A tornado is a rapidly rotating column of air that makes contact with the Earth’s surface and a cumulonimbus cloud. This atmospheric phenomenon is characterized by a visible funnel cloud and high-velocity winds. While generating a full-scale tornado is impossible, small-scale simulations can explore the core fluid dynamics of its formation. These demonstrations use liquids and air to model the rotational forces and pressure differences that define a true atmospheric vortex, illustrating the science of rotation and fluid mechanics.
Simulating a Tornado Using Liquid
The simplest way to model a vortex is the “Tornado in a Jar” demonstration, using a clear container and water. Fill a jar about three-quarters full with water, then add a few drops of dish soap and a teaspoon of white vinegar. The soap reduces surface tension, and the vinegar helps maintain the vortex shape, making the funnel more visible. Adding a pinch of fine glitter can also enhance visibility by acting as tracers that follow the liquid flow.
To create the vortex, securely tighten the lid and swirl the jar rapidly in a circular motion for several seconds. When the jar stops moving, the applied centrifugal force causes the water to rotate and push outward against the walls. This outward movement, combined with gravity, forces the liquid to spin around a central, hollow core, forming a funnel shape. This rotating column of water demonstrates a vortex in a confined space.
A slightly more involved liquid method is the “Tornado in a Bottle” experiment, which uses two identical plastic bottles. Fill one bottle about two-thirds full with colored water. The bottles are connected mouth-to-mouth using a specialized connector tube or a washer and duct tape to create a watertight seal.
Turn the assembly upside down so the water-filled bottle is on top. The water flow is initially slow because air must move up to replace the draining water, creating a bottleneck. To initiate the vortex, rapidly swirl the top bottle for a few seconds. This action starts the water spinning, forming a smooth, hollow funnel that allows the air to rush up and the water to drain quickly.
Creating a Visible Air Vortex
Generating a vertical, funnel-shaped air vortex requires a more controlled environment than liquid simulations. This demonstration involves constructing a housing, such as a clear container or vertical pipe, with controlled air movement. The setup uses a fan for a vertical updraft and angled air inlets for rotational motion, mimicking the wind shear and updraft of a thunderstorm.
To make the air vortex visible, dense fog or smoke is introduced into the base of the housing. This is achieved by placing dry ice in warm water, which produces thick carbon dioxide vapor. When the fan is activated, rotating air currents pull the fog into the column, revealing the funnel shape. The stability and width of the vortex can be adjusted by changing the fan speed or the angle of the air inlets.
Air Vortex Cannon
The air vortex cannon creates a toroidal, or donut-shaped, vortex ring. It is constructed from a box or bucket with a circular hole cut into one side and a flexible plastic sheet covering the opposite side. Striking the flexible side forces a pulse of air through the hole, and friction causes the air to roll in on itself, forming a stable, spinning ring.
Visualizing the ring requires filling the box with fog or smoke before striking. This demonstrates how rotation and pressure differences stabilize a mass of fluid. For safety, wear gloves when working with dry ice, and handle electrical components like fans with care.
The Physics of Vortex Formation
The formation of a vortex, whether in a jar or the atmosphere, is governed by the principles of fluid dynamics, primarily angular momentum and pressure gradients. Angular momentum tends to be conserved in a rotating fluid system. This means that as the rotating fluid mass moves inward toward the center of rotation, its speed must increase to maintain the same rotational momentum.
This relationship explains why the center of a vortex spins much faster than the outer edges. As the radius of the funnel decreases, the tangential velocity of the fluid particles increases significantly, a process known as vortex stretching. This effect is demonstrated in the experiments, where the fluid at the narrow core of the funnel moves at its maximum speed.
The other major factor is the pressure gradient. A vortex creates a region of very low pressure at its center compared to the higher pressure of the surrounding fluid. This difference in pressure acts as a centripetal force, constantly pushing the fluid inward to maintain circular motion around the central axis. This low-pressure core gives the vortex its characteristic hollow or funnel shape, drawing in nearby matter to perpetuate the rotation.