Polar mesospheric clouds (PMC) and noctilucent clouds (NLC) are terms assigned to layers of scattering particles (believed to be water ice) at the cold summertime high-latitude mesopause. The two types of clouds, believed to be composed of water ice, are clearly related but have been observed under very different circumstances. PMC have been observed by satellites in full daylight, in the afternoon and late morning. NLC are observed from the ground within 2 or 3 hours of local midnight when the sun is below the horizon. We suggests that the differences in their optical properties and in their observed heights can be attributed to a diurnal variation in the properties of a single type of cloud. Using two published models of the diurnal and semidiurnal variations of temperature, vertical wind speed, and eddy diffusion coefficient at high latitude, we simulate the evolution of ice clouds over a 24-hour period. The microphysical processes of particle genesis, growth, sedimentation, and sublimation are included in our model. We find that the minimum in temperature at about 2000 hours LT causes a sharp maximum in scattered brightness to occur about 1 hour before local midnight, with up to a factor of 7 variation in cloud brightness between noon and midnight. Variations in vertical wind speed ad eddy diffusion enhance the variation in cloud brightness, but only slightly. We also find that when the model is adjusted to agree with the observed height for PMC (85 km), it predicts a lower height of abut 83 km for ice clouds near midnight, in agreement with triangulation determinations of NLC heights. These predicted NLC brightness variations may be verified by future ground observations of NLC. However, it will be necessary to make careful corrections for changing illumination and solar extinction conditions throughout the night. ¿ American Geophysical Union 1989