fueled (H2-O2) rocket launches from the Kennedy Space Center in Florida. The major exhaust constituent of these rockets is H2O, with few particulate contaminants present. Admittedly, launches from Kennedy might produce clouds far to the east of Florida where they would be difficult to observe at evening twilight. Yet, we have not uncovered a single report of unusual night-luminous clouds in the aftermath of a Saturn V launch. We must conclude, therefore, that little observational evidence exists linking large midlatitude mesospheric injections of (pure) water vapor with extensive, persistent, optically thick clouds. 4. MESOSPHERIC CLOUD PERTURBATIONS Whitten et al. (2,17) have forecast the accumulation of water vapor in the mesosphere as a result of SPS HLLV traffic at northern midlatitudes (—30 °N). Their predictions indicate that, in the region of highest probability of cloud formation (i.e., 80-90 km; 48-54 mi), global water vapor enhancements could be in the range of 109^20%. The NLC calculation shown in Fig. 3 for a doubling of H2O gives roughly a tripling of the cloud optical depth. Thus, at latitudes where NLCs commonly occur, one might expect the clouds to be ~30%-60% more opaque. Certainly, the absolute increase in cloud optical depths at visible wavelengths would be less than 1 x 10 4 (NLCs typically have optical depths s 10“4). The corresponding effect on the average surface temperature of the Earth is therefore estimated to be less than 0.01 K (18,19), which is climatically insignificant. The extension of noctilucent clouds to low latitudes, likewise, should not be greatly affected by SPS rockets. Normally, the clouds occur sporadically at latitudes as low as 45°. Theon et al. (8) noted that NLCs do not automatically occur whenever the mesosphere is extremely cold. They proposed that another factor, probably either an enhanced water vapor concentration or a suitable condensation nucleus, is needed to form NLCs. Hence, the accumulation of water vapor induced by SPS rockets might slightly increase the frequency of appearance of NLCs below 70° latitude, without a measurable climatic effect. Above 70° latitude, Donahue et al. (20) detected (from the OGO-6 satellite) a persistent circumpolar cloud layer near 80 km (48 mi) in summer. Inasmuch as models of atmospheric circulation indicate substantial upward convection of air at the summer poles (e.g., 21), the likely source of water for the circumpolar clouds is the polar upper stratosphere. Whitten et al. (17) found that SPS rockets will not noticeably disturb the stratospheric water vapor reservoir at high latitudes. Thus, rocket H2O emissions should have an even smaller effect on the circumpolar clouds than on midlatitude NLCs. To study the formation and persistence of rocket condensation trails, account must be taken of the deposition and expansion of the rocket exhaust gases. Whitten et al. (2) discuss an empirical model which describes the horizontal dispersion of a rocket trail. They show how a simple expansion scheme may be utilized in a onedimensional model to make rough simulations of a growing HLLV launch plume. In effect, the complex aerodynamical processes governing the early development of the rocket trail are ignored, and emphasis is placed on the long-term interaction of the trail with the ambient atmosphere. After a short relaxation time (—100 sec), it is assumed that the trail has reached temperature and pressure equilibrium with the ambient atmosphere and has a cross-sectional area of — 1 km2 (0.4 mi2). At this time, the exhaust water vapor is presumed to be uniformly distributed across the plume,
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