A tornado 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 thunderstorm and work its way down to the surface, or it may begin at the surface and work its way up to the base of the thunderstorm.  A majority of tornadoes in North America rotate counterclockwise (cyclonic); however, a few have been seen rotating clockwise and are referred to as anticyclonic.

November 7, 2011 - Tipton, OK tornado

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 tornado 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!

May 4, 2007 - Greensburg, KS EF-5 Tornado

While there isn’t an official “tornado 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 thunderstorm…most often within a rotating thunderstorm 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 atmosphere 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.

Colder and more dense air slides under and forces warm moist air to rise

But for a tornado to develop within a thunderstorm, one extra special ingredient is required…Wind Shear.  Before I define wind shear, you will need to first understand that our atmosphere 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 surface.  The prevailing surface winds in Texas tend to blow from the south or southeast.  Just above the surface, they tend to blow from the southwest.  Another layer above that, they tend to blow from the west.  That is wind shear, specifically directional wind shear…winds turning with increasing height above the ground.  These winds, turning with increasing height, push on the thunderstorm air mass causing it to start twisting and creating a rotating thunderstorm capable of spinning up a tornado.  One way to visualize this would be similar to stirring cream into your morning coffee.

Looking North - Arrows indicate directional change with increasing height

Now that we have all the ingredients, moisture, instability, lift and wind shear, lets talk about the mechanics of tornado formation, or tornadogenesis.  Before thunderstorms even develop, wind shear creates an invisible horizontal spinning effect within the lower atmosphere.  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 atmosphere in the presence of developing thunderstorms and wind shear.

Horizontal shear

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 thunderstorm’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 mesocyclone.  The rotation of the updraft lowers the air pressure within the mesocyclone and acts to further increase the strength of the updraft.

The mesocyclone, however, is still not a tornado.  The mesocyclone may produce a feature known as a “Wall Cloud”…a distinct lowering at the base of the thunderstorm…and it may look like a tornado, but there’s still one additional process needed to assist with tornadic development and that is the the rear flank downdraft, or RFD.  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 RFD, 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 thunderstorm. RFDs can either be buoyant (warm) or non-buoyant (cold).  Buoyant RFDs work to refocus the surface winds 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 RFD, if strong and continuous, will re-circulate the surface inflow into the developing tornado again and again, creating a positive feedback loop and potentially a very violent and long lasting tornado.  Look at it this way…the buoyant RFD is sortof like the string you pull to spin a toy top.  Keep pulling the string effectively, and the top will keep spinning.

Rear Flank Downdraft

Wall cloud with tornado

So now all the ingredients have come together and a tornado has formed.   You’re probably wondering how bad can a tornado get?  Well, fortunately, most tornadoes that form are considered weak and are within the EF0 to EF1 range and approximately 95% of all tornadoes are rated EF3 or lower.  As you can see from the EF-Scale chart below, the EF0 to EF1 tornadoes would have winds 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, tornado winds have been estimated as exceeding 200mph. On May 3, 1999, scientists using a Doppler on Wheels (DOW) radar measured the fastest tornado wind speed ever recorded, 318 mph, in the tornado that struck Moore, Oklahoma.  Before that, another portable Doppler radar had recorded a 288 mph wind in the tornado that hit Red Rock, Oklahoma, on April 26, 1991.  How are tornadic winds typically measured?  Unless scientific equipment is there in the presence of a tornado, 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 estimated to be…”

Current EF Scale - In use since February 1, 2007

Because most tornadoes are related to the strength of a thunderstorm, 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!

NOAA Weather Radios save lives!!!!

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 tornado 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 atmosphere is the same way…always seeking that perfect balance, but (thankfully) not always finding it.