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).