File:Cirrus clouds on mars.jpg

English: Clouds add drama to the sky above "Endurance Crater" in this mosaic of frames taken by the navigation camera on NASA's Mars Exploration Rover Opportunity at about 9:30 a.m. on the rover's 290th sol (Nov. 16, 2004). The view spans an arc from east on the left to the southwest on the right.

These clouds are part of a band that forms near the equator when Mars is near the part of its orbit that is farthest from the Sun. For Opportunity (and Spirit and the rest of the southern hemisphere), this occurs in late fall and early winter. During this period, atmospheric temperatures and the amount of water vapor combine to form large-scale clouds. These clouds look like Earth's cirrus clouds and share other similarities with cirrus clouds in that they are believed to be composed entirely of water-ice particles with sizes on the order of several micrometers (a few ten-thousandths of an inch).

The images that are combined to produce this view have been processed to remove geometrical distortion associated with the camera's 45-degree field of view. In addition, special image processing has been applied to enhance the clouds and make them visible across the entire mosaic. The rim of Endurance was processed using the same technique, illustrating how much enhancement was done. Glare from the Sun washed out the clouds on the left in the original images; this glare was removed.