Rain: It never rains on Mars because liquid water is unstable under Martian atmospheric conditions. To learn why, see "Is there liquid water on Mars?"

Snow: Viking spacecraft images of the limb of Mars (i.e. the horizon, viewed from orbit) have revealed cloud layers between 20 and 80 km above the surface, apparently composed of water ice. These clouds are similar to the thin clouds above the winter Antarctic on Earth in terms of their temperature and tenuous nature. Ice particles can slowly precipitate from almost clear Antarctic skies (known as diamond dust) and collect on the surface. It may be possible that for particular locations on Mars very fine "snow" particles could fall on Mars in a manner similar to Antarctic precipitation. As yet, we do not have enough observational data to determine conclusively whether such "snow" could actually reach the Martian surface before it sublimates (i.e. turns to vapor). But theoretical calculations seem to suggest that generally such snow would not tend to reach the surface.

An alternative form of "snow" on Mars could be solid carbon dioxide snow. During winter at the poles, the atmosphere (which is predominantly carbon dioxide (CO₂)) condenses. There are two extreme cases: (1) the CO₂ condenses only at the cold surface to form a dense ice layer (2) the CO₂ condenses in the atmosphere, perhaps on dust nucleation sites, and forms a "snow" which falls to the surface. As yet there is insufficient data to know the exact microphysics that really occurs in the Martian near-surface polar regions, but solid CO₂ snow seems very likely in the polar winter.