Actual weather patterns.
http://mars.jpl.nasa.gov/gallery/global/20020418f_g.html
[in late northern summer, which is also late southern winter. At this time of year, the south polar cap (bottom, white feature in each image) is very large, extending from the south pole northward to 60°S. Also at this time of year, clouds of water ice crystals are common over the four largest volcanoes in Tharsis.]
Marcia
-just wondering... about precipitation again. You did validate my thoughts above when you replied to Tom.