Also known as dust devils, willy-willies are dust filled vortices that can spiral upwards from the ground, varying in height from a few feet to over 1000 feet.

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 area where the ground is dry and high surface temperatures produce strong updraughts.

The initial rotation may be caused by irregularities in the surface.