It can be easy to forget, but Mars has clouds very similar to the the ones you find here on Earth. Don't believe me? Here, check out the first photograph ever taken of an overcast Martian sky; or this picture of some stratus clouds, captured about 40 minutes before a Martian sunrise. The thin sheets of cotton-white goodness spread across the image featured up top? Yep, those are clouds.
The first two images were photographed years ago by NASA's Mars Pathfinder. The image up top, however, was captured much more recently by NASA's HiRISE — the most powerful camera we've ever sent to another planet.