Tornadoes... Mother Nature's best kept secret!

If you ask meteorologists what got them interested in the studying the weather in the first place, you'll find that a majority of them will mention something about a tornado.  And why not?  Tornadoes are often found to be the most fascinating of weather phenomenon amongst both professional meteorologists and amateur weather enthusiasts.  At the same time, they are actually one of the least understood of all weather phenomenon... and that's because they are so dangerous and destructive!

So what exactly causes tornadoes?
Tornadoes most commonly form in supercell thunderstorms (although they can form in thunderstorms along squall lines, near the ends of thunderstorm bow echos, and within land falling hurricanes).  These type of thunderstorms form in areas of high vertical wind shear, where winds increase in speed as you increase in altitude.  This results in rotation about a horizontal axis inside the thunderstorm.  When the updraft within the storm hits this rotation, it tilts it into the vertical (as shown to the right), creating a rotating updraft located in the area encircled by the red, dashed line in the image below.  This is a part of a storm circulation known as a mesocyclone.

Diagram of the structure of a supercell thunderstorm.

Now the mesocyclone really gets spinning.  In a process called stretching, air is stretched in a narrower and narrower column, causing it to rotate faster and faster (imagine how a skater spins faster by drawing in their arms and legs towards the axis of rotation).  Once the forward flank downdraft and the rear flank downdraft meet under the mesocyclone , the rear flank downdraft surrounds and isolates the rising updraft at the mesocyclone's center.  The low-level part of the updraft circulation now rises more slowly than the updraft aloft causing the entire air column to stretch.  Just as in the skater scenario, as the low-level rotation is stretched, it's rotational speed is increased dramatically (this is known as vortex stretching). 

Now it gets complicated.  Since the exact process of tornadogenesis (the process of making a tornado) is still uncertain, there are three different theories.  The first is known as the "dynamic pipe effect" in which the tornado descends from the mid-levels of the storm and then emerges from the base of the wall cloud.  To understand this concept, think of a narrowly constricted flow in the middle atmosphere that might develop when the mid-level mesocyclone is stretched.  Imagine this as a pipe.  Air entering this narrowly constricted pipe region from below must itself constrict as it approaches the entry point.  That constriction actually extends/lowers the pipe downward.  This process can then continue to the ground as long as the air below the pipe is rotating.  When it reaches the ground, a tornado is born.

The dynamic pipe effect.

The second concept is more of a bottom-up approach.  This process is believed to occur due to the tilting of the horizontal circulation along the forward flank gust front as it moves under the mesocyclone's updraft.  The air behind the gust front is cooler and sinking, while the air ahead of the gust front is warm and rising, which leads to a sense of rotation along the boundary.    If the rotation advances under the strong updraft of the mesocyclone, it can be tilted into the vertical, leading to rapid rotation very close to the ground.  With further vortex stretching, this rotation can become a "bottom-up" tornado.

The final concept actually came from the original VORTEX (Verification of the Origins of Rotation in Tornadoes EXperiment) study from the 1994-1995 storm chasing season.  This concept describes a process called "vortex breakdown" in which a tornado expands to a very large size.  As it gets extremely large, a downdraft develops at it's core.  As this downdraft progresses down to the vortex to the surface, the tornado expands and can become up to a half mile wide.  Then, smaller vortices known as suction vortices develop within the circulation of the tornado when the central downdraft merges with the outer rising air.  The most violent part of the tornado are the suction vortices where the strongest winds occur.  During the VORTEX study, a similar process was observed in the mesocyclone during tornadogenesis.  In this case, a central downdraft occurred within the mesocyclone circulation.  The resulting tornado occurred when the central downdraft merged with the rotating air in the outer part of the surface mesocyclone.  In a similar scenario as suction vortices, one of the resulting vortices in the mesocyclone spun up to form the tornado.

So what about tornadoes?  Any interesting facts?

Well, tornadoes are generally on the ground for a few minutes, but can last as long as an hour or even more!  The lifespan of a tornado generally depends on how quickly the rear flanking downdraft can wrap completely around the tornado's circulation, cutting off it's updraft.  However, supercell thunderstorms that do produce a tornado can go through the tornadogenesis process several times during its lifetime.  Each cycle is associated with a new updraft that develops just outside of the precious cell that produced the earlier tornado.  The resulting tornadoes form a group known as a tornado family, with more than one member of the family being possible on the ground at the same time.

How are tornadoes rated?
Recently, the National Weather Surface updated their criteria for tornado intensity, upgrading the classic Fujita scale to the new Enhanced Fujita scale (or EF scale).  Below you can see how the two scales compare in wind speeds... and then you can see the Enhanced Fujita scale in detail.

Okay... so your head is probably ready to explode after all of that.  This post got a bit lengthy and technical to boot.  If you have any questions, post a comment and I'll do my best to answer them there!  Until next time, keep your eye on the sky!