We have investigated the effects on the observed 20 &mgr;m brightness temperatures measured by the IRTM instrument, of the presence of cloud layers of CO2 ice particles. The results show that such clouds have a profound effect, and a layer corresponding to visible thickness of ~0.5, can create brightness temperatures of 120-130K, even when the surface is at 150K. This suggests that these clouds may also contribute to the effect of brightness temperatures below the CO2 frost point, which have been observed over the southern winter polar region.