Activity/Demo Title: Tornadoes (Water Vapor and Fire)
Activity/Demo overview:
In these demos, tornadoes are formed in two different ways. First, liquid nitrogen is mixed with warm water to create steam within a closed box that has a fan fixed to the top. The rotating air pulls the vapor into a tornado. In the second demonstration, a fire is lit on a rotating platform surrounded by a mesh cage. As the platform rotates, the rotating air within the cage pulls the fire into a tornado. These tornadoes can be used to demonstrate the physics of weather, air pressure, fluid dynamics, and combustion. The demos can be performed together or separately.
Activity/Demo Instructor Notes:
Demo 1: Water Vapor Tornado
Materials: about 2 cups of water, a dewar of liquid nitrogen (about 1 liter is enough), glass dish (about 10 cm in diameter), tornado chamber (see Haddie), lamp
Setup:
Prior to the demonstration, the water should be heated. Best results can be achieved if the water is just under boiling at the time it is used, so boiling the water beforehand and then storing it in a thermos until needed works well. Microwaving the water for 5-7 minutes is also fine.
The tornado chamber should be set up and plugged in prior to the demonstration. At the top of the chamber (in the lid), there is a fan. This fan should be left turned off until the demonstration is done. A transparent side of the chamber should face the audience. The side of the chamber with the door should similarly face a way that is easy for you to access without blocking the audience’s view. You can also place the lamp on the side with the door and turn it on beforehand to illuminate the inside of the chamber. It is worth running the demo beforehand by yourself to see how best to angle the lamp (and see if it gets in the way of accessing the door).
Place the glass dish in the tornado chamber directly beneath the fan. Optionally, you can place the dish on a piece of Styrofoam to prevent it from freezing to whatever surface is underneath.
The liquid nitrogen should be placed nearby for easy access. If needed, use safety glasses and gloves when handling the liquid nitrogen and warm water. You can adjust the quantity of water and liquid nitrogen used to achieve the best results.
Demonstration:
To begin, you can explain how tornadoes form. This explanation can also be done while you perform the demonstration. For example, one could say something like:
“Tornadoes form when warm and cold air mix in the atmosphere. If you have cold, dry air high in the atmosphere that moves over warm, wet/humid air closer to the earth’s surface, the warm air will start to rise and mix with the cool air. This causes the water in the air to condense into storm clouds. The mixing process can also cause the air to swirl around, and if there are strong enough winds, the swirls can form into intense vortices that reach all the way to the ground. Just like water draining in a bathtub, the storm clouds can be pulled down into the vortex, creating what we see as a tornado.”
Depending on the context, you can also include more information about tornado wind speeds and size, or how this relates to other air pressure, weather, fluid dynamics, etc. principles. In this demonstration, the liquid nitrogen is used to simulate the cool, dry air, while the hot water is used to simulate the warm, wet air. You can point this out as you perform the demo to make the connection to reality more explicit.
To perform the demonstration, open the door to the chamber and pour liquid nitrogen into the glass dish. About 1-1.5 cups should be enough, or to a depth of 1 cm. Now, take the thermos of warm water and pour about 1 cup into the dish onto the liquid nitrogen. A large cloud of water vapor should form. Quickly turn on the fan on the chamber lid. If you want the tornado to form quickly, use the highest fan speed. A slower speed works too and can be used if you want the tornado to last longer or take longer to form. Once the fan is on, quickly close the chamber and watch your tornado form!
Demo 2: Fire Tornado
Materials: fire tornado platform with mesh cage (see Steve), lighter fluid, lighter/matches, brown paper towels (like the kind you find in Chamberlin bathrooms)
Setup:
The platform should be placed on a flat surface away from anything flammable. In the center of the platform is a tin cup. Inside the cup, put 3-4 sheets of loosely crumpled paper towel. Pour a small amount (a few tablespoons) of lighter fluid evenly over the paper towels. Have the matches and mesh cage nearby.
Demonstration:
You can start by explaining how fire tornadoes form. That could be something like this:
“In forest fires, temperatures can get incredibly high. If there is a lot of wind in an area where a forest fire is burning, cooler air from outside the fire mixes with the hot air inside. As the cold and warm air mix, it can spiral around and form vortices. With sufficiently high wind speeds, these vortices can pull in the fire itself and create fire tornadoes.”
Fire tornadoes look cool, but they aren’t as large or long-lasting as real tornadoes, which you can mention to the audience. You can also describe any relevant air pressure, weather, fluid dynamics, etc. principles. It’s also worth mentioning not to attempt this demonstration at home, given the flammable materials used. Here, the burning paper towels simulate a forest fire, while the spinning mesh cage is used to pull the air into a rotating column. It is worth mentioning this as you set up and perform the demo.
To perform the demo, use a lighter or match to ignite the paper towels on the platform. Quickly place the mesh cage onto the platform. There is a notch on the platform with a corresponding notch on the bottom of the cage to secure it. Spin the cage/platform gently. About 1 rotation per second is sufficient. Spinning too fast can cause the air within the cage to become too turbulent to form a vortex. After spinning, step back and watch a fire tornado form!
Once the demonstration is complete, take off the cage and put out the fire. The platform comes with a lid that can be placed over the cup containing the burning paper towel to snuff it out. Watch for ash and embers, as the burning paper towel can easily fly through the air. After allowing the fire to die and the cup cools, you can remove the lid and clean out the ash.