Living where we do we get lots of willy-willies. They range from a few leaves swirling around on the ground to columns of dust and leaves that you can definitely feel. When I was in Halls Creek we used to call them ’30 second cyclones’ because they could be strong enough to knock over things in your yard.
Willy-willy is the Australian name for minor whirlwinds and dust-devils. Although they look like smaller versions of tornadoes they form differently. Tornadoes are linked to super-cell thunderstorms or other powerful storms. It is the storm that begins spinning to form the tornado.
Willy-willies are formed by local winds. There may be a windstorm or windblast that helps them or where we are it is just the heat. A rising column of air is caused by a local hotspot, which may be something like a road. Any unevenness or an existing wind can start the column spinning. This is what is happening with the most common small swirls of leaves.
Larger ones can last longer because they become self sustaining. As the air column rises it stretches and gets thinner. This makes it spin faster because of the conservation of angular momentum. This is the trick when an iceskater or dancer pulls their arms and legs in and speeds up, you can also try this on spinning playground equipment. Leaning out will slow you down, leaning in will speed you up.
As it spins faster it can start to pull more air in because of the spin, just like in an emptying bathtub. Plus the rising air leaves low pressure underneath it which is filled by incoming air.
They can become quite large, however they rarely last more than a minute or so. We have fun spotting them out the car window, or if we are outside we try to run and jump in the middle of them. Which instantly makes them collapse! It’s lots of fun though.
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