Current theory suggests that the only variation in H20 mixing ratio to be expected above 20 km is that due to oxidation of CH^, methane. Since CH^ probably enters the stratosphere through the tropical tropopause at a mixing ratio of approximately 1.6 ppmv, it could at most add 3.2 ppmv to the HgO mixing ratio passing through the tropical tropopause "cold trap." At -80 C and 100 mb this is 5.5 ppmv and would lead to ~ 9 ppmv as a maximum possible F^O mixing ratio in the upper stratosphere. However, as indicated above, current data indicates a decrease in mixing ratio from the tropical tropopause to a value near 4.5 ppmv, near 19-20 km. This decrease suggests a stratospheric sink for ^0, but in the absence of an identified sink it must be regarded as currently unexplained. Since the methane concentration also decreases between the tropopause and 20 km, it can no longer add as much as 3.2 ppmv of H20 by oxidation at higher levels. Thus one can arrive at a value of ~ 8 ppmv as a maximum possible mixing ratio for water at any level above 20 km. Similarly, from the minimum temperatures observed over the winter poles at 25-30 km and over the summer poles at the mesopause, one can arrive at a minimum possible water mixing ratio for the upper atmosphere. The numbers obtained from this exercise are near 3 ppmv. Any observations of upper atmospheric water vapor concentrations outside the range of 3-8 ppmv must be regarded as due to observational error or to unknown H20 sources, sinks, or redistribution mechanisms. During the last year (1978), limited satellite measurements of water vapor have been made with an earth limb scanner on Nimbus VI (Gille and Russell) and with a pressure-modulated radiometer (PMR) on Nimbus VII (Houghton) . Both of these measurements (neither of which has yet been reported fully) have been made in the infrared at 6.3 micron wavelength. For the 1980s, there are plans to measure atmospheric water vapor using a variety of instruments, and thus it is hoped that ten years from now the situation will be much better than it is at present, when not even an adequate global mean value exists for mesopheric water vapor, to say nothing of variations with time and latitude. 2.4 HIGH-ALTITUDE CLOUDS 2.4.1 Noctilucent Clouds (Ellsaesser, Turco) These are thin clouds observed occasionally at the high latitude summer (cold) mesopause, where the temperature drops so low (below 150 K) that condensation can occur even though water vapor mixing ratios are no greater than several ppmv. These mesopheric clouds have been the subject of scientific investigation for nearly a century. Noctilucent clouds (NLCs) have been observed from the ground (Fogle and Haurwitz, 1966), from satellites (Donahue et al., 1972), and have been sampled in situ (Hemenway et al., 1964). Current theories favor a crystalline ice composition (Reid, 1975) covering a meteoritic dust nucleus, although hydrated metallic ions have also been suggested as the nucleating agent (Goldberg and Witt, 1977). D'Angelo and Ungstrup (1976) recently reported an anticorrelation between NLCs and ionospheric electric fields (which lead to ohmic heating of the mesosphere), suggesting that NLCs are composed of a condensible substance such as H20. This theory tends to confirm the idea that the conditions necessary for NLC
RkJQdWJsaXNoZXIy MTU5NjU0Mg==