What you call them will depend on where in the world you come from. Willy-Willy, Dust Devil, Cockeyed Bob, Whirlwind and Sand Auger are just some of the names they go by. But how do they form? And are they the same as a tornado? Well, not really. Although they both have a rotating vortex, willy-willy’s are fair weather events. Tornadoes are formed in thunderstorms, super-cells or cold fronts. Water spouts are different again (see article here).
Willy-willy’s form when hot air near the ground rises quickly through a layer of cooler air above it. If conditions are just right, the rising air can begin to rotate.
As the air rapidly rises, the column of warmer air is stretched vertically and more warm air rushes in along the ground to the bottom of the newly forming vortex. As more warm air rushes in toward the developing vortex to replace the air that is rising, the spinning effect becomes further intensified and self-sustaining. As the warm air rises, it starts to cool, losing its buoyancy. This cooler air then descends outside the core of the vortex. This cool air returning to the ground acts as a balance against the spinning warm-air outer wall and keeps the system stable.
Image Credit: unknown
The spinning effect, along with surface friction, produces a forward momentum. Usually, willy-willy’s are very small and weak, often less than 1 metre in diameter with maximum winds speeds averaging about 70 km/h. Most dissipate less than a minute after forming. But occasionally, they can reach a diameter of up to 90 metres with winds in excess of 100 km/h. If the conditions are right they can last for 20 minutes or more before dissipating. Willy-willy’s can suddenly just ‘disappear’ leaving the dust they were carrying to float to the ground. The following image shows a large willy-willy near Port Hedland. The stack on the left is 116m high!
Image Credit: ©Troy Bourne / Perth Weather Live
Interestingly, unlike tornadoes or cyclones, willy-willy’s will rotate either clockwise or anti-clockwise. Due to their size, the earth’s rotation has no effect of the direction of the spin and each direction occurs with equal frequency.
Even more interestingly, willy-willy’s have been observed on the surface of Mars! Check it out here.
Image Credit: Courtesy NASA/JPL-Caltech
Here are some more images sent into us by PWL viewers:
Image Credit: ©Dean Thompson / Perth Weather Live. This image was captured by Dean on his phone , not far from Roy Hill, in Western Australia’s Pilbara region..
Image Credit: ©Name Supplied / Perth Weather Live. Another image from the North West of Western Australia.