Sunday, June 26, 2011

The Tornado

          Hi it's Rebecca again, I'm back with another installment of my blog series. Whenever I'm around people, once they find out that I like to chase storms, the conversation inevitability changes to questions about storms, and especially what is it like to see a tornado.  This post will go in quite a bit of detail on what a tornado is, how it forms, and what  it looks like. I will also touch on a few cousins of the tornado.

What is a tornado:

            Do a web search and you will find most explanations start with: a tornado is a violent, dangerous, rotating column of air that is in contact with both the ground and a cumulonimbus cloud.

            In a paper by Charles A. Doswell III a severe convective storms researcher; he said he did not like the phrase tornado touchdown. I must confess I feel the same way, in my option, the word leads to a big and dangerous misconception.  When most people hear that a tornado is forming they expect to see a funnel coming down out of the clouds. In my opinion, this idea can put lives in danger. I want to make one thing very clear, there is nothing coming down, in the sense that something is falling out of the cloud.   Doswell said:  "What actually goes on when a vortex is present in the atmosphere is that the vortex either (a) is already present at the surface, or (b) wraps around itself, like a smoke ring". So while the vortex can build downward. it's not a tornado descending.  A vortex can also intensify the exact opposite way, by that I mean from the ground up; this is how many landspouts (weak tornadoes) form. The funnel cloud  is also called a condensation funnel, because what your seeing is really the condensation of water vapor into cloud material. I will go into this in more detail later.

            So let's look back at the web definition of a tornado, a  tornado is a violent rotating column of air extending from a thunderstorm to the ground. Perhaps a better description would be, a tornado is a vortex of air extending upward from the surface  into a cloud base that has deep moist convection, that is intense enough at the surface to cause damage. They can appear suddenly without warning and can be invisible until dust and debris are picked up.

 How does a tornado form:

            In a regular garden variety thunderstorm warm moist air shoots upward meeting colder, dryer air. Because the warm moist air is lighter than the cold dry air it will form a strong updraft within the thunderstorm.  During the storm, the cold air and warm air combine in a set pattern: the cold air drops as the warm air rises. As the warm moist air rises, it may meet varying wind directions at different altitudes (wind shear). The wind shear creates an invisible horizontal spinning effect in the lower atmosphere. Now because we have warm moist air in the updraft, it hits this horizontal tube of spinning air and tilts it into a vertical position. As the updraft tightens the spin and it speeds up (much like a when an ice skater pulls in their arms and spins faster.  The warm air eventually twists into a spiral and forms the funnel cloud that we all associate with a tornado.

            There are two types of tornadoes: those that develop out of a supercell thunderstorm and those that form out of a regular thunderstorm.

Supercell tornadoes:

            Tornadoes that form from a supercell thunderstorm are the most common, and often the most dangerous. In the blog post "Types of Thunderstorms" I talked about Supercells and a little about the tornadoes they form. This kind of tornado  has a life cycle. First, the mesocyclone , along with the rear flank downdraft( RFD), starts moving towards the ground. At this time a small funnel appears to build up at the base of the wall cloud. Once the RFD reaches the ground, the surrounding dirt rises up, causing damage to objects on the ground. The funnel touches the ground immediately after the RFD, forming a tornado.

            The next stage starts when the RFD, begins to cool. The distance the tornado covers, depends on the rate at which the RFD cools. The long lived tornadoes during the Super Dixie Outbreak were a good example of what happens when there is plenty of warm moist air for the tornado to feed on.  Once the RFD cannot provide any more warm air to the tornado, it begins to die. The lack of a warm air supply causes the  vortex to weaken and contract . As the tornado weakens, the mesocyclone also starts to dissipate. There is one important thing to keep in mind,  a new mesocyclone can start very close to the dying one.  So you don't want to let your guard down too quickly.


                                                                    Diagram courtesy of Weatherzone

Non-supercell tornadoes:
            These are circulations that form without the aid of the rotating updraft found in supercells. Non supercell tornadoes develop in normal thunderstorms in the process I described above.  One type of non-supercell tornado is the gustnado, typically they look like a swirl of dust or debris along the leading edge of the thunderstorm outflow (the gust front).  There is usually no condensation funnel or other visible connection to the cloud base above.  Gustnadoes, like all tornadoes, are potentially dangerous to both life and property. While most are very weak, a few have been known to reach EF1 strength. Gustnadoes are most commonly seen in lines of thunderstorms, especially bow-echoes. Another non-supercell tornado is the landspout.   Landspouts are most commonly seen in lines of towering cumulus clouds or on the backside of weak thunderstorms. Unlike a gustnado, landspouts are normally visible; most of them have a narrow, rope like condensation funnel extending from the base of the cloud to the ground. These tornadoes are typically short-lived and weak. However, it is not unheard of for them to reach EF2 status.

