A repeated weather phenomenon occurs each year near the start of southern winter over Arsia Mons. Just before southern winter begins, sunlight warms the air on the slopes of the volcano. On the leeward slope, water ice condenses, forming a cloud which can extend westward for more than 1000 km. The autumn of 2018 saw a particularly pronounced version of this orographic cloud, as the planet-wide dust storm finally subsided. The presence of some dust undoubtedly emphasised the phenomenon.