Mars Express HRSC image of the volcano Arsia Mons with a ~1500 km orographic cloud that is forming downwind of the mountain.

Although clouds are common in the region, this cloud is out of season. Clouds over Arsia Mons typically occur when Mars is furthest from the Sun (local summer), because the atmosphere is colder (closer to water's condensation point) and the northern ice cap supplies ample amounts of water vapor to the atmosphere. However, this long cloud streamer formed as Mars was approaching the Sun.

Similar long, out-of-season clouds have been spotted over Arsia Mons around the same time of Mars' year, most recently in 2015. This cloud repeats every Martian year due to a delicate balance of conditions. First, the south pole is approaching its summer equinox, and as it shrinks back the atmosphere becomes denser and more moisture-laden. Second, the atmosphere in the region of Arsia Mons is still in the process of warming up as Mars approaches the Sun. Third, the warming skies in the southern hemisphere start lofting dust, which provides small particles for water to condense onto. (The southern spring/summer is notoriously dusty -- almost every global dust storm ever observed has started during this time of year.) The last condition is wind. Shortly after the Sun rises, winds start blowing west across the region. This wind pushes air up the eastern slope of Arsia Mons. The 20 km difference in elevation from base to summit causes the water to condense out as the air ascends and cools.

These conditions are only in balance for 30-60 days of the Martian year. Too early, and the air is too dry because the southern ice cap has not released enough water. Too late, and Mars' approach to the Sun makes it too warm for water to condense. Without dust, which lowers the atmospheric temperature and provides a focus for condensation, these conditions might not be met for long, if at all.

This image was taken during Mars Express' 18,627th orbit of Mars, September 21, 2018. It combines images taken through the HRSC camera's IR, green and blue filters to produce a natural color image.