A <strong class=’StrictlyAutoTagBold’>tornadostrong> is defined as a rapidly rotating column of air that blows around a small area of intense low pressure with a circulation that reaches the ground.  This circulation may begin within the <strong class=’StrictlyAutoTagBold’>thunderstormstrong> and work its way down to the <strong class=’StrictlyAutoTagBold’>surfacestrong>, or it may begin at the <strong class=’StrictlyAutoTagBold’>surfacestrong> and work its way up to the base of the <strong class=’StrictlyAutoTagBold’>thunderstormstrong>.  A majority of tornadoes in North America rotate counterclockwise (cyclonic); however, a few have been seen rotating clockwise and are referred to as anticyclonic.

[<strong class='StrictlyAutoTagBold'>captionstrong> id="" align="aligncenter" <strong class='StrictlyAutoTagBold'>widthstrong>="600" <strong class='StrictlyAutoTagBold'>captionstrong>="November 7, 2011 - Tipton, OK <strong class='StrictlyAutoTagBold'>tornadostrong>"]strong class=’StrictlyAutoTagBold’>widthstrong>=”600″ height=”338″ />[/<strong class='StrictlyAutoTagBold'>captionstrong>]

 

Tornadoes can develop in many parts of the world, but the United States wins the prize for the greatest number of tornadoes reported each year (about 1,000).   You would think Kansas would be the state with the largest annual <strong class=’StrictlyAutoTagBold’>tornadostrong> count, but it isn’t!  Texas, again, wins that prize with an annual average of 155.  Kansas takes second with an annual average of 96; however, they do get the prize for the greatest average number of strong (EF3-EF5) tornadoes.  So, while Texas gets more due to sheer square footage, Kansas grows them bigger!

[<strong class='StrictlyAutoTagBold'>captionstrong> id="attachment_6911" align="aligncenter" <strong class='StrictlyAutoTagBold'>widthstrong>="400" <strong class='StrictlyAutoTagBold'>captionstrong>="May 4, 2007 - Greensburg, KS EF-5 Tornado"]strong class=’StrictlyAutoTagBold’>widthstrong>=”400″ height=”278″ />[/<strong class='StrictlyAutoTagBold'>captionstrong>]

 

While there isn’t an official “<strong class=’StrictlyAutoTagBold’>tornadostrong> season” recognized in the same manner as Hurricane season, tornadoes are most abundant in spring and early summer during the months of March, April, May and June.  This is due to the fact that thunderstorms capable of producing tornadoes are more frequent during these months.  So, what causes them to form?  All tornadoes develop within a <strong class=’StrictlyAutoTagBold’>thunderstormstrong>…most often within a rotating <strong class=’StrictlyAutoTagBold’>thunderstormstrong> also known as a “supercell”.  We’ll get to the rotating part in just a second.   But first, you will recall from our previous article “Thunderstorm 101”, that thunderstorms develop when the <strong class=’StrictlyAutoTagBold’>atmospherestrong> is unstable, there is an abundance of warm moist air, and there’s something, such as a frontal boundary, to cause the warm moist air to rapidly lift as it collides with colder and drier air.

[<strong class='StrictlyAutoTagBold'>captionstrong> id="attachment_6922" align="aligncenter" <strong class='StrictlyAutoTagBold'>widthstrong>="288" <strong class='StrictlyAutoTagBold'>captionstrong>="Colder and more dense air slides under and forces warm moist air to rise"]strong class=’StrictlyAutoTagBold’>widthstrong>=”288″ height=”216″ />[/<strong class='StrictlyAutoTagBold'>captionstrong>]

 

