The substantially semiannual character of the density variation and its rather weak latitudinal dependence are puzzling features of the seasonal variation of density in the thermosphere. These features are puzzling because the annual harmonic of the seasonal variation of thermospheric heating is considerably greater than the semiannual and because the heating for both harmonics is predominately latitude dependent. A model of the globally uniform component of the semiannual oscillation in thermospheric density is developed. This component is a conduction mode of oscillation modified by thermal damping. It is shown that this oscillation is essentially driven by Joule heating at high latitude. Model calculations give results which agree well with observations. It is suggested that the observed prominance of the semiannual oscillation is a consequence, primarily, of (1) the stronger forcing for the semiannual conduction mode over the annual and (2) the suppression of the latitudinally dependent components by seasonal variations in composition.