Outback Phenomena

Willy Willies, also commonly known as dust devils are an amazing natural phenomena that predominantly occur inland and has a similar appearance to a tornado. Like tornados, they have a twisting movement of air which commonly carries dust and other particles in its wake.

Unlike tornados, Willy Willies can be formed on a perfectly blue sky with no clouds in site anywhere. Willy Willies sometimes occur on extremely hot and still days when the sun superheats the ground. This super-hot ground heats layers of air above that, which in turn heats other layers above that. Since hot air expands and is therefore less dense, it will rise through cooler and denser air quite rapidly. Any sideways movement during this initial rise in hot air, may create a vortex and result in the twisting column we know as a Willy Willy.

Willy Willies can be set in motion with a gust of wind at the right time and this rotation can be in either direction.

Damage to Structures

Generally, Willy willies are relatively small (a few metres in height), short-lived and pose no danger, however, there have been instances where they have caused damage to weak structures. On rare occasions, there has been evidence of huge Willy Willies peeling off tin roofs and dragging up insulation batts as high as the eye can see. Tin and wood fragments have been observed at over 40 metres high in rural Victoria and grass has been observed at over 2km high in Western Queensland. Although the majority of Willy Willies are small and harmless, you can never underestimate the forces of nature.

Dangers to Aircraft and Vehicles

Willy Willies can pose a hazard to planes, especially when the aircraft is near the ground because the moderate winds and dust could potentially cause damage. It can also be daunting to pilots trying to take-off or land because the thick dust column would impair visibility. A resident of Mt Isa used to witness Willie Willies forming at a bend in the road near the airport. When trucks would roar around this bend, two Willy Willies would form in either direction and head towards the airport. People towing caravans, boats or trailers should also be careful when a large Willy Willy is approaching and pull over to the side of the road until it passes.

Remember those old cartoons, where a character would be walking through the desert, run up and dive into a lake and then suddenly realise it was a mirage all along? You may think that mirages, such as the ones portrayed in the cartoons, are an optical illusion – this however, is not all true. Unlike optical illusions – which are visually perceived images, mirages can actually be physically photographed. It's the mind that creates the illusion and tries to reinforce the idea that there may be water in the distance. Since we have accepted the existence of mirages, we have learnt to distinguish the difference.

A mirage is a natural-occurring refraction phenomenon whereby light rays are bent to produce a displaced image usually in the distance. Mirages commonly appear on roads when the weather conditions are sunny, very hot and dry.

High temperatures during a very hot sunny day will heat up the ground itself, which in turn will heat up the layers of air above the ground. If the layer of air sitting a fraction above the ground is hotter than any layers of air above that, then the light rays will bend in an upward concave path. The traveller looking through the bent or refracted light rays will now see a mirrored image of the sky above, appear like a water body on the ground.

If you place a pencil in a glass of water, you will notice that the pencil appears to be bent at the air and water boundary. This is how a mirage works because the density difference in the air layers (which is caused by the temperature differences) bends the light in a similar way.

A general rule of thumb when it comes to the temperature differences between the ground air and the air above in regards to mirages is outlined below:

Inferior Mirages

When an image is seen under the real object such as an image resembling water on a highway, then this is considered an inferior mirage. The mirage image is actually the mirror reflection of real objects above it such as: the sky, clouds, shrubs and maybe other vehicles in the distance. Inferior mirages commonly appear on bitumen on hot sunny days because the road absorbs a lot of heat and warms the layers of air above it. As the layers of air of varying temperatures mix (hot air rising through the cold air), it creates a turbulent and vibrating look, which can reinforce the notion of water shimmering in the distance.

Superior Mirages

In the case where the air near the ground is cooler than that higher up, the light rays will curve and bend downwards instead, producing a superior image. This mirage image appears above the true object such as a setting sun and may produce a large distorted look. Since the air layer on the surface has to be much cooler than the air layers above, these mirages are more likely to appear in the colder climates. Superior mirages are not as common as inferior mirages especially in a predominantly hot country like Australia. Examples of superior mirages include: a larger distorted sunset, distant shorelines which appear to be towering and trees which are normally invisible below the horizon are now visible.

A majority of us have seen Willy Willies in the outback and mirages on the road as it’s not that uncommon. The next time you encounter one of these amazing natural phenomena, take a quick thought and appreciation of nature’s wonders and the science behind it.