What’s happening with the Tornado Model?
The whirling fan as the top creates a spinning “updraft” or vortex. This pulls air in at the bottom of the container and out at the top of the plant saucer. Dry ice is made of frozen carbon dioxide. It is very cold. As it warms. It turns from a solid (ice) into a gas (this is called sublimation). It cools the air above it, causing a little cloud of water vapor to condense from the air. The little cloud enters the updraft, allowing us to see the vortex which resembles a small funnel cloud.
How does this relate to weather?
In a natural environment, how would the column of air begin to rotate without a huge fan placed on top of the thunderhead? This is not completely understood by scientists, but one way the rotation appears to happen is when winds at two different altitudes blow at different speeds creating wind shear. For example, a wind at 1000 feet above the surface might blow at 5 miles per hour (mph). Wind at 5000 feet might blow at 25 mph. This causes a horizontal rotating column of air. If this column gets caught in a supercell updraft, the updraft tightens the spin, and it speeds up. This is much like a skater's spin speeds up, (much like an ice-skater spins faster when the arms are pulled in close to the body). A funnel cloud is created. The rain and hail in the thunderstorm cause the funnel to touch down. This creates a tornado.
How do Tornadoes Form?
Tornadoes only form when a thunderstorm has a particular combination of winds. Air rising in thunderstorms can begin to spin when it's affected by winds blowing it in different directions. It starts to rise and is pushed to the side by wind. It rises a bit more and is jostled again by wind moving in another direction. Winds moving in different speeds and directions at different altitudes cause the rising air to start spinning.
A tornado can form in a thunderstorm where the rotating air of an updraft (shown in purple) meets the rotating air of a downdraft (shown in aqua), which has turned upward.
Credit: Paul Markowski, Penn State University
Air that spins as it rises is typical in supercells, the strongest type of thunderstorm, but not all spinning air creates a tornado.
For a tornado to form, there also needs to be spinning air near the ground. This happens when air in the storm sinks to the ground and spreads out across the land in gusts. Gusts of warmer air rise as they blow. Gusts of cooler air sink as they blow across the land. If there are enough rising and sinking gusts, the air near the ground starts spinning.
The spinning air near the ground speeds up as it’s drawn inward toward its axis of rotation. This happens in the same way that figure skaters spin faster when their arms are drawn in rather than when their arms are outstretched. This is called conservation of angular momentum.
The rotating air moves horizontally across the land and can be tilted vertically by the force of the rising, rotating air. That allows a tornado to form.
Tornado Diagram
1. Humid air is trapped beneath cold, dry air."
2. A “cap” between the two air bodies is disturbed by wind or a weather front.
3. Lower air rises and expands toward the reduced pressure aloft, punching a hole in the cap.
4. Moisture condenses, releasing latent heat which warms the air, causing it to rise by convection at up to 150 mph. The cloud has now formed a thunderstorm.
5. The thunderstorm may die out in intense rain and/or hail, or it may spawn a tornado.
6. Interactions between air at various altitudes, humidities, and temperatures cause rain, lightning, air circulation, and a strong, rotating updraft, now called a “mesocyclone.” This rotation is almost always counter-clockwise (seen from above) in the Northern Hemisphere.
7. A tornado may form below the mesocyclone. As the spinning air column narrows, it rotates faster and the funnel cloud extends higher into the storm.
Information about Tornado Alley:
In the spring and summer the middle of the United States it is quite hot which allows the air to hold a large amount of water. There are always warm, moist air masses moving up from the Gulf of Mexico; there is always cool, dry air masses moving at high altitudes from the North and from the West (Rocky Mountains). The Jet Stream helps to keep moving the air masses across the land. When you have a storm front (where differing air masses collide) the conditions are perfect for these big storms, or supercells, to form. All these conditions collide here in a way that results in very powerful storms and the highest chances of destructive tornadoes.
The term “Tornado Alley” is a nickname that has been given to the area of the United States where the incidences of significant tornadoes appear to be most prevalent, in other words where the greatest danger from tornadoes exists. Tornadoes can and do happen in areas outside of Tornado Alley, but they are most likely to be less dangerous and result in less damage than those that occur within the Tornado Alley region. There are also many tornadoes in Florida, but they are often smaller and less dangerous.
Related Resources
Credits
This activity has been adapted from Web Weather for Kids by Melissa Rummel at the UCAR Center for Science Education.