plume related to infrared and ultraviolet radiation, photochemistry, and latent heat, as a proper treatment of these effects is beyond the scope of the present work.) As the rocket trail continues to expand at each height, the ratio of gaseous to condensed water increases steadily (assuming a fixed ambient temperature profile). Eventually a point is reached where the particles begin to evaporate rapidly to maintain the equilibrium water vapor pressure. At a given altitude, this occurs when the plume has expanded to a size such that the total water vapor injected, if uniformly distributed over the area of the plume, would just produce ice saturation. Hence, after a short time, the contrail is confined to the coldest region of the mesosphere (assuming that the rate of horizontal expansion is roughly independent of height). The contrail also disappears abruptly when the water vapor in the coldest region becomes sufficiently diluted (allowing that the ambient water vapor concentration is at least moderately undersaturated). No persistent clouds are predicted because pure ice crystals cannot exist very long in the absence of water vapor saturation. Although meteoric dust is present in the mesosphere (7), it is probably too small in size (<0.005 p.m) to act as a water vapor condensation nucleus unless the supersaturation exceeds approximately 10 (5). Figure 5 summarizes a plume calculation corresponding to an HLLV launch at 45 °N latitude in July. The early-time optical depth along the axis of the contrail [which has a vertical extent of roughly 15 km (9 mi)] exceeds 100. Because the optical depth is dependent on particle size — which cannot be precisely calculated with our model — and viewing geometry, the results in Fig. 5 should be interpreted only as a rough
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