A time-lapse image of the dust storm's progress from the Opportunity rover, from June 14, 2007 through July 19, 2007. The images are approximately true color composites, generated using data from the rover's panoramic camera's various filters. The numbers across the top of the image report a measurement of atmospheric opacity, called "tau." The lower the number, the clearer the sky. Credit: NASA/JPL-Caltech/Cornell