Spectral results from a vertical sounding of temperature and wind velocity performed with an open stratospheric balloon in Argentina near the Andes mountains between 12 and 25 km of altitude, are reported. The use of sonic anemometers allows for a higher resolution than in previous experiments. The data records are studied in successive subintervals, yielding a good spectral correlation between the ascent and the descent around and below the tropopause. The possibilities of an orographic origin for large amplitude modes observed in the spectra and of wave generation by non linear interactions between them are discussed. ¿ American Geophysical Union 1996