              If you recall my discussion on weather radar, you will remember, I said doppler radar can't see wind; it can only see objects like rain, hail, or even birds. Also, doppler radar in general cannot see tornadic scale rotation, it is much too small. What we see on radar is the much larger scale rotation of the entire thunderstorm rotating. Gustnadoes and landspouts pose a very significant challenge to forecasters. not only because they can form in rather benign environments. But also, most of the time they form before precipitation is detected on radar. Another thing is most of the rotation occurs close to the ground, which is below where the radar can see.Rarely does radar give us a good view of non-supercell tornadoes. Because of this, non-supercell tornadoes are next to impossible to predict.


               


I saw this landspout a little over a week ago, on the 17th, around 6:17 PM on the Tughill. It just spun up out of nowhere and with no warning; there was some thunder going on but that was about it. I saw the  condensation funnel start to form on the backside of the storm.  As it got a little lower, I saw the circulation come up from the ground to meet it..It hung out for about two minutes and then dissipated. The next day I went down and checked the area: some of the hardwood trees had branches up to two inches in diameter snapped. Also there was a few small trees uprooted. I estimate the 3 sec max winds were 65 mph or so. That would rate this an EF0.

What does a tornado sound like: 
                       I'm sure you've heard people mention the sound of a freight train, when they describe a tornado. While this loud rumbling is true of some tornadoes, it's not true of all. In fact tornadoes can have many different sounds; it depends on many factors: closeness, intensity, and what it's eating at the time. Besides the continuous rumble, I've heard them shriek like some crazed banshee. Sometimes, a tornado produces a loud whooshing sound, like what you hear when the car windows are open, only much louder. In fact tornadoes can make noises that range from whistles to humongous roars. no matter what it sounds like, if the tornado is close by the air rushing into the storm is impossibly loud. For some the sound is awe-inspiring, for others it inspires terror. But, I can assure you it's a sound you will never forget. 
 What does a tornado look like:
            Most people' mental image of a tornado is like the one in the "Wizard of Oz", I guess this is because for most of us this was our first glimpse of a tornado. However tornadoes come in a variety of shapes and sizes. . A tornado often goes through a life cycle starting as a classic funnel shape, then broadening and widening in its mature stage. Then it enters the dissipating stage where it becomes thinner, long and often very distorted. This is called the rope stage. I should add, that the size and shape of a tornado is no indication of it's strength. Below I will briefly discuss the major shapes.
The wedge tornado:
            A wedge tornado doesn't have your typical classic funnel shape.  They have especially large funnels, which can be over two miles wide. the distance between the ground and the cloud base can be very short.
                                                                            A Wedge Tornado
Elephant trunk:
            These look just like the name suggest. The funnel starts out wide and gradually gets narrower as it gets closer to the ground; it has a slight curve to the shape as well.
                                                                 Picture of an Elephant trunk tornado
   


Rope Tornado:


















            Sometimes they appear as roiling billows of smoke, other times a twisting rope, or a barely visible swirl of dust.
                                                                        A image of a  Rope tornado

Stovepipe:
            A stovepipe tornado typically has straight sides. the top of the tornado has about the same width as the base of the tornado.
                                                                 A picture of a Stovepipe tornado
Multiple Vortex Tornado:

              There is a lot we don't completely understand about tornadoes especially near the base of the tornado. A multiple vortex tornado (sub-vortices or suction vortices) is one that has mini vortices inside the bigger main vortex. I think most toradoes have these suction vortices. Most of the time no one can see them because they are rain wrapped or hidden by debris in the funnel. I've seen a few multi vortex tornadoes. In the ones I've seen these sub-vortices formed at the base of the tornado. Inside the main vortex there are several forces at play: inflow and outflow angle, rotational motion, centrifugal forces, pressure gradient forces, and even the winds in and around the tornado vortex. I think these complex forces form relatively calm areas inside a tornado, therefore areas inside the parent tornado will be spinning faster that others. Sub-vortices can cause narrow areas of extreme damage inside the main tornado damage path. Even though a tornado can range from less than one hundred yards to over two miles in width, these smaller vortexes may only be 60 or 70 feet in diameter and follow one another, this is often referred to as training. The winds in these sub-vortexes can easily spin in excess of 150 mph and are most likely responsible for a majority of a tornadoes destruction. They are one of the reasons people think tornadoes can skip over one house and hit another house across the street.
           

                                                                           Multiple vortex tornado
 
Satellite tornado:
            Something that's similar in nature to multiple vortexes is the satellite tornado. It is different from a multiple-vortex tornado in that it's a separate but weaker tornado which forms close to the main tornado within the same mesocyclone. As its name implies it orbits the main tornado like a satellite. Putting my chaser hat back on, satellites can be extremely dangerous. If you're  not paying attention to the main inflow band  of the tornado; you might have an uninvited guest sneak in from behind.  From my experience, satellite tornadoes like to form within striated rain bands. This is because within the bands are small shear/convergence zones that can easily spin-up a tornado. So it's always a good idea to stay out of them, if you can.
Outbreaks:
            Not too long ago, a close and dear friend asked me, what is an outbreak and how many tornadoes does it take to make one. Because of that, I thought I would mention it here. A tornado outbreak occurs when you have at least six within a 24-36 hour time frame from the same general weather system.   There are two major kinds of outbreaks; cluster outbreaks and corridor outbreaks.  A cluster outbreak is when you have four or more tornadoes which occur within a roughly circular area of between 5,000 and 6,000 square miles. Whereas, corridor outbreak is when there are three or more tornadoes that generally move from west to east within a narrow corridor of land. Over half of the corridor outbreaks occur between March 1 - May 15 which peaks during the last half of April. On the other hand, Over 70% of the cluster outbreaks happen from May 16 - June 30 with a well defined peak in early June. Tornado outbreaks are often sub-divided into three groups.

