Optical Depth The most direct and probably reliable estimates of opacity are those derived from Viking lander imaging of the Sun. Figures 1 and 2 show the seasonal variation of the optical depth at the Viking lander locations VL1 and VL2, respectively. The season is indicated by the value of aerocentric longitude of the Sun, measured in the orbital plane of the planet from its vernal equinox (Ls = 0° and 180° corresponding to spring and fall equinox for the northern hemisphere, respectively; and Ls = 90° and 270° corresponding to northern and southern summer solstices, respectively). Figures 1 and 2 were derived from references by Pollack [1,2] and Zurek [3] and were discretized for each 5°. As mentioned before, the optical depth is assumed to remain constant throughout the day. Opacities are minimum during the northern spring (Z.s = 0°-90°) and summer (Z.s = 90°-180°), and maximum during southern spring (Ls= 180°-270°) and summer (Z.s = 270°-360°), the seasons during which most local and major dust storms occur. When dust storms are not present, the optical depth are typically about 0.5. Two global dust storms occurred during the period of the observation as indicated by the high values of the optical depth (lower bound values). Global and Local Dust Storms The intensity of Martian global and local dust storms is defined in terms of opacity of the dust it raises. Global dust storms are those which obscure planetary-scale sections of the Martian surface for many Martian days (sols), whereas local dust storms are less intense, and form and dissipate in a few days or less. From the photovoltaic system design point of view, the intensity, frequency, and duration of these storms may be viewed as ‘partially cloudy' and ‘cloudy' days for which additional energy storage in the photovoltaic system must be taken into account. The characteristics of global and local dust storms are listed below. Global Dust Storms (1) One, or occasionally two global dust storms of planetary scale may occur each Martian year. The duration may vary from 35 to 70 days or more. Although global dust storms do not occur every year, their occurrence is fairly frequent.
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