Science In Real Life: Tornadoes
A tornado is one of the most striking reminders that the backdrop of life on the surface of the planet Earth is a collection of extremely powerful forces every bit as dynamic as the complex network of small creatures that scurry about attempting to find things like food and happiness. Most of the time these larger forces keep to themselves and don't cause any trouble, but when they collide they cause mayhem and chaos on a scale your average comic book villain can only dream of. In the case of a tornado, it's a collision between massive bodies of moving air in just such a way as to produce gaseous drills of death near the surface.
One of the most important and useful concepts in physics and engineering is equilibrium. A system not in equilibrium will continue to evolve until its various parts are exerting only equal and opposite forces on each other. Tilt a see-saw, and let it go. In the instant you release, the upper end is pulling on the lower end harder than the lower end can pull back; thus does the see-saw return to its original position. Objects not fully supported fall until they are, springs de-compress until they reach their normal extent, and eventually the Democrats win an election.
What does this have to do with tornadoes? Suppose I have a nice warm Kansas, fresh out of the July oven. Lots and lots of hot air coming up from the surface. Up above is a giant mass of cold air - this is a system out of equilibrium. The hot and cold air mix in a very messy fashion, causing clouds to form, which then gives rise to a huge thunderstorm as the mixing continues. But wait, there's more! Turns out, this is no ordinary thunderstorm: it has wind shear, the very latest in fashionable accessories for today's meteorological phenomenon on the go. Wind is traveling in one direction near the ground, and in the opposite direction high above.
Now we're on a roll, literally. The opposing wind fronts cause the air to spin like a wheel rolling along the road. The updraft of warm air tilts the rotation upward, and suddenly we have a vertically spinning column of air. Warm air from near the ground enters the cloud, and rises in an upward spiral called a mesocyclone. When it gets to the top, it displaces colder air, pushing it down on almost all sides of the mesocyclone (leaving a gap for new warm air to come in). Here's the kicker: the descending cold air pulls the cyclone down towards the ground and compresses the tip. That makes it spin faster (Advanced note: conservation of angular momentum). It picks up dust and water droplets, becoming visible, and by this point it's going so fast that it starts picking up damn near everything else. Congratulations, it's a bouncing baby tornado. It'll keep going as long as there's a fresh supply of warm air entering at the bottom.
To summarize: A tornado occurs when warm air and cold air mix in such a way that the warm air spins its way upwards, and the surrounding cold air forces a very tight, rapid spin. If you're still wondering where the initial spin comes from, imagine two ice skaters facing each other across the ice. They start skating toward each other, almost head on - if they kept going straight they'd brush past each other. But at the last second, they reach out and link arms. Suddenly they're spinning around, the crowd goes wild, and the Winter Olympics have begun.
The reason tornadoes occur so frequently in the midwestern region surrounding Kansas is a geographic accident. What you have there is a nice flat plain between a northern source of cold air and a tropical source of warm air. It's a perfect laboratory for sliding warm and cold air past each other. The eddies that form are a natural consequence of the mixing. The fact that the eddies often attain wind speeds upwards of 150 miles per hour is a reminder that the air we think nothing of breathing can pack a serious wallop once it really gets moving.
What I've explained here is only part of the story - what are called supercellular tornadoes. They are the most common, but there are other mechanisms that cause similar phenomena. Atmospheric fluid dynamics is an exceedingly complicated subject, and research is ongoing. Finally, I found these three animations instructive. Take a look if you're having trouble visualizing how the air moves.
http://www.msnbc.com/modules/tornado/default.asp
http://esminfo.prenhall.com/science/geoanimations/animations/Tornadoes.html
http://access.ncsa.uiuc.edu/Stories/supertwister/page3.htm
As always, your questions and suggested topics are welcome.
One of the most important and useful concepts in physics and engineering is equilibrium. A system not in equilibrium will continue to evolve until its various parts are exerting only equal and opposite forces on each other. Tilt a see-saw, and let it go. In the instant you release, the upper end is pulling on the lower end harder than the lower end can pull back; thus does the see-saw return to its original position. Objects not fully supported fall until they are, springs de-compress until they reach their normal extent, and eventually the Democrats win an election.
What does this have to do with tornadoes? Suppose I have a nice warm Kansas, fresh out of the July oven. Lots and lots of hot air coming up from the surface. Up above is a giant mass of cold air - this is a system out of equilibrium. The hot and cold air mix in a very messy fashion, causing clouds to form, which then gives rise to a huge thunderstorm as the mixing continues. But wait, there's more! Turns out, this is no ordinary thunderstorm: it has wind shear, the very latest in fashionable accessories for today's meteorological phenomenon on the go. Wind is traveling in one direction near the ground, and in the opposite direction high above.
Now we're on a roll, literally. The opposing wind fronts cause the air to spin like a wheel rolling along the road. The updraft of warm air tilts the rotation upward, and suddenly we have a vertically spinning column of air. Warm air from near the ground enters the cloud, and rises in an upward spiral called a mesocyclone. When it gets to the top, it displaces colder air, pushing it down on almost all sides of the mesocyclone (leaving a gap for new warm air to come in). Here's the kicker: the descending cold air pulls the cyclone down towards the ground and compresses the tip. That makes it spin faster (Advanced note: conservation of angular momentum). It picks up dust and water droplets, becoming visible, and by this point it's going so fast that it starts picking up damn near everything else. Congratulations, it's a bouncing baby tornado. It'll keep going as long as there's a fresh supply of warm air entering at the bottom.
To summarize: A tornado occurs when warm air and cold air mix in such a way that the warm air spins its way upwards, and the surrounding cold air forces a very tight, rapid spin. If you're still wondering where the initial spin comes from, imagine two ice skaters facing each other across the ice. They start skating toward each other, almost head on - if they kept going straight they'd brush past each other. But at the last second, they reach out and link arms. Suddenly they're spinning around, the crowd goes wild, and the Winter Olympics have begun.
The reason tornadoes occur so frequently in the midwestern region surrounding Kansas is a geographic accident. What you have there is a nice flat plain between a northern source of cold air and a tropical source of warm air. It's a perfect laboratory for sliding warm and cold air past each other. The eddies that form are a natural consequence of the mixing. The fact that the eddies often attain wind speeds upwards of 150 miles per hour is a reminder that the air we think nothing of breathing can pack a serious wallop once it really gets moving.
What I've explained here is only part of the story - what are called supercellular tornadoes. They are the most common, but there are other mechanisms that cause similar phenomena. Atmospheric fluid dynamics is an exceedingly complicated subject, and research is ongoing. Finally, I found these three animations instructive. Take a look if you're having trouble visualizing how the air moves.
http://www.msnbc.com/modules/tornado/default.asp
http://esminfo.prenhall.com/science/geoanimations/animations/Tornadoes.html
http://access.ncsa.uiuc.edu/Stories/supertwister/page3.htm
As always, your questions and suggested topics are welcome.