Dust storm

This website has some great pictures and video of springtime dust storms on Mars.

Two Hubble telescope images of Mars, taken about a month apart, reveal a state-sized dust storm churning near the edge of the northern polar cap. The polar storm is probably a consequence of large temperature differences between the polar ice and the dark regions to the south, which are heated by the springtime sun.

http://hubblesite.org/newscenter/archive/releases/1996/34