In this lecture we'll briefly look at the internal structures of severe and supercell thunderstorms.  Then we'll look at some general characteristics of tornadoes.


Severe storms are more likely to form when there is vertical wind shear.  Wind shear (pt 1) is changing wind direction and/or wind speed with distance.  In this case, the wind speed is increasing with increasing altitude, this is vertical wind shear.

A thunderstorm that forms in this kind of an environment will move; we'll assume it moves  at an average of the speeds at the top and bottom of the cloud (pt. 2).  The thunderstorm will move to the right more rapidly than the air at the ground which is where the updraft begins.  Rising air that is situated at the front bottom edge of the thunderstorm will find itself at the back edge of the storm when it reaches the top of the cloud. 

This movement from front to back produces a tilted updraft (pt. 3).  The downdraft is situated at the back of the ground.  In a moving thunderstorm like this the updraft is continually moving to the right and staying away from the downdraft.  The updraft and downdraft coexist and do not "get in each others way" as was the case in air mass thunderstorms.  A severe thunderstorm can last longer and get larger and stronger than an air mass thunderstorm.

Wind shear can also cause the tilted updraft to rotate.  A rotating updraft is called a mesocyclone (pt. 4).  Meso refers to medium size (thunderstorm size) and cyclone means winds spinning around low pressure.  Low pressure in the core of the mesocyclone creates an inward pointing pressure gradient force needed to keep the updraft winds spinning in circular path (low pressure also keeps winds spinning in a tornado). 

The cloud that extends below the cloud base and surrounds the mesocyclone is called a wall cloud (pt. 5).  The largest and strongest tornadoes will generally come from the wall cloud. 

Note (pt. 6) that a tilted updraft provides a way of keeping growing hailstones inside the cloud.  Hailstones get carried up toward the top of the cloud where they begin to fall.  But they then fall back into the strong core of the updraft and get carried back up toward the top of the cloud.  Formation of large hailstones (3/4 inch or larger) is one of the criteria meterologists use to identify a severe thunderstorm.
A wall cloud can form a little bit below the rest of the base of the thunderstorm; the figure above tries to explain why that is true.  Clouds normally form when air rises, expands, and cools as shown above at left.  The rising air expands because it is moving into lower pressure surroundings at higher altitude. 

At right the air doesn't have to rise to as high an altitude to experience the same amount of expansion and cooling.  This is because it is moving into the core of the rotating updraft where the pressure is a little lower than normal for this altitude.  Cloud forms a little bit closer to the ground. We'll see later that the same kind of thing happens when moist air moves horizontally into the low pressure core of a tornado.

A mesocyclone is also a key part of a supercell thunderstorm.




Here is a relatively simple drawing showing some of the key features in a supercell thunderstorm.  In a supercell the rotating updraft (shown in red above) is moving upward with enough momentum that it is able to penetrate a little ways into the stratosphere.  This produces the overshooting top or dome feature above.  A wall cloud and a tornado are shown at the bottom of the mesocyclone.  The flanking line is a line of new cells trying to form alongside the supercell thunderstorm.  This is basically downdraft winds colliding with prexisting winds just like what can occur along a gust front.


Here is a second slightly more complicated drawing of a supercell thunderstorm.  A typical air mass thunderstorm (purple) has been drawn in to give you an idea of relative size.

Thunderstorms with rotating updrafts often have a distinctive radar signature called a hook echo.


A hook echo is sketched above (shaded in brown), normally this is about all you would see on a radar image of the storm (remember the radar signal is reflected by precipitation particles but not cloud droplets or ice crystals).  Some of the other thunderstorm features have been added here, however.  FFD and RFD show the locations of the front flank and rear flank downdrafts, respectively.  U is the rotating updraft  T in the figure shows where tornado formation is most likely.  Point G indicates something that resembles a tornado, a gustnado, that will sometimes form along the gust front (flanking line).  These are not true tornadoes and often do not extend all the way from the ground to the cloud.  Though winds in a gustnado are sometimes strong enough to produce light damage.  The cold front symbol has been used to show a gust front where existing winds collide with downdraft winds.  This is the flanking line.  Storm motion is toward the ENE.   Someone interested in observing this storm (a "storm chaser") should position themselves SE of the storm.  There they would be able to view the wall cloud and any tornadoes that form.  Many of the interesting features would be hidden by rain if you were on the NW side of the storm.

Here are some actual radar images of supercell thunderstorms with prominent hook echoes.



Note a portion of the flanking line seems to be visible in this image.



This second image is from a May 3, 1999 storm that produced an F5 (perhaps an F6) tornado that hit Oklahoma City.  Winds in the tornado may have exceeded 300 MPH at an altitude about 100 feet above the ground (wind speeds were measured with a mobile doppler radar).

While the hook echo shape on a conventional radar suggests rotation, Doppler radar can confirm it (Doppler radar uses a shift in the frequency of the reflected signal to determine wind direction and speed).  The sketches below shows simplified images of a supercell thunderstorm obtained with both conventional and Doppler radar (this is the May 3 1999 Oklahoma City tornado again). 





The orange and yellow colors on the left image indicate the intensity of the reflected radar signal and tell you something about precipitation amount or intensity.  The colors on the Doppler image tell you about wind direction and speed.  The green, blue, and purple colors on the Doppler image indicate winds blowing toward the radar (the radar was east of the image).  The red, orange, and yellow show winds blowing away from the radar.  The different colors correspond to different wind speeds.  Winds blowing in opposite directions in such close proximity indicate rotation and are called a velocity couplet or a tornado vortex signature.  In this case the winds are spinning in a counterclockwise direction (the Doppler radar isn't able to detect the north and south components of the spin because those winds aren't pointing toward or away from the radar).




