To see a tornado is to experience nature in one of its most violent and fascinating acts.
The two scientists in the movie Twister are convinced of this as they set out to drive into the centre of the vortex. Unfortunately very few people survive such an adventure.
In May 1996 over 800 people were killed by a tornado in Bangladesh. The survivors described it as a "funnel of fire".
Some of these lives may have been saved if we could predict tornadoes. Scientists know that a tornado is a large vortex in the atmosphere, similar to the swirling fluid flow you see in the bathtub after you pull the plug. It starts as a supercell thunderstorm, which grows in strength and intensity, typically producing large damaging hailstones and lots of rain. A swirling patch of cloud then begins to form into a funnel shape. The funnel drops, becoming more intense and when it eventually hits the ground it is a fully fledged tornado.
Why does the cloud form into the shape of a funnel? Scientists can't answer this question by studying actual data: Most scientific equipment doesn't survive the furious winds of the centre of a tornado.
However using
the theory of energy conservation and the fact that the cloud is swirling
they can predict the funnel shape. In fact the total energy (kinetic plus
potential) remains constant. If we consider a unit mass of air moving at velocity
V the kinetic energy is V2/2=S2/2r2, where
S is a constant, since the velocity is dependent on the radius, r of the vortex.
The potential energy is proportional to the height, h of the air above ground
and equal to gh (g=9.8m/s2 is the acceleration due to gravity).
Energy conservation then implies that S2/2r2+gh = a
constant. Now plot h as a function or r and you get a funnel shape.
High up in a tornado where the radius is large, the swirling air moves slowly and most of the energy is potential. As the air drops the potential energy changes to kinetic, the radius decreases (V2/2=S2/2r2) and the velocity increases. Thus a furious vortex develops.
What remains a mystery, and makes predicting tornadoes difficult, is why the swirling cloud forms in the first place. In search of an understanding of this scientists are developing models using equations describing energy conservation, fluid flow, moisture transportation and heat balance.
Using super computers to solve these large sets of equations scientists have been able to build three dimensional reconstructions of how tornadoes develop. You can see some of their results in an impressive Scientific American article in the August 1995 issue.
By the way you may think you have little change of seeing a tornado in Australia. However the photograph show here was taken at South Kolan, near Maryborough, Australia. Luckily this tornado left only a track of flattened trees in its wake.
Some good
tornado sites:
http://.whyfiles.org/013tornado/
http://www.ingenioustv.co.uk/stormchase/index.htm
