(2) Global dust storms begin near perihelion, when solar insolation is maximum (southern spring and summer) in the southern mid-latitude. (3) The first global dust storm (1977) spread from a latitude of 40° S to a latitude 48° N in about 5-6 days. (4) The opacity during the global dust storm is greater than 1. Local Dust Storms (1) Local dust storms occur at almost all latitudes and throughout the year. However, they have been observed to occur most frequently in the approximate latitude belt 10°-20° N and 20°-40° S, with more dust clouds seen in the south than in the north, the majority of which occurred during southern spring. (2) Based on Viking orbiter observations, it is estimated that approximately 100 local storms occur in a given Martian year. (3) Local dust storms last a few days. (4) The opacity of local dust storms may be assumed about 1.
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