Observations from the Mars Orbiter Laser Altimeter (MOLA) indicate a distinct difference in the nature of north and south polar clouds. The majority of north polar clouds are diffuse with relatively low optical extinction (channel 4 clouds). In contrast, a significant number of south polar clouds provide very strong laser echoes suggesting optical extinction as high as 300 km-1 (channel 1 clouds). These high extinction clouds were almost exclusively observed in the polar night between 70¿ and 85¿S. We have previously shown that the low extinction clouds are composed of a small number of large carbon dioxide particles, which form in relatively gentle updrafts, or lee waves. A possible explanation for the high extinction clouds is that they are composed of more, but somewhat smaller, carbon dioxide particles forming in high-velocity convective updrafts. Numerical simulations using a microphysical cloud model show that the cloud extinction depends most strongly on the availability of ice nuclei (IN), assumed to be dust, and the updraft velocity. Radiative cooling by the CO2 clouds can further destabilize the air column. Cloud extinction consistent with MOLA observations is produced for IN concentrations greater than 5 cm-3 and updraft core velocities of ~2 m s-1. The discrepancy in north and south cloud extinction may be the result of differing IN concentrations and stability regimes between the north and south polar regions. Many channel 4 clouds may also be convective but formed in air with either low updraft velocities or low abundances of ice nuclei.