This plot shows the estimated change in dust levels from December 2003 to early January 2004 at Gusev Crater (red curve) and Meridiani Planum (black curve), the two Mars Exploration Rover landings sites. The measurements, retrieved from Mars Global Surveyor Thermal Emission Spectrometer, indicate that a large regional dust storm beginning in mid-December raised significant dust near Meridiani. Smaller amounts of dust were spread globally by winds, the effects of which were seen at Gusev Crater. For comparison, a dust optical depth value of 1.0 would correspond to a very smoggy day in Los Angeles or Houston, and a value of 0.1 to a relatively clear day in Los Angeles.