What causes tornadoes to be so variable in size and intensity

[STurner]

**** I have been wondering for sometime what causes some tornadoes to shrink and become violent but on the other hand a tornado may get larger but become weaker. I have heard one term called the "ice skater effect" but am not sure why this happens with tornadoes. There have been a couple events I have puzzled over for quite some time and I will discuss them. The Oakfield, Wisconsin on July 18, 1996 was around 300-400 yards wide when doing F3 and F4 damage but narrowed to 100 yards and started doing very intense F5 damage. The other one was the Parkersburg, Iowa tornado this year on May 25, 2008 which did EF5 damage at a little over a half-mile-wide but at one point grew to 1.2 miles wide but only did EF2-EF3 damage. I am sure there are other events not listed here but was wondering others thoughts on this subject.

 

[rdale]

You need to start with the scale you are measuring with first... The EF scale is not a measurement of a tornado, it's the measurement of the damage it produces. If a wedge passes over a corn field, it might just be rated EF1, while if you cloned that same storm and moved it through a suburb -- it would be EF3.

 

[Skip Talbot]

Shane, there are dozens of reasons why tornadoes vary in size and intensity, and many of these are not well understood by science. The ice skater effect you mentioned is the conservation of angular momentum:

http://en.wikipedia.org/wiki/Angular_momentum

The conservation of angular momentum explains the angular acceleration of an ice skater as she brings her arms and legs close to the vertical axis of rotation. By bringing part of mass of her body closer to the axis she decreases her body's moment of inertia. Because angular momentum is constant in the absence of external torques, the angular velocity (rotational speed) of the skater has to increase.

For tornadoes, the general thought is that the storm's updraft stretches the tornado vortex upwards, narrowing the funnel, and increasing the rotational speed of the winds in the vortex.

 

[Adam Lucio]

There could be a relation to the energy available to the space it has to work with.

Picture a water hose. Water flows out at a general slow rate of speed. What happens when you put your finger over it? The same amount of water [or in the tornadoes case...air] has less space to flow through, so the pressure/intensity increases.

Its been awhile since Ive had physics class.

 

[rdale]

That would tell us thinner tornadoes are stronger than wide ones, but I think most would feel a wedge beats a rope given the same buildings underneath.

 

[Wes Carter]

To expand on Rdale's answer, the size of the tornado is not the best indicator of it's intensity.

However, the ice skater analogy you are referring to is an example that is often used in describing the law of conservation of momentum. If you have ever studied physics, you will remember that momentum has two components, magnitude and direction. If an external force acts upon an object in motion to change it's direction it also changes the magnitude of it's momentum. However, the vector quantity of momentum remains constant.

I know that raises more questions than it answers, so here are some links that explain it much better than I can.

http://www.grc.nasa.gov/WWW/K-12/airplane/conmo.html
http://www.britannica.com/EBchecked/topic/388641/conservation-of-momentum
http://www.accessscience.com/abstra.../www.accessscience.com/content.aspx?id=157800