Alternatively, you can buy a 'tornado tube' or 'vortex valve' from a science or toy store that will join the bottles together and create a tornado easily, but here at Science by Email we call that cheating!
What to do
Fill one of the bottles around two-thirds full of water. You can add food colouring at this stage if you want to.
Place the washer over the top of the bottle. Place the empty bottle upside-down on top of the washer and tape the two bottles firmly together with the washer in the middle. Make sure the taping is tight and use plenty of it, as you want the join to be water tight.
Turn the tornado tube upside-down so the bottle filled with water is on top.
Watch what happens as the water tries to flow to the bottle below, note the rate of flow.
Now, smoothly rotate the bottle a few times to start the water swirling. The easiest way to do this is to steady the bottom bottle with your hand as you rotate the top bottle in a smooth, steady arc. Watch the water flow into the bottle below as a vortex forms. Watch the rate of flow as the water flows in a vortex.
What's happening?
If the water is spun, a vortex forms. When water is moving in a circle, forces called centripetal forces are acting on the water. Centripetal force is a centre seeking force and it is pulling the water towards the centre of the vortex. Without this force, the water would simply move in a straight line.
Things tend to want to keep going in the same direction as they started. To make an object move in a circle it has to constantly change direction and to do this there has to be a force applied - in this case, a centripetal force.
The water at the narrow part of the vortex is moving faster then the top. At the narrow part of the bottle, water is closer to the centre and, just like an ice skater with their arms pulled in, it does not have a great distance to cover, and will move faster.
This vortex drains rapidly into the bottle below as the hole in the vortex allows air to move through easily and the water can drain smoothly.
Applications
Vortices occur in nature in many forms. They are found in whirlpools, weather systems, tornadoes and even galaxies.
Scientists have recently found a huge vortex spinning off the west coast of Australia. The vortex is around 200 kilometres across and 1000 metres deep.
The water in the vortex is spinning at speeds of up to 5 km/hr. This is not fast enough to suck in a boat, in fact you may not even notice it if you were bobbing around above the eddy in a dingy. All you would notice is the warm, tropical atmosphere created by the warm water swirling below you.
However, this vortex is moving fast enough to sweep fish and lobster larvae offshore. Dr Anya Waite, a biological oceanographer from University of Western Australia said "Essentially, the eddy could be either a death trap for larval fish, or a nursery for them. For most finfish larvae, offshore transport away from their adult habitat is probably detrimental. But it's possible that long-lived rock lobster larvae can benefit from the enhanced offshore productivity in eddies, increasing their survival rate in the open ocean."
Australia's marine ecosystems and underwater weather systems are as unique and diverse as our land ecosystem - but far less studied overall by Australian scientists.
