Previous observations of atmospheric oscillations with zonal wave number 1 have consistently located waves of periods between 5 and 7 days. This study presents a global and seasonal analysis of the 6.5-day wave in High Resolution Doppler Imager daytime mesosphere and lower thermosphere horizontal winds, temperatures, and nighttime atomic oxygen at 95 km. The horizontal structures of all these atmospheric variables are similar to the gravest symmetric wave number 1 Rossby wave, i.e., the (1,1) mode. The seasonal and spatial analysis displays possible modification by the zonal mean wind. Finally, the observed vertical structures of the 6.5-day wave indicate that it is an internal Rossby wave, not an external or Lamb wave. ¿ 2001 American Geophysical Union