A theoretical spectral model is presented to describe the mean (zonally averaged) circulation and latitudinal distributions in the temperature and composition. The results show that solar differential heating by radiation at low latitudes and Joule dissipation at auroral latitudes are primarily responsible for the observed temperature and density variations. Due to energy advection by the mean meridional circulation, the (latitudinal) variations in the total pressure field are relatively small, but significant temperature and composition gradients are maintained through wind induced diffusion, consistent with the empirical model of Hedin (1983). In the dissipative medium of the upper thermosphere, pressure gradients are not very effective in driving the mean circulation; thus the contribution to superrotation is negligible. At low and mid-latitudes, the largest contribution to superrotation comes from correlations between the horizontal wind field of the diurnal tide and the diurnal variations in the ion density (drag) which give rise to a momentum source (see the review of Rishbeth (1972)). This mechanism, and to a lesser extent the correlations associated with the viscous shears and pressure gradients of the diurnal tide, produce a superrotation rate with a maximum of Δ&OHgr;/&OHgr;~0.03 at 200 km, falling off to 0.01 at exospheric heights. At lower altitudes, less than 200 km, our model is in reasonable agreement, with the satellite drag data but departs drastically at higher altitudes where King-Hele and Walker (1983) are quoting a value of 0.22 (350 km). At these altitudes, our theoretical results are consistent with the wind measurements on the Dynamics Explorer 2 by Spencer et al. (1982) and Wharton et al. (1984), who deduce a small average superrotation rate of <0.01.