The martian season during Cycle 6 corresponds to the occurrence of the largest amounts of water vapor in the martian atmosphere in the northern arctic region. This season is less well documented historically than others because it is observable only near aphelic oppositions. Cycle 6 data will be crucial in establishing the interannual variability of this phenomenon, which may be an important component of the water cycle. One of the major goals of the Mars HST project has been to observe the development and distribution of dust storms on Mars. But the greater density and resolution of this summer solstice season possible during Cycle 6 will enhance this probability.
Discrete clouds are important indicators of meteorological processes on Mars.
Martian topographic relief exceeds an atmospheric scale height and will produce fixed eddies in certain locations and seasons which should be revealed by persistent cloud activity. Some of the most interesting and prominent clouds, historically called W clouds, occur in the Tharsis and Valles Marineris regions. We have observed interesting changes in the relative size and optical depth of the components both seasonally and interannually. At these short wavelengths, essentially all of the reflected intensity results from atmospheric scattering because of the negligible surface albedo. In the absence of aerosols and ozone, the intensity results from Rayleigh scattering, which can be exactly calculated at both wavelengths.