Recent satellite observations have shown the existence of a persistent layer of light-scattering particles in the vicinity of the polar mesopause during the summer. The suggestion has been made that this layer consists of ice particles, and that noctilucent clouds are a sporadic manifestation of the layer near its low-latitude edge. The consequences of this proposal in terms of the water vapor content of the mesosphere are explored in this paper through the development of a model for such a cloud, in which the water vapor mixing ratio is assumed to be 1 to 10 ppm at 60 km, and the temperature is assumed to drop to values well below 140 K at the mesopause. Water vapor is transported upward by eddy diffusion, and is photodissociated by solar ultraviolet radiation. Sublimation nuclei of radius 10Å are assumed to exist in the vicinity of the mesopause, and the growth of the ice particles and their terminal speed of descent are calculated from simple kinetic theory considerations. The results indicate that radii of order 1000Å cannot be attained by spherical cloud particles unless there is much more water vapor in the mesosphere than current estimates suggest, or unless there is a substantial and persistent upward motion of the mesosphere as a whole. Non-spherical shapes are also considered, however, and it is found that needle-shaped and disc-shaped particles can readily grow to linear dimensions of order 1000Å.