Fire tornados occur in nature during intense fires but can be replicated on a small scale on the tabletop. It makes for a good learning tool and a visually mesmerizing vortex of flame that will take your breath away.
What is a fire tornado?
A fire tornado is a spinning vortex of fire that spirals upward away from the source due to whirling eddies of air. They occur naturally during some fires where the intense heat pulls air and ash, or other fuel, upwards and begins to spin due to whirling eddies of air. Fire tornados have been recorded as tall as 1 kilometer (.6 miles) in wildfires. (To see the giant fire tornado I make in my backyard, click here)
How does a fire tornado work?
Fire tornados are the result of the same physics that causes tornados, hurricanes and water spouts. First, think of a hot air ballon - as the air in the balloon is heated it begins to rise. This is because hot air is less dense than cooler air. As the density goes down (and temperature increases) the more dense (cooler) air is pulled toward the ground by gravity causing the less dense (warmer) air to be pushed up; it is just like an ice cube floating on water - ice is less dense than the water around it.
Not all rising air causes a tornado, you can see this in the video when the flame is sitting on the desktop by itself, there is no fire tornado, just rising hot air. In order for the tornado to occur the air around the hot rising air must start to spin. In a hurricane this spinning air is due to the wind patterns of the earth caused by the Coriolis force; in tornados it is the rushing air interacting with the earth (air at the surface is slowed down by friction, air up high is not - this causes rotation to begin).
For our demonstration we use the Lazy Susan (rotating plate) and the wire mesh garbage can to get the air to rotate. When the fire inside the can heats the air around it, the air begins to rise. Air from outside the can is pulled inward to fill the space where the heated used to be. This causes this cooler air to heat up and the process continues. When the basket starts to spin, the air rushing in toward the flame starts to move in a circular fashion, this causes vortices or eddies of air to form around the flame. As the heated air moves upward in the basket it interacts with the other air rushing inward and the sides of the spinning basket, increasing its angular momentum. All of these factors together create a fire tornado.
How to make a fire tornado
Supplies (links redirect to Amazon)
Secure the wire waste basket to the rotating plate with poster putty. Place the fire resistant fabric in the bottom of the waste basket (this prevents the heat from melting your supplies). Pour a small amount of isopropyl alcohol into a heat resistant dish (petri dish, evaporating dish, pyrex measuring cup, etc) and place inside the waste basked, on top of the fire fabric. Light the isopropyl alcohol on fire and slowly spin the rotating dish.
DO NOT TRY THIS AT HOME! Instructions are for science professionals and science teachers who have a safe space to show this demonstration. Isopropyl alcohol is extremely flammable and the fire tornado demonstration can cause extreme heat and or flame resulting in injury or fire.
Craig Beals shows the hosts of Montana This Morning on KTVQ-CBS the science behind the Fire Tornado.
Looking to make an even larger version of the fire tornado? Then check out the firenado I started in my kids pool! If, for some reason, you decide to make one of your own let me make one suggestion - don't try to take a dip in the pool while it is on fire, it is just a bad idea...a really bad idea.
Keep on Learning!