We infer from the observed occurrence frequency of polar mesosphere summer echoes and from the three-dimensional (3-D) modeling of conditions in the high-latitude mesopause region that a persistent layer of icy particles exists in midsummer at all latitudes poleward of about 60¿N at and a few kilometers below the mesopause. All of these icy particles are transported equatorward by the climatological mean winds. At the same time, many of the larger icy particles possess a high enough sedimentation velocity to induce a net downward transport of water vapor. Both types of particle motions cause the mesopause region to become substantially dryer than without these transports of icy particles. We follow the interactions between water vapor and icy particles by means of a 3-D dynamical and chemical model, which includes a module for the formation, growth, and sublimation of icy particles. For midsummer conditions and poleward of 67¿N latitude, the model predicts (1) a strongly dehydrated region, typically above 84 km, in which the water vapor mixing ratio can fall below 0.2 ppmv and (2) atmospheric regions with enhanced water vapor abundance near both the lower and the equatorward borders of the icy particle layer.