Hubble images of the Sinus Meridiani region taken on October 28, 2005 show evidence of a regional dust storm. A comparable Hubble image taken on June 26, 2001 of the same region shows a storm-free environment. The dust storm, which is about 930 miles (1500 km) long, is about the size of the states of Texas, Oklahoma, and New Mexico combined. Ground-based amateur telescopes have been watching the storm grow and evolve.
This storm has been churning in the planet's equatorial regions for several weeks now, and it is likely responsible for the reddish, dusty haze and other dust clouds seen across this hemisphere of the planet in views from Hubble, ground-based telescopes, and the NASA and ESA spacecraft studying Mars from orbit. The occurrence of the dust storm in close proximity to the NASA Mars Exploration Rover Opportunity's landing site in Meridiani Planum has the potential to cause problems for the rover. Specifically, if the dust in the atmosphere gets thick enough it could block some of the sunlight needed to keep the rover operating at full capacity.
The 2005 image was taken with Hubble's Advanced Camera for Surveys. The 2001 image was taken with Hubble's Wide Field Planetary Camera 2. Both images were taken within weeks of closest approach/opposition of the planet. The smallest resolvable features in the images (small craters and wind streaks) are about 12 miles (20 km) across.