This is a nice picture showing what is probably a relatively weak tornado extending downward from the bottom edge of a wall cloud (from the University Corporation for Atmospheric Research).





The United States has more tornadoes in an average year than any other country in the world (over 1000 per year).  The central US has just the right mix of meteorological conditions. (T. Fujita, "Tornadoes Around the World, Weatherwise, 26, 56-83, 1973)



In the spring, cold dry air can move all the way from Canada to the Gulf Coast (with being blocked by mountains) and collide with warm moist air from the Gulf of Mexico.  Strong thunderstorms can form along the resulting cold fronts.  It also helps if winds change direction and speed with altitude (vertical wind shear). 


It seems counterintuitive, but a mid-level inversion layer can also contribute to severe thunderstorm development.  An inversion layer keeps a lot of relatively weak storms from forming.  Instead only a few, stronger-than-average storms form.  If they are able to "punch through" the inversion layer they encounter cold dry and unstable air above and grow explosively.  Jet stream winds overhead can help by providing upper level divergence.


Tornadoes have been observed in every state in the US, but tornadoes are most frequent in the central plains, a region referred to as "Tornado Alley" (highlighted in red, orange, and yellow above; the numbers are tornadoes per year within a circle of one degreee latitude by longitude)




Here are some basic tornado characteristics.

1.  About 2/3rds of tornadoes are F0 or F1 tornadoes (the F refers to the Fujita scale, we'll learn more about that in Lecture 32) and have spinning winds of about 100 MPH or less.  Microburst winds can also reach 100 MPH.  Microbursts are fairly common in Tucson in the summer, tornadoes are rare.  Microbursts can inflict the same level of damage as most tornadoes. 

2.  A very strong inwardly directed pressure gradient force is needed to keep winds spinning in a circular path.  The pressure in the center core of a tornado can be 100 mb less than the pressure in the air outside the tornado.  This is a very large pressure difference in such a short distance.  The PGF is much stronger than the Coriolis Force (CF) and the CF can be neglected.

3.  Tornadoes can spin clockwise or counterclockwise, though counterclockwise rotation is more common. 

4, 5, 6.  Tornadoes usually last only a few minutes, leave a path on the ground that is a few miles long, and move at a few 10s of MPH.  There are exceptions, we'll look at one shortly.

7, 8.  Most tornadoes move from the SW toward the NE.  This is because tornado-producing thunderstorms are often found just ahead of a cold front.  Winds ahead of a cold front often blow from the SW.   Most tornadoes have diameters of tens to a few hundred yards but tornadoes with diameters over a mile have been observed.

9, 10.  Tornadoes are most frequent in the Spring.  The strongest tornadoes also occur at that time of year.  Tornadoes are most common in the late afternoon when the atmosphere is most unstable.





The figure traces out the path of the 1925 "Tri-State Tornado" .  The tornado path (note the SW to NE orientation) was 219 miles long, the tornado lasted about 3.5 hours and killed 695 people.  The tornado was traveling over 60 MPH over much of its path. It is the deadliest single tornado ever in the United States.



Tornadoes often occur in "outbreaks."  The paths of 148 tornadoes during the April 3-4, 1974 "Jumbo Tornado Outbreak" are shown above.  Note the first tornadoes were located in the upper left corner of the map.  The tornadoes were produced by thunderstorms forming along a cold front (see the weather map below).  During this two day period the front moved from the NW part toward the SE part of the figure.  Note that all the tornado paths have a SE toward NE orientation.
The recent April 25-28, 2011 outbreak is now being called the largest tornado outbreak in US history.  A total of 327 tornadoes in 21 states have been confirmed.  At least 344 people were killed during the outbreak. 


At the present time about 75 people are killed every year in the United States.  This is about a factor of ten less than a century ago due to improved methods of detecting tornadoes and severe thunderstorms.  Modern day communications also make easier to warm people of dangerous weather situations.  Lightning and flash floods (floods are the most serious severe weather hazard) kill slightly more people.  Hurricanes kill fewer people on average than tornadoes.



Tornadoes begin in and descend from a thunderstorm.  You would usually see a funnel cloud dropping from the base of the thunderstorm.  Spinning winds will probably be present between the cloud and ground before the tornado cloud becomes visible.  The spinning winds can stir up dust at ground level.  The spinning winds might also be strong enough at this point to produce some minor damage.  Here's a pretty good example of the beginning stages of a tornado: tornado in Laverne Oklahoma.

In Stage 2, moist air moves horizontally toward the low pressure in the core of the tornado.  This sideways moving air will expand and cool just as rising air does (see figure below).  Once the air cools enough (to the dew point temperature) a cloud will form. 

Tornadoes can go from Stage 2 to Stage 3 (this is what the strongest tornadoes do) or directly from stage 2 to stage 5.  Note a strong tornado is usually vertical and thick as shown in Stage 3.  "Wedge tornadoes" actually appear wider than they are tall.

The thunderstorm and the top of the tornado will move faster than the surface winds and the bottom of the tornado.  This will tilt and stretch the tornado.  The rope like appearance in Stage 5 is usually a sign of a weakening (though still a dangerous) tornado.


The tornado cloud forms when moist air moves into lower pressure in the core of the tornado.  The air expands and cools to the dew point and a cloud forms.  This is just like the cloud that forms when air rises (and moves into lower pressure and expands).