Tornado_close
Tornado
Tornado
It is notoriously difficult to render tornadoes in Pov-Ray, because of the way it handles media such as
smoke or clouds, but Bot has made some progress, as shown in the examples above. (I intend to
experiment with more tornado morphologies in the near future).

Tornadoes

Most severe tornadoes form in severe thunderstorms called
supercells. A supercell thunderstorm develops
rotation about a vertical axis, is several km in diameter, and known as a
mesocyclone. In a supercell the
updraft normally associated with thunderstorms is so strong that it punches through the overlying stable
layer of air and mixes with cold, dry air, resulting in the development of tremendous turbulence. The
formation of tornadoes is incompletely understand and probably involves both wind turbulence and
electromagnetic phenomena. However, the most popular model given below will serve for our purposes:

1. Surface winds and higher winds cross each other at an angle.
2. This sets air rotating about a horizontal axis. As the tube gets wrapped up more tightly, it rotates faster
(rather like an ice skater who spins faster by drawing in their arms - what we call
conservation of angular
momentum
).
3. Updraft lifts the vortex tube into the vertical position, so it now rotates around a vertical axis and reaches
the storm clouds above and the storm evolves into a tornado or funnel cloud (a tornado that does not
touch the ground).
4. The cloud funnel may or may not touch the ground, becoming a tornado and throwing up a funnel of
dust.

Wind speeds in a tornado may reach 300 mph (480 km/h). The rotating winds create a low pressure zone,
which is filled by air rushing in from outside, creating powerful winds in the vicinity of a tornado.
One of the main problems with tornadoes is that their formation is very hard to predict, making it hard to
give people life-saving warnings.

Most tornadoes spin counterclockwise in the Northern Hemisphere and clockwise in the Southern
Hemisphere, but some rotate in the opposite sense.

Waterspout: a tornado that forms out at sea (though usually not associated with supercell thunderstorms).

Electrical activity associated with tornadoes

Some people have reported seeing lightning and glowing lights inside tornadoes and scientists are doing
research into the electrical properties of tornadoes. (Film footage does indeed seem to confirm the
occurrence of electrical phenomena inside tornadoes). Most thunderstorms produce what’s known as
negative lightning (which is negatively charged) but occasionally produce positive lightning (positively
charged). However, some storms unusually start off by producing mostly positive lightning, and these
storms tend to produce large hail and tornadoes when they switch to producing negative lightning part-way
through their life-span. This suggests that electrical energy may actually be the main driving force for
tornado production.

To measure electrical activity inside tornadoes, scientists have used automated probes (called “turtles”)
designed to survive inside a tornado and to measure such things as pressure, temperature and electrical
activity and record the data on an internal computer for later retrieval. These devices have so far
measured pressure drops inside tornadoes and also a drop in the electrical field, rather than an increase.
The meaning of this drop in electrical activity inside a tornado are not known, and it is possible that not all
tornadoes are the same. The research continues!

Why do scientists know so little about tornado formation?

This is a technical problem concerning atmospheric phenomena that involve what we call
non-linearity.
Non-linear effects occur in physical systems in which feedback takes place, in this case we have what
appears to be positive feedback - once a tornado seed begins to grow it gathers force and momentum and
if electromagnetic forces are involved then these will couple to the fluid flow properties. One set of
mathematical equations governs the flow of the fluid (air) - the so-called
Navier-Stokes equations which
are non-linear. Another set of equations governs the electromagnetic phenomena - the
Maxwell equations
and both these equations combine in a non-linear manner to govern the flow of electrically charged fluids.
Suppose that electrically charged particles in the air, such as ice, moisture, gas or dust, get swept up
inside a swirling mass of air, then as these particles collide with other particles, they may induce more
electric charge in other particles (rather as static electric charge builds up on objects rubbed together,
such as a comb and someone's hair). As more and more charge accumulates we may have a partial
plasma rather than a simple fluid. A plasma is an electrically charged fluid. The key is, that in a plasma, the
electric charge effects the fluid flow as the charged particles of the fluid are attracted toward or repelled by
one-another. Consequently bulk fluid movements can push the charges around. Thus we have a feedback
coupling the Maxwell equations to the Navier-Stokes equations, since affecting one will change the other.
Such systems of equations are notoriously difficult to solve and require massive computing power. Only a
detailed computer simulation could help resolve the issue concerning the role of electrical charge in
tornado formation and behaviour. This is easier said than done! More experiments are also required to
obtain accurate
parameters for such a model, such as electric field strength and fluid flow velocity inside
tornadoes. The research continues.

Hazards of Tornadoes

Tornadoes are the most violent winds on Earth, reaching speeds of up to 480 km/h. They may reach some
6 or 7 km in height and may travel for more than 150 km before they dissipate. On average the USA gets
100 000 thunderstorms per year and about 1000 of these produce tornadoes. Each year about 50-70
people, in the USA, are killed because of tornadoes. The worst series of tornadoes occurred on March 18,
1925, when eight tornadoes in Missouri, Illinois, Indiana, Kentucky, Tennessee and Alabama caused 689
deaths.

Hazards of Hail

Hail may reach over 150 km/h by the time it hits the ground! Giant hailstones may be over 10 cm in
diameter! Supercells are the storms most likely to generate baseball sized hailstones as well as tornadoes.

The mesocyclones associated with most tornadoes (and often mistaken for tornadoes) are a danger in
themselves! They are usually accompanied by hail and exhibit strong downdrafts that can take down an
aircraft. They are accompanied by strong winds and torrential downpours.

Mammatus and wall cloud

Mammatus is a bubbly cloud formation that forms when a downdraft collides with the flanking line of the
mesocyclone. A strong enough downdraft lowers the flanking line to form a
wall cloud. A wall cloud is a
cloud formation in front of a severe shower or thunderstorm and is a dark cloud wall often exhibiting visible
rotations which may intensify into tornadoes.

Dust Devils

Dust Devils are whirlwinds that throw up a cloud of accompanying dust. Dust Devils do not funnel from the
clouds to the ground. Dust Devils are earth-bound whirlwinds that are usually a few metres in height and
they are much less forceful than tornadoes, but are capable of throwing one off one's feet and the dust
they blow about can be a hazard for electrical equipment, and they can sometimes be dangerous. Some
Dust Devils reach monstrous proportions, 100-200 metres in height and 10 metres in width. Measurements
reveal intense electrical fields (up to several thousand volts per metre) inside and near to at least some
dust devils, possibly caused by friction between the swirling grains of sand or dust. It may even be that
electrical potential differences (differences in electric charge between the air and the ground) may actually
initiate the swirling of the air. Glowing lights have been reported within Dust Devils, a signature of electrical
activity. Dust Devils form when the Sun heats the ground enough to cause a convective instability and the
rising warm air that results lifts dust into the air. (Convection is the bulk movement of pockets of air caused
by temperature differences - less dense warm air rises as cooler and denser air sinks). Again the role of
electrical charge in their formation is not clear, but may have something to do with the dust or sand where
they form. When two particles of dust come into contact they may exchange electric charge by a
quantum
mechanical
effect called tribocharging. Mars is famous for its enormous Dust Devils that reach gigantic
proportions (though are not very forceful due to the low atmospheric density on Mars).

More information on other atmospheric storm phenomena are coming soon...