Martian Clouds

Although not as pronounced as on Earth clouds are common features on Mars. The Martian atmosphere has only a trace of water vapor; however, the temperature and pressure is such that the atmosphere is usually close to saturation and produces clouds. Even from Earth based telescopes, clouds have been observed by transient brightening on the surface of Mars. Numerous cloud patterns have been seen from the Marineer and Viking spacecraft and have been classified into various categories.

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http://www.solarviews.com/eng/marscld.htm