This is an image of a Martian storm front.
Image from: NASA
This is an image of a storm moving across the Martian terrain. The camera is looking down upon the storm and the storm front forms a spiral pattern, the same way terrestrial storms are presented on the evening news. The storm provides evidence of water, and a water cycle on Mars.
As early as 1796 scientists were reporting "yellow", and "white" or "bluish" clouds in the Martian atmosphere. However, it wasn't until the Mariner 9 mission that clouds of water were positively identified.
Clouds seem to be plentiful only in the middle latitudes, and many of the cloud formations seem to be due to topographic forcing by Olympus Mons. More study is needed to understand just how the clouds come and go in the Martian atmosphere. For example, even though clouds have been found, there is still little evidence when and where it actually rains on Mars, if at all.
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