The air descending from the thunderstorm will often hit the ground and be forced out ahead of the cell creating a gust front. The common desert term for such a phenomenon is haboob . Haboobs are most frequent in SW North America during the month os May through September, with most frequent occurrence in June, but they can occur in every month. Their average duration is less than three hours. The average maximum wind velocity is over 30 mph and dust may raise to heights exceeding 3000 feet.
Downdrafts can be dry or wet.
Dry downbursts will not necessarily show a solid perturbation from the base of the cloud to the characteristic curl. Instead, a dry downburst is generally only visible when the vertically descending winds hit the ground and pick up substantial quantities of dust. These types of downbursts are common in Arizona and will be particularly evident during the early portion of the monsoon season when there is still little precipitation associated with thunderstorms. Dry Monsoon Thunderstorm Schematic Wet downbursts, on the other hand, have the characteristic precipitation curl tracing out the vortex-ring circulation that surrounds the concentrated downdraft within the rain shaft. In essence, a gustnado is a tornado-like vortex that appears to develop on the ground and extend several hundred feet upward.