The defining characteristic of a tornadic vortex is that the tightest radius is on the ground. From there, the radius expands in the direction of the flow. The constriction of the radius at the ground is caused by an extreme low pressure that supplies the necessary centripetal force. Above the ground, the low pressure relaxes in the direction of the flow, eventually faring into the lesser pressure deficit within the parent thunderstorm, and the radius expands with the loss of centripetal force approaching the source of the low pressure. (See Figure 1.)
Tornado in Mulvane, KS, 2004-06-12
Figure 1. Tornado in Mulvane, KS, 2004-06-12, credit Eric Nguyen, courtesy Corbis Corporation.
The tight radius at the ground is not just how we distinguish tornadoes from other types of vortexes. The concentration of energy at the ground is what makes tornadoes so destructive. In spite of skin friction, air moving along the ground in response to the extreme low pressure achieves its greatest speeds entering the vortex, and the revolution rate as well as the angular velocity relax with altitude. If the tightest radius and fastest air speeds were at the source of the low pressure inside the cloud, damage on the ground would be far less. So understanding the destructive power of a tornado necessitates identifying the force that constricts the radius at the ground.
In rough terms, a tornado can be considered a vacuum vortex, with a flow field motivated by the low pressure in the cloud above. But the extreme low pressure at the ground, away from the source of the low pressure in the cloud, is unexpected in an open thermodynamic system. If energy can neither be created nor destroyed, and if entropy always increases with distance from the source of the energy, the lowest pressure in any open-air vacuum vortex must always be at the source of the low pressure, which would be inside the cloud. There shouldn't be a way of getting an extreme low pressure away from the source of the low pressure, nor should air speeds be the fastest where the friction is the greatest (i.e., on the ground). Hence fluid dynamic principles do not allow the inverted funnel shape in a open system. Therefore, a tornado is some sort of closed system, in which one or more non-fluid dynamic forces have modulated the flow field.
The only "non-fluid dynamic forces" in the atmosphere (and especially in thunderstorms) are electromagnetic. So while the vacuum vortex is caused by fluid dynamic factors (i.e., the low pressure inside the cloud), the constriction of the radius at the ground can only be due to EM factors, as they are the only other physical forces present.
For EM factors to influence the behavior of air in a fluid dynamic vortex, they (obviously) have to be capable of exerting forces on the air. Since air is only infinitesimally responsive to the magnetic force, it can be confidently ruled out. Hence the constriction of the radius at the bottom can only be due to the electric force. We can also say with absolute certainty that for the electric force to alter the behavior of the air, the air has to be charged.
Because the tornadic inflow is clear, we know that it is free of water aerosols and rain drops. Relative humidity readings in the air are typically ~20%, meaning that the water content is less than .2% by volume. Liquid and solid water particles are the primary negative charge carriers in the storm, while the gaseous nitrogen and oxygen molecules are not good at hosting net negative charges. Hence the absence of liquid or solid water particles in the tornadic inflow suggests that any substantial space charge would have to be positive, not negative. This will be confirmed by other means later, but it is more straightforward to identify the sign of the space charge when first acknowledging that the air is, in fact, charged.
All other factors being the same, there are many ways that a space charge could influence the behavior of a gas, but we can limit the solution domain to only one possibility if we stick closely to definitions. We know that we are attempting to explain the constriction of the radius of a vacuum vortex, away from the source of the low pressure, with the fastest air speeds where friction is the greatest (i.e., on the ground). While such is impossible in an open thermodynamic system, these are the defining characteristics of a bottleneck flow in a closed system. (See Figure 2.)
Figure 2. A fluid pulled through a bottleneck has its fastest speed and lowest pressure at the bottleneck.
The air flows the fastest through the bottleneck, as the same volume of air has to move at a greater speed to get through a smaller aperture. In an ideal gas, with no friction, there would be no pressure gradient. But skin friction at the bottleneck increases with the square of the velocity, and this impedes the flow of air. Once past the bottleneck, the air accelerates rapidly, leaving an extreme low pressure at the bottleneck. Then the low pressure relaxes as the air approaches the source of the low pressure. Demonstrations of such behaviors use an apparatus similar to that in Figure 3.
Figure 3. An apparatus that creates a bottleneck vortex.
Figure 4 shows the results at different "swirl ratios" (i.e., the angular velocity divided by the vertical velocity). In the 1st panel, slight angular velocity enables a narrow vortex that stays organized. In the 2nd panel, with a larger swirl ratio, we see a phenomenon known as "vortex breakdown." Rotating rapidly while surrounded by stationary air, the vortex is subjected to friction, which begets turbulence. This allows the surrounding air, which lacks centrifugal force (because it is not rotating), to flow into the vortex. Once inside, it seeks the extreme low pressure at the base. A "downdraft" inside the vortex relieves the low pressure, and thereby reduces the centripetal force. This results in the rapid widening of the vortex just prior to its breakdown. Note that even in tightly-controlled conditions, this configuration is extremely unstable. In the 3rd panel, with an even higher swirl ratio, vortex breakdown occurs at soon as the air exits the hole. And in the 4th panel, the turbulence is so robust that it shrouds the vortex.
