Severe storms are more likely to form when there is vertical wind shear (the picture above is on p. 154a in the ClassNotes).  Wind shear (Point 1) is changing wind direction and/or wind speed with distance.  In the case shown above, 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 at about the speed of the mid level winds in the picture (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 by the time it reaches the top of the cloud. 

This means the storm will have a tilted updraft, the updraft will tilt toward the back of the storm (pt. 3).  The downdraft is situated at the back of the ground.  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."  If you remember in air mass thunderstorms, the downdraft gets in the way of the updraft and leads to dissipation of the storm.

Sometimes the tilted updraft will begin to rotate.  A rotating updraft is called a mesocyclone (pt. 4).  Meso refers to medium size (thunderstorm or city size) and cyclone means winds spinning around low pressure (tornadoes are sometimes called cyclones).  Low pressure in the core of the mesocyclone creates an inward pointing pressure gradient force needed to keep the updraft winds spinning in a circular path.

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.  We'll see some pretty dramatic videos of wall clouds on Friday.

Note (pt. 6) that a tilted updraft also 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.  The longer hailstones can stay in the cloud the larger they can become.

A wall cloud can form a little bit below the rest of the base of the thunderstorm.   Clouds 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.  Only when the air has risen high enough, moved into low enough pressure, expanded and cooled enough will a cloud form.

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 formation can occur a little bit closer to the ground.




Here's a picture of a portion of the bottom of a thunderstorm with a wall cloud and, what appears to be, a relatively weak tornado (narrow diameter and almost transparent). Photo from the University Corporation for Atmospheric Research

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Now on to tornadoes.
The United States has roughly 1000 tornadoes in an average year.  That is more than any other country in the world .
A year's worth of tornado activity plotted on a world map.  Note the name at bottom left: T.T. Fujita, "Mr. Tornado."  The scale used to rate tornado strength and intensity is named after him. 

Part of the reason why the central US has some many tornadoes is just a consequence of geography. 

This map (found on p. 161 in the ClassNotes) shows the average frequency of tornado occurrence in the US.  Tornadoes have been observed in every state (including Alaska), but they are most frequent in the Central Plains, a region referred to as "Tornado Alley" (the large area highlighted in red, orange, and yellow above). 



Here are some basic tornado characteristics (the figure above is also on p. 161)

1.  About 2/3rds (maybe 3/4) of tornadoes are F0 or F1 tornadoes (this is referring to the Fujita Scale, which we'll learn more about on Friday) and have spinning winds of about 100 MPH or less.  Microburst winds can also reach 100 MPH.  Microbursts are much more common in Tucson in the summer than tornadoes and can inflict the same level of damage. 

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. 

The same pressure drop can be found in the strongest hurricanes but it takes place over a much larger distance.  The PGF isn't as strong and the CF does play a role.

3.  Because the Coriolis force doesn't play a role, tornadoes can spin clockwise or counterclockwise, though counterclockwise rotation is more common.  This might be because larger scale motions in the cloud (where the CF is important, might determine the direction of spin in a tornado).

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 where winds often blow from the SW.   Most tornadoes have diameters of 10s to a few 100s of yards but tornadoes with diameters over a mile have been observed.  Tornado diameter can also be much larger near the base of the thunderstorm than it is near the ground.

9, 10.  Tornadoes are most frequent in the Spring.  The strongest tornadoes also occur at that time of year.  You don't need to remember the specific months.  Tornadoes are most common in the late afternoon when the atmosphere is most unstable.



At the present time about 75 people are killed every year in the United States by tornadoes.  This is significantly 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 than tornadoes.  Hurricanes kill fewer people on average than tornadoes.  The increase in the number of tornadoes observed per year is probably more due to there being more people in locations that are able to observe and report a tornado rather than a true increase in tornado activity. 

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We didn't have time to cover this material in class today but I included it here anyway. 
This 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.  So this was far from an average tornado.  The tornado was traveling over 60 MPH over much of its path. It is still today the deadliest single tornado ever in the United States (you'll find a compilation of tornado records here).  The Joplin Missouri tornado (May 22, 2011) killed 162 people making it the deadliest since 1947 and the 7th deadliest tornado in US history (another list of the deadliest tornadoes in US history)