Viking infrared thermal mapper observations of Mars in the 15-μm CO2 band reveal global atmospheric thermal behavior at the 0.3- to 0.6-mbar level. Dust entrained by storms produces major modification of diurnal and latitudinal structure in the brightness temperature T15. In the dust-laden atmosphere of southern spring and summer 1977, T15 was a maximum in late afternoon at a latitude well south of the subsolar latitude. Diurnal amplitude was as great as 30 K, while diurnal mean temperatures exceeded 220 K. Over the northern winter polar cap, T15 increased dramatically following the second global dust storm of 1977; even in regions of polar night the change was up to 80 K. Inversions of similar magnitude resulted, and the change in downward radiance was sufficient to modify substantially the rate of CO2 condensation at the surface.