Convection in Earth's mantle is dominated by long-wavelength structure, as evidenced by the very ''red'' spectra of both seismic velocity heterogeneity in the deep mantle and the non-hydrostatic gravity field, or geoid. Here we show that this large-scale structure may be a consequence of two factors that influence the scale of mantle convection. First, the existence of surface plates, which tend to organize the flow. Second, a substantial increase in lower mantle viscosity for which there is considerable independent geophysical evidence. Combining these two factors in 3-D spherical mantle convection models explains rather well the observed seismic spectrum of mantle heterogeneity. ¿ American Geophysical Union 1996