Last time we discussed straight-line winds and how they become severe. We talked about wind loading, and how severe wind events evolve.
This time we are going to discuss the most severe weather event that can happen, the tornado. We will first discuss what a tornado is, then we will discuss ideas about how they form, and finally we will go over where you are likely to find such storms.
As stated in Lesson 2 a tornado is a violently rotating column of air pendant from a thunderstorm and in contact with the surface.
The first aspect of this definition is the part about violently rotating air. What constitutes violent rotation? The current standard is the Fujita scale, but this is a damage scale with wind estimates based on the damage. If you have a very fast moving F5 tornado it might not have time to do F5 damage along its path. Similarly a very slow-moving F1 tornado may do extensive damage since potential targets are under the action of the wind for longer periods. So, we need to settle on a definition for violent. I propose using the severe wind limit as the start of the tornado scale:
Weak Tornado: 50 knots - 100 knots.
Strong Tornado: 101 knots - 200 knots.
Violent Tornado: 201 knots +.
While these are arbitrary, so are all of the other scales. This has the advantage of being simple.
The idea that tornadoes are pendent from a thunderstorm seems wrong in the face of current data. It seems that many tornadoes form from the ground up (see tornadogenesis below). If this is true, then the tornado is not pendant from the thunderstorm. This is a problem. The thing that connects the tornado to the thunderstorm is the updraft of the thunderstorm.
Perhaps a better definition would be something like this: "A tornado is a violently rotating column of air in contact with the ground and driven by vertical forcing."
The vertical forcing gives the tornado a vertical component to its winds. This is one reason why vehicles are unsafe in strong or violent tornadoes. Since the air is driven upwards it exerts a force on the underside of any object. After some critical value is reached, the object will become weightless. After that the object becomes debris, whether it is a playing card, a truck, or a building.
In 1999 on 3 May the town of Moore in Oklahoma was hit hard by a nearly mile-wide violent tornado. In a damage photograph taken a mile from the tornado damage path, a metal folding chair can be seen neatly drive through a 4x4. This 4x4 was twelve feet from where it had been located. It had been one of two vertical supports holding up a balcony. The folding chair was thrown a mile from the tornado with enough force to knock a 4x4 vertical support out from under a balcony and drive the leg of the chair all the way through it! Thus is the power of a violent tornado...
Here we look down from above on a tornado.
Inflow jets are very dangerous places, winds can be sufficient to drag objects into the tornado. They are not limited to the surface, there have been some spectacular videos of tornadoes with inflow jets higher up in the vortex.
Here is the vertical structure of a tornado.
The funnel cloud forms inside the actual tornado. In a sense this is triggered by the reduced pressure from air being centrifuged out of the base of the vortex on the inside. This produces an internal downdraft (observed by mobile Doppler radar) and it lowers the pressure sufficiently to produce saturation inside the funnel. If there is insufficient moisture there will be no visible funnel cloud.
Discuss the multivortex tornado, and how this might come about.
The way a tornado forms is still a matter of both intense debate and research. It seems to involve the following elements:
There are some other factors that may play a part:
A tornado will continue so long as there is a source of vertical motion and inflow. If strong winds undercut the vortex, the tornado will dissipate. If the thunderstorm updraft weakens, or the convergence ceases, the tornado will dissipate.
It is possible for the conditions that produced the tornado to move away from the tornado. This may result in the development of another tornado further away. The first tornado may still exist as other tornadoes form nearby.
It is also possible for multiple tornadoes to develop around a very strong tornado. This effect is sometimes called a tornado cyclone, and a large region of the air under the thunderstorm will be seen to be rotating. This can become coupled with (or even produce) a mesocyclone.
Large tornadoes undergo a process where the flow inside the tornado becomes disrupted, but does not dissipate. The result is a turbulent vortex where smaller sub-vortices are produced and rapidly dissipate within the larger tornado. Such a tornado is called a multivortex tornado and might look like this from the top down.
Speculate on how this information can tell you where a tornado is likely to occur.
The most likely region of a thunderstorm for tornado formation and development is in the updraft region, but only if there is a good source of inflow. Tornadoes that form here are the most likely to be long-lasting and violent.
Another place where tornadoes occur, though less frequently, are in the convergence zones where outflow boundaries are intersecting. This can occur in storm trains where the outflow from the dissipating lead cell encounters outflow from the maturing cell and a brief strong updraft forms between them. Tornadoes that form in convergence areas tend to be disorganized and weak, but they can occur in large families, and occasionally they can develop strong tornadoes. Another event that occurs occasionally is a weak tornado will form on the outflow and be drawn into the inflow region and will strengthen there.
Another type of vortex occurs on the outflow. This type fails to reach the cloud level, or so it is thought (we do not really know for sure). It is called a gustnado. The only part of it that is visible is the debris cloud, so it is not really possible to tell how high up the vortex extends. These vortices occasionally reach the strength of a weak tornado.
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