Many areas on Mars experience the passage of giant dust devils. A thin coating of fine bright dust covers most of the Martian surface. When a dust devil goes by it blows away the coating and exposes the underlying dark surface, which within a few weeks assumes its former bright colour, either from being re-covered through wind action or some form of oxidation through exposure to sunlight and air. Dust devils occur when the sun warms up the air near a flat, dry surface. The warm air then rises quickly through the cooler air and begins spinning while moving ahead. This spinning, moving cell may pick up dust and sand and leave behind a clean surface. These dust devils have been seen from the ground and high overhead from orbit. They have even blown dust off the solar panels of the two Rovers on Mars, thereby greatly extending their usefulness. The pattern of the tracks has been shown to change every few months. A study that combined data from the High Resolution Stereo Camera and the Mars Orbiter Camera found that some large dust devils on Mars have a diameter of 700 metres and last at least 26 minutes.