A haboob is a type of intense dust storm carried on an atmospheric gravity current. Haboobs occur regularly in arid regions throughout the world. This wall of dust can be up to 100 km (62 mi) wide and several kilometers in elevation. At their strongest, haboob winds often travel at 35–100 km/h (~20–60 mph), and they may approach with little or no warning. Often rain does not appear at ground level as it evaporates in the hot, dry air (a phenomenon known as virga).