meteors. A full treatment of the microphysical evolution of the dust is given. The dust particles act as water vapor condensation nuclei, and may also coagulate with ice crystals. In regions where ice crystals evaporate, the dust is released and may be recycled into the cloud. It turns out that, for the normal supersaturations expected in the coldest regions of the summer mesosphere, only dust particles with radii exceeding — 15 A will nucleate (because of the Kelvin surface energy barrier). Figure 2 shows the altitude zones in the mesosphere where bulk liquid water and ice can exist for different temperature profiles and water vapor abundances (8,9). It appears that, with typical water vapor concentrations —3 parts per million by volume (ppmv), an ice cloud can exist only at a very cold mesopause. If the water vapor concentration is increased artificially, water or ice clouds can exist over a broader range of altitudes and at higher ambient temperatures (which implies, at lower latitudes). In Fig. 3, model predictions of the particle size distributions in a typical noctilu- cent cloud are compared with observations (10). The cloud particles are generally smaller than —0.1 jum and are likely to have a cubic or hexagonal shape. [According to Turco et al. (5), the crystals are expected to be equidimensional regardless of shape.) When both meteoric dust and air ions are present, dust appears to be the favored nucleus (5). In the absence of dust, water vapor supersaturations tend to increase and ion nucleation may commence. However, considering the substantial uncertainties in theoretical predictions of ion nucleation rates, it is difficult to establish the primacy of dust or ions as the condensation sites in noctilucent cloud genesis (5).
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