
strength of the wind speed in the upper atmosphere in winter is correlated to
the distance of the site from the centre of the polar high. The Richardson
number is employed to investigate the stability of the free atmosphere and,
consequently, the probability to trigger thermodynamic instabilities above the
three sites. We find that, in a large majority of the cases, the free
atmosphere over the Internal Antarctic Plateau is more stable than at
mid-latitude sites. Given these data we can obtain a ranking of the three sites
with respect to wind speed, in the free atmosphere as well as in the surface
layer, and with respect to the stability of the atmosphere, using the
Richardson number.