But for a <strong class=’StrictlyAutoTagBold’>tornadostrong> to develop within a <strong class=’StrictlyAutoTagBold’>thunderstormstrong>, one extra special ingredient is required…Wind Shear.  Before I define <strong class=’StrictlyAutoTagBold’>windstrong> <strong class=’StrictlyAutoTagBold’>shearstrong>, you will need to first understand that our <strong class=’StrictlyAutoTagBold’>atmospherestrong> is made up of many layers.  Winds can blow from different directions at each layer.  Winds also increase in speed with height due to the decreasing effects of friction from the earth’s <strong class=’StrictlyAutoTagBold’>surfacestrong>.  The prevailing <strong class=’StrictlyAutoTagBold’>surfacestrong> <strong class=’StrictlyAutoTagBold’>windsstrong> in Texas tend to blow from the south or southeast.  Just above the <strong class=’StrictlyAutoTagBold’>surfacestrong>, they tend to blow from the southwest.  Another layer above that, they tend to blow from the west.  That is <strong class=’StrictlyAutoTagBold’>windstrong> <strong class=’StrictlyAutoTagBold’>shearstrong>, specifically directional <strong class=’StrictlyAutoTagBold’>windstrong> <strong class=’StrictlyAutoTagBold’>shearstrong>…<strong class=’StrictlyAutoTagBold’>windsstrong> turning with increasing height above the ground.  These <strong class=’StrictlyAutoTagBold’>windsstrong>, turning with increasing height, push on the <strong class=’StrictlyAutoTagBold’>thunderstormstrong> air mass causing it to start twisting and creating a rotating <strong class=’StrictlyAutoTagBold’>thunderstormstrong> capable of spinning up a <strong class=’StrictlyAutoTagBold’>tornadostrong>.  One way to visualize this would be similar to stirring cream into your morning coffee.

[<strong class='StrictlyAutoTagBold'>captionstrong> id="attachment_6923" align="aligncenter" <strong class='StrictlyAutoTagBold'>widthstrong>="300" <strong class='StrictlyAutoTagBold'>captionstrong>="Looking North - Arrows indicate directional change with increasing height"]strong class=’StrictlyAutoTagBold’>widthstrong>=”300″ height=”310″ />[/<strong class='StrictlyAutoTagBold'>captionstrong>]

 

Now that we have all the ingredients, moisture, instability, lift and <strong class=’StrictlyAutoTagBold’>windstrong> <strong class=’StrictlyAutoTagBold’>shearstrong>, lets talk about the mechanics of <strong class=’StrictlyAutoTagBold’>tornadostrong> formation, or tornadogenesis.  Before thunderstorms even develop, <strong class=’StrictlyAutoTagBold’>windstrong> <strong class=’StrictlyAutoTagBold’>shearstrong> creates an invisible horizontal spinning effect within the lower <strong class=’StrictlyAutoTagBold’>atmospherestrong>.  Several years ago when I attended my first Storm Spotter training session, the instructor presented a PowerPoint slide depicting this as giant horizontal blue tube suspended over a hayfield.  He told us if we ever saw one of those giant horizontal blue tubes coming towards us, we needed to run like heck.  Obviously, you’ll never see one of those, but just mentally picture that effect in the lower <strong class=’StrictlyAutoTagBold’>atmospherestrong> in the presence of developing thunderstorms and <strong class=’StrictlyAutoTagBold’>windstrong> <strong class=’StrictlyAutoTagBold’>shearstrong>.

[<strong class='StrictlyAutoTagBold'>captionstrong> id="attachment_6910" align="aligncenter" <strong class='StrictlyAutoTagBold'>widthstrong>="430" <strong class='StrictlyAutoTagBold'>captionstrong>="Horizontal <strong class='StrictlyAutoTagBold'>shearstrong>"]strong class=’StrictlyAutoTagBold’>widthstrong>=”430″ height=”376″ />[/<strong class='StrictlyAutoTagBold'>captionstrong>]

 

As the storm’s updrafts begin to form, these updrafts begin to pull and stretch this tube of horizontally spinning air into the updraft.  As this column of air is stretched vertically, it begins to rotate even faster.  Imagine holding on to both ends of a piece of yarn.  Spin the yard around like a jump rope few times to get it rotating, then pull your hands apart and watch the rotation get tighter and faster.  Basically that is what happens when the <strong class=’StrictlyAutoTagBold’>thunderstormstrong>’s updraft catches hold of the tube of horizontally spinning air and stretches it upward.  This rising and rotating column of air, typically on the southern end of the storm cell, is known as the <strong class=’StrictlyAutoTagBold’>mesocyclonestrong>.  The rotation of the updraft lowers the air pressure within the <strong class=’StrictlyAutoTagBold’>mesocyclonestrong> and acts to further increase the strength of the updraft.

