Localized tropical dust storms occur on Mars during all seasons. 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. The storm is near the edge of the shrinking south polar cap.
All dust storms require high surface winds to start and to be sustained.
Also great dust storms, in particular, always occur close to southern summer on Mars which is the season when Mars is nearest to the sun and there is maximum solar heating. Several hypotheses have been advanced to explain the onset of regional and planet-wide dust storms but the sheer lack of data means that we cannot single out one theory in particular. Higher winds lift more dust causing further amplification of the storm in positive feedback. For a dust storm to extinguish itself and the dust to fall back to the surface, the winds must drop. Computer simulations show that as the atmospheric dust load increases, the vertical mixing of the dust decreases but winds near the surface continue to increase.