A dust devil is a strong, well-formed, and relatively short-lived whirlwind. Its size ranges from small to large—0.5 m (18 in) and a few yards or metres tall to more than 10 m (30 ft) wide and more than 1 km tall. The primary vertical motion is upward. Dust devils are usually harmless, but can on rare occasions grow strong enough to endanger both people and property.