strong class=’StrictlyAutoTagBold’>widthstrong>=”600″ height=”188″ />

 

The <strong class=’StrictlyAutoTagBold’>mesocyclonestrong>, however, is still not a <strong class=’StrictlyAutoTagBold’>tornadostrong>.  The <strong class=’StrictlyAutoTagBold’>mesocyclonestrong> may produce a feature known as a “Wall Cloud”…a distinct lowering at the base of the <strong class=’StrictlyAutoTagBold’>thunderstormstrong>…and it may look like a <strong class=’StrictlyAutoTagBold’>tornadostrong>, but there’s still one additional process needed to assist with tornadic development and that is the the rear flank downdraft, or <strong class=’StrictlyAutoTagBold’>RFDstrong>.  Thunderstorm downdrafts, in general, are formed when cloud precipitation begins to evaporate and chill the air around it.  This air, now colder and heavier than the air around it, begins to descend as a downdraft increasing in speed as it descends.  The <strong class=’StrictlyAutoTagBold’>RFDstrong>, though, is a special kind of downdraft because it is thought to form via a different mechanism than simple evaporative cooling and colder sinking air.  The current thinking, subject to modification with further scientific study, is that this downdraft is caused by air pressure differences between the updraft areas within the <strong class=’StrictlyAutoTagBold’>thunderstormstrong>. RFDs can either be buoyant (warm) or non-buoyant (cold).  Buoyant RFDs work to refocus the <strong class=’StrictlyAutoTagBold’>surfacestrong> <strong class=’StrictlyAutoTagBold’>windsstrong> inward towards the storm’s rotating updraft.  Non-buoyant RFDs will do the exact opposite and force the inflow air down and away from the updraft.  A buoyant <strong class=’StrictlyAutoTagBold’>RFDstrong>, if strong and continuous, will re-circulate the <strong class=’StrictlyAutoTagBold’>surfacestrong> inflow into the developing <strong class=’StrictlyAutoTagBold’>tornadostrong> again and again, creating a positive feedback loop and potentially a very violent and long lasting <strong class=’StrictlyAutoTagBold’>tornadostrong>.  Look at it this way…the buoyant <strong class=’StrictlyAutoTagBold’>RFDstrong> is sortof like the string you pull to spin a toy top.  Keep pulling the string effectively, and the top will keep spinning.

[<strong class='StrictlyAutoTagBold'>captionstrong> id="attachment_6930" align="aligncenter" <strong class='StrictlyAutoTagBold'>widthstrong>="391" <strong class='StrictlyAutoTagBold'>captionstrong>="Rear Flank Downdraft"]strong class=’StrictlyAutoTagBold’>widthstrong>=”391″ height=”289″ />[/<strong class='StrictlyAutoTagBold'>captionstrong>]

 

[<strong class='StrictlyAutoTagBold'>captionstrong> id="attachment_6924" align="aligncenter" <strong class='StrictlyAutoTagBold'>widthstrong>="400" <strong class='StrictlyAutoTagBold'>captionstrong>="Wall cloud with <strong class='StrictlyAutoTagBold'>tornadostrong>"]strong class=’StrictlyAutoTagBold’>widthstrong>=”400″ height=”267″ />[/<strong class='StrictlyAutoTagBold'>captionstrong>]

 

