  The atmospheric properties above three sites (Dome A, Dome C and the South
Pole) are investigated for astronomical applications using the monthly median
of the analyses from the ECMWF (European Centre for Medium-Range Weather
Forecasts). Radiosoundings extended on a yearly time-scale at the South Pole
and Dome C are used to quantify the reliability of the ECMWF analyses in the
free atmosphere as well as in the boundary and surface layers, and to
characterize the median wind speed in the first 100 m above the two sites.
Thermodynamic instabilities in the free atmosphere above the three sites are
quantified with monthly median values of the Richardson number. We will present
a ranking of the sites with respect to the thermodynamic stability, using the
Richardson number, and with respect to the wind speed, in the free atmosphere
(using ECMWF analyses) as well as in the surface layer (using radiosoundings).