Figure 4. Laboratory demonstration of laminar and turbulent vortexes, courtesy C. R. Church.
All of these distinctive forms have been observed in tornadoes.
Bottleneck Vortex Comparison 1
Figure 5. Vortex breakdown midway through the vortex. Note the evaporation as the low pressure relaxes in the direction of the flow.
Bottleneck Vortex Comparison 2
Figure 6. Vortex breakdown just above the boundary.
Bottleneck Vortex Comparison 3
Figure 7. Vortex breakdown shrouded by turbulence that it created.
So the laboratory research demonstrated that vortex breakdown can only occur if the low pressure is relaxing in the direction of the flow, and that the fastest air speeds occur at the lower boundary, not in spite of skin friction, but because of it, as this is what creates the bottleneck. The researchers successfully recreated all of the distinctive tornadic forms, but they failed to demonstrate how the properties of bottleneck vortexes were relevant to the study of tornadoes, which are assumed to be open systems, incapable of bottleneck flows. The reason is that in the 1970s, they did not have the EM data and the EHD principles necessary to understand how the electric force could introduce closed-system properties into an atmospheric vortex. This can now be accomplished.
We have already acknowledged that the tornadic inflow is charged, and that this somehow results in the constriction of the radius at the ground. We have seen that in a bottleneck flow, the constriction comes from skin friction at the bottleneck. So we know that for charged air to create a bottleneck flow, somehow it has to accentuate skin friction. If the air is charged, it will induce an opposite charge in the ground, resulting in an attractive force. With the air pulled down to the ground, skin friction then impedes the flow, producing the bottleneck.
Open & Closed Vortexes
Figure 8. Vortexes, open & closed.
We only need one more piece to have a complete description of the phenomenon. In the laboratory apparatus, the air encountered skin friction as it moved toward and through the lower aperture. In a tornado, the air encounters skin friction as it moves along the ground. But we need an "aperture" in which the inflow is released from its attraction to the ground — otherwise, the air would simply cling to the ground, and the low pressure aloft would get its air from elsewhere.
The properties of this "aperture" can be deduced with confidence. There is no changing the conductivity of the Earth, which supports an induced charge if exposed to charged air. So the only way to release the air from its attraction to the ground is to neutralize its charge. To neutralize a space charge, we need an equal supply of the opposite charge. We previously identified the sign of the space charge as positive. So we need a supply of electrons to neutralize the positive charge in the tornadic inflow.
There are two possible sources of electrons, and there is evidence that both are active electron donors. The first is the Earth itself. But it is not a flow of free electrons out of the Earth. Very few of the molecules in the tornadic inflow actually come into contact with the ground. Those get neutralized, but the neutralizing electrons do not spread readily through the low conductivity of the air. The most effective charge neutralization comes from charged dust that is lofted by the electric force into the tornadic inflow. This produces a mixture of positive and negative ions, where the charges haven't actually recombined, but the electric force binding the air to the ground is effectively neutralized, because the mixture is net neutral. The other electron donor is the massive negative charge region inside the storm, and somewhat surprisingly, this appears to be the more reliable source of electrons. The reduced pressure inside the vortex lowers the electrical resistance, thereby enabling a faster Townsend avalanche. The electrons can also flow faster through condensed water in the vortex wall than they can through the clear air at the ground. The electron drift within tornadoes has been confirmed by the magnetic field that it generates, by radio frequency interference, and in extreme cases, by glow discharges within the vortex. All of the data indicate that the current density is in the range of 100~250 amps.1,2,3,4
So the tornadic inflow is positively charged, hence it induces a negative charge in the Earth, and is thereafter attracted to the Earth, until the charges are neutralized, near or inside the vortex, at which time the air is free to ascend. Thus the electric force, and the neutralization thereof, instantiate a bottleneck vortex in the atmosphere.
Figure 9. Tornadic potential energy is the product of the electric force holding the air down, when it would have curved upward in response to the low pressure aloft.
1. Winn, W. P.; Hunyady, S. J.; Aulich, G. D., 2000: Electric field at the ground in a large tornado. Journal of Geophysical Research, 105(D15): 20145-20153
2. Brook, M., 1967: Electric Currents Accompanying Tornado Activity. Science, 157(3795): 1434-1436
3. Watkins, D. C.; Cobine, J. D.; Vonnegut, B., 1978: Electric Discharges Inside Tornadoes. Science, 199: 171-174
4. Berson, F. A.; Power, H., 1972: On the geo-electromagnetic aspects of tornado initiation. Pure and Applied Geophysics, 101(1): 221-230