So now all the ingredients have come together and a <strong class=’StrictlyAutoTagBold’>tornadostrong> has formed.   You’re probably wondering how bad can a <strong class=’StrictlyAutoTagBold’>tornadostrong> get?  Well, fortunately, <strong>moststrong> tornadoes that form are considered weak and are within the EF0 to EF1 range and approximately 95% of <strong>allstrong> tornadoes are rated EF3 or lower.  As you can see from the EF-Scale chart below, the EF0 to EF1 tornadoes would have <strong class=’StrictlyAutoTagBold’>windsstrong> estimated between 65 and 110 mph.  While that’s certainly strong, and can be handy at dontating some patio furniture to your neighbors, it’s not strong enough to cause major loss of life or catastrophic damage.  The very violent EF4 and EF5 tornadoes only account for about 2% of all tornadoes.  While rare, <strong class=’StrictlyAutoTagBold’>tornadostrong> <strong class=’StrictlyAutoTagBold’>windsstrong> have been estimated as exceeding 200mph. On May 3, 1999, scientists using a Doppler on Wheels (DOW) radar measured the fastest <strong class=’StrictlyAutoTagBold’>tornadostrong> <strong class=’StrictlyAutoTagBold’>windstrong> speed ever recorded, 318 mph, in the <strong class=’StrictlyAutoTagBold’>tornadostrong> that struck Moore, Oklahoma.  Before that, another portable Doppler radar had recorded a 288 mph <strong class=’StrictlyAutoTagBold’>windstrong> in the <strong class=’StrictlyAutoTagBold’>tornadostrong> that hit Red Rock, Oklahoma, on April 26, 1991.  How are tornadic <strong class=’StrictlyAutoTagBold’>windsstrong> typically measured?  Unless scientific equipment is there in the presence of a <strong class=’StrictlyAutoTagBold’>tornadostrong>, using specially designed radar equipment to monitor the storm, the best estimator we have at this time is the degree of destruction it causes.  This is why the EF Scale (Enhanced Fujita Scale) was developed, and  is why you will frequently hear meteorologists say “Winds were <strong>estimatedstrong> to be…”

[<strong class='StrictlyAutoTagBold'>captionstrong> id="attachment_6925" align="aligncenter" <strong class='StrictlyAutoTagBold'>widthstrong>="360" <strong class='StrictlyAutoTagBold'>captionstrong>="Current EF Scale - In use since February 1, 2007"]strong class=’StrictlyAutoTagBold’>widthstrong>=”360″ height=”266″ />[/<strong class='StrictlyAutoTagBold'>captionstrong>]

 

Because most tornadoes are related to the strength of a <strong class=’StrictlyAutoTagBold’>thunderstormstrong>, and thunderstorms normally gain most of their energy during peak daytime heating, it is not surprising that most tornadoes occur in the afternoon and evening hours.  However, tornadoes have occurred at all hours of the day, and nighttime tornadoes are especially dangerous giving sleeping residents of a community little or no warning.  This is why we always remind and encourage all of our readers to have a working NOAA Weather Radio, or one of the Weather Radio smart-phone apps that will wake you up at night if a Tornado Warning is posted for your area!

[<strong class='StrictlyAutoTagBold'>captionstrong> id="attachment_6941" align="aligncenter" <strong class='StrictlyAutoTagBold'>widthstrong>="448" <strong class='StrictlyAutoTagBold'>captionstrong>="NOAA Weather Radios save lives!!!!"]strong class=’StrictlyAutoTagBold’>widthstrong>=”448″ height=”272″ />[/<strong class='StrictlyAutoTagBold'>captionstrong>]

So why don’t all thunderstorms produce tornadoes?  We have days where all the atmospheric ingredients seem to be in place, yet not a single <strong class=’StrictlyAutoTagBold’>tornadostrong> develops!  We call that day a “bust”, but I’m sure most everyone else is breathing a sigh of relief!  This is what atmospheric scientists and meteorologists are still investigating and trying to determine…what exact combination of atmospheric conditions pull the trigger on the formation of tornadoes.  Maybe weather itself should be included as an eighth “Natural Wonder” of the world for all its destructive power, even amidst all its beauty and life giving attributes.  I guess that for now, you can think of it like trying to hula hoop.  Ever tried that?  If you have, you know that you have to get your hips and that hoop in perfect rhythmical balance in order to levitate and sustain the hoop.  I imagine that our <strong class=’StrictlyAutoTagBold’>atmospherestrong> is the same way…always seeking that perfect balance, but (thankfully) not always finding it.