Bottled tornado

Follow these instructions to make your own bottled tornado and learn about how tornadoes are formed.

• 11 January 2007 | Updated 14 October 2011

• What you need
• What to do
• What's happening

A major tornado strikes Australia every couple of years, but you don't have to wait for a natural occurrence - you can make one of your own at home.

Tornadoes are twisting funnels of wind that are larger than dust-devils (sometimes also called willy-willies) but smaller and more short-lived than tropical cyclones.

In the United States, tornadoes are referred to as twisters, but in Australia we call them willy-willies, or cock-eyed Bobs. You might have seen one in a film such as Twister or The Wizard of Oz.

Warning: in this activity you need to drill or punch a hole in stiff plastic. Ask an adult to help you as drills and nails can cause injuries.

What you need

To do this activity you will need:

• two large plastic soft-drink bottles
• glue
• a nail or drill
• water
• food colouring
• glitter.

What to do

The hole in the bottle lids is the key.

1. Remove the lids from the soft-drink bottles.
2. Stick the tops of the lids together with a strong glue then ask an adult to help you punch a large hole through the middle of the lids using a nail or drill. Experiment to find the best width (a hole the size of a drinking straw is ideal).
3. Fill one of the bottles with water. Add some food colouring and glitter to make the water currents easier to see.
4. Screw the lids onto the full bottle, and screw the empty bottle on top. Make sure that the seal is tight and waterproof!
5. Turn the bottles upside down, so that the bottle full of water is on the top. Spin them so that the water starts to rotate inside. As the water twists down into the empty bottle, the air will rise up through the water in the top bottle, and look just like a tornado. Where have you seen this shape before? (Hint: think of the bath.)

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. Try turning the bottles over without spinning them. Does the water drain from the top bottle as quickly?

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