Latest set of NavCam pics from Opportunity show quite a few clouds in the sky:
http://qt.exploratorium.edu/mars/opportuni...cam/2004-07-14/
visble in these 3 images:
http://qt.exploratorium.edu/mars/opportuni...21P1555L0M1.JPG
http://qt.exploratorium.edu/mars/opportuni...21P1555L0M1.JPG
http://qt.exploratorium.edu/mars/opportuni...21P1555L0M1.JPG