This phenomenon, most commonly known today as dust devils, is an upward spiralling, dust filled vortex of air that may vary in height from a few feet to over 1,000. They are usually several metres in diameter at the base, then narrowing for a short distance before expanding again. They mainly occur in desert and semi-arid areas, where the ground is dry and high surface temperatures produce strong updrafts.

The initial rotation may be caused by irregularities in the surface. Unlike tornadoes, willy-willies grow upwards from the ground, rather than down from clouds. In the stronger willy-willies, a cumulous cloud can be seen at the top of the rising column of warm air. Willy-willies only last a few minutes because cool air is sucked into the base of the rising vortex, cooling the ground and cutting off its heat supply.

Although they may resemble 'mini-tornadoes', willy-willies are nowhere near as powerful or destructive. They travel across the ground and, besides dust, they may also carry other loose debris such as hay; as seen in the clip below.

On September 13th 2000 a willy-willy formed at the Coconino County Fairgrounds and caused property damage and injuries. Winds were estimated as high as 75 mph. In another rare incident, three children in a bouncy castle were carried over 10 feet in east El Paso, Texas.