Although Martian atmosphere is thin relative to the Earth, it is dense enough to share many of the same dynamic features observed within Earth's atmosphere, such as clouds and cyclones. Note the bright white cloud formation in the middle of the image to the right (Figure 1.2). On Earth, clouds represent the condensation of water vapor in air. On Mars, however, clouds can be the condensation of either water vapor or, more often, carbon dioxide vapor. Above the polar regions, clouds are assumed to be mostly carbon dioxide clouds. In latitudes closer to the equator, clouds are possibly water vapor clouds because of the higher temperature. In this image(Figure 1.2), also note the large vortex at the upper left corner. This is a Martian cyclone, equivalent to a hurricane on Earth. This cyclone is about 250 km across.

The picture on the left is another cyclone in the Martian atmosphere (Figure 1.3). This cyclone is about 600 km in diameter. Judging from the shadow cast on the Martian surface, scientists are able to determine that these cyclone clouds are about 6 to 7 kilometers above the surface. Notice the counterclockwise rotation of the cyclones. Like the Earth, cloud motions are driven by Coriolis Forces that arise within a rotating system. (Coriolis force is an apparent force a moving body feels in a spinning system. It causes the body to move to the right in the Northern hemisphere on both the Earth and Mars, and to the left in the Southern hemisphere.) Because Mars spins just as fast as the Earth, Coriolis Forces on Mars are expected to be equally efficient in driving atmospheric dynamics.