Local outbreak: normally this is at a county or state level.

Line outbreak: in this case the tornadoes form around the same time along a line. A line outbreak can be at the state level. However, normally it encompass several states.

Progressive outbreaks: are when several tornadoes form over a 12 to 72 hour time frame. this kind of outbreak progresses toward the NE, E, or SE. A progressive outbreak is like the one we just saw in the end of April.
 
Cousins of the tornado:
Waterspouts:
            These are similar to landspouts, except they occur over water. So called "fair weather waterspouts" grow from the bottom up. The first sign of a waterspout is a dark spot on the water's surface;  it's a good sign that a invisible vortex is present from the surface to the cloud base. As the waterspout grows stronger, it begins kicking up a ring of sea spray around the dark spot. As the spout grows it begins to carry the spray upward in a circular pattern known as a spray vortex.  As low air pressure inside the vortex falls the funnel begins to become more and more visible. once it reaches the cloud base the spout is at its peak, and is moving across the surface of the water. Once it's warm water supply is cut off it begins to dissipate. The rain behind the spot will cool the air thereby killing the waterspout. Most waterspouts are weak. However, tornadic waterspouts, are tornadoes which moved from the land to the water, or form over water in the first place. They are very dangerous, Tornadic waterspouts form under a rotating storm or supercell.
                                                                       Image of a waterspout
Dust Devils:                                                            
            Dust devils are created when air near the surface becomes a lot warmer than the air above it. This causes a lot of instability which allows the warm to rise quickly. They form on hot days, generally over areas of fairly bare ground, including parking lots. They are not associated with thunderstorms. Dust devils rarely cause anything more than minor damage.
                                                                           Picture of a dust devil

Fire whirls:

          Fire whirls sometimes called fire tornadoes or fire devils, are seen in intense fires. Very strong updrafts over the firefront result in rapid upward air movement. Because the air was displaced strong horizontal winds form as air rushes in to replace that in the updraft. Under these conditions small intense whirlwinds will often form,  outlined by bright flames, along the fire front.
            Well that's about it, I hope you found this both educational and enjoyable to read. It's not meant to replace SKYWARN training, instead it's a supplement to it. Now while I feel the SKYWARN program must be revamped, in my option it has a few serious gaps. I feel the classroom time should be increased, with more emphasis on such things as video. However, it is still an excellent program that I encourage each of you to take.  I have a strong believe that reading books, watching video, reading stuff like this blog series, along with SKYWARN training will keep you safe when severe weather strikes. I was going to end the series with this installment. However, I've decided to add one more, the next one will be on such things as watches, warnings, and what to do in different severe weather events.

Rebecca Ladd.


7 comments:

  1. I've loved reading this blog series, I love to learn new things. This was both enlightening and easy to read. Rebecca, I do hope you keep writing in this blog.

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  2. Ms. Ladd,

    I stumbled onto this blog while performing a web search. It is both concise and informative. I've sent you an e-mail; I think I remember you,from Picher, OK. Again, I enjoyed reading this, Ms. Ladd and Mr. Gregorio, I've bookmarked this blog for future reference.

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  3. Such beautiful pictures and a very informational. I'm in the process of creating an Extreme Weather Unit for my three special needs kiddos, whom I homeschool. My five year old autistic son is absolutely obsessed with tornadoes. I was wondering if it would be okay to use your photos of an elephant trunk tornado and a rope tornado for a printable I'm creating for them. There's no money involved in the printables at all. Your blog and this post would be sited on the printable for any and all who saw them. Thank you so much for such spectacular photos! If there's a problem with me using the two photos for personal use, please contact me at renaemarienae@gmail.com.

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  4. I'm sorry , I didn't see your comment until now.... No, there would be know problem using any of the picture, or any of them from the Tornado post.

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  5. Is there any research on how different tornadoes form? I didn't know if that type of research was possible

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  6. The truth is that we don't fully understand tornadoes. But, the most destructive and strongest tornadoes occur from mesocyclone supercells. Tornado formation is believed to be dictated mainly by things which happen on the storm scale, in and around the mesocyclone. Recent theories and results from the VORTEX2 program suggest that once a mesocyclone is underway, tornado development is related to the temperature differences across the edge of downdraft air wrapping around the mesocyclone. projects like Vortex 2 are very important; we still have lots of work to do.

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Thank you for taking the time to comment, I will answer as soon as I can.