Arsia Mons and its "clouds" (2) - 24 April 2003, Ls 173° (Extremely Saturated Natural Colors; credits NASA/JPL/MSSS)

Original caption:"This image was taken on April 24, 2003, such as in late Southern Autumn on Mars. These pictures prove that some parts of Mars experience weather phenomena that repeat each year, at about the same time. In some Regions, the repeated event may be a dust storm that appears every year, like clockwork, in such a way that we can only wish the weather were so predictable on Earth. One of the repeated weather phenomena occurs each year near the start of Southern Winter over Arsia Mons, which is located near 9° South Latitude, 121° West Longitude. Just before Southern Winter begins, sunlight warms the air on the slopes of the volcano. This air rises, bringing small amounts of dust with it. Eventually, the rising air converges over the volcano's caldera, the large, circular depression at its summit. The fine sediment blown up from the volcano's slopes coalesces into a spiraling cloud of dust that is thick enough to actually observe from orbit".