Timeline of observed planet-encircling and regional dust storms, clouds, and obscurations. Earth dates are indicated at the top, with perihelion of Mars at the bottom. Second timeline indicates period of observations from Earth-based telescopes, shaded by percentages of L _s of photographic coverage. Third timeline shows the apparent angular size of Mars as seen from Earth, scaled from 0 to 30 arcseconds (figure from Zurek and Martin, 1993).