Localized tropical dust storms occur on Mars during all seasons. A typical storm is about 1 million km2 size and comprises microscopic particles which move with speeds of 15-30 m/s (33--66 mph) before dissipating after a few days. Dust devils, about 2 km width and a few kilometers high, have also been observed in the tropics by the Viking orbiters. However, the most dramatic aspect of the Martian climate is when a dust storm expands to encompass nearly one or both hemispheres. Indeed, sometimes these great dust storms can become completely global.
