The Mars Orbiter Laser Altimeter (MOLA) experiment, carried on the Mars Global Surveyor spacecraft, has observed echoes from cloud tops on roughly 2.5% of the total data taken above 70 ¿N over the northern winter Martian polar hood from March to June 1998. Sloping wavefronts are commonly seen at latitudes above 70 ¿N, implying the presence of propagating buoyancy waves. Since these wavefronts frequently extend from the surface up to 10 km at a time when CO2 is known to be condensing on the polar surface, it seems likely that the laser-scattering particles consist primarily of CO2 ice, and that the near-surface temperature lapse rate is 0.85 K km-1, set by the vapor pressure of dry ice. From the observed wavelengths, where available, we have calculated the corresponding phase velocities; some of these waves appear to be correlated with surface discontinuities, and may represent lee waves. ¿ 2000 American Geophysical Union