Clouds and fog have been observed on Mars. While the martian atmosphere has only a trace of water vapour, low temperatures and atmospheric pressure means that the atmosphere is usually close to saturation and produces clouds.
Low temperatures means that clouds are formed by water-ice particles, otherwise known as 'ice clouds'. At higher altitudes and extremely low temperatures, carbon dioxide freezes out from the atmosphere to create 'dry-ice' clouds.
Fog usually occurs in low areas such as valleys, canyons and craters. Image of fog in Valles Marinieris.
Clouds over a sandy landscape as photographed by the Mars River, Opportunity.
Clouds and fog form near volcanoes in the Tharsis region. Note Olympus Mons at top left.
Pink stratus clouds consist of water-ice condensed on reddish dust-particles suspended in the atmosphere.
Rare, high-altitude clouds on Mars. (probably dry-ice clouds)