The end of a Mars global dust storm on the day after perihelion (closest approach to the sun in Mars' orbit), at 3 pm local time! The storm is near the edge of the shrinking south polar cap. This picture was taken by Viking Orbiter 2: the storm is about 200 km across, the frame width is 710 km, and the frame center is at 64 deg S. (Viking Orbiter Image 248B57)
Source: NASA