A study of cloud observations since the 1960s suggests that the appearance of water ice crystal clouds and surface fogs is more frequent in the northern spring and summer than in the southern spring and summer. A decade later, it was proposed that the clouds were orographic in origin, and this was confirmed in 1971 when Mariner 9 images showed the clouds forming as wind passed over the volcanoes Olympus Mons, Ascraeus Mons, and Pavonis Mons. Another cloud formation of note is the Syrtis Blue Cloud. The cloud usually appears during the late spring and early summer of Mars' northern hemisphere and was noticed at the last two apparitions in 1995 and 1997. Mars exhibits morning and evening clouds.
Morning clouds are bright patches of surface fog or frosty ground.
The fog dissipates by mid-morning, but the frost can persist for most of the day. Evening clouds are usually larger and more numerous. These are double-edge swords to watch. On the one hand, they can be exciting to observe as they form and grow, but if they get too big, they can hide surface features for many months. Some dust storms have been known to cover the whole planet.