A dust devil is a strong, well-formed, and relatively long-lived whirlwind, ranging from small (half a meter wide and a few meters tall) to large (more than 10 meters wide and more than 1000 meters tall). The primary vertical motion is upward. Dust devils are usually harmless, but can on rare occasions grow large enough to pose a threat to both people and property. They're not true tornadoes - dust devils form as a swirling updraft under sunny conditions during fair weather, rarely coming close to the intensity of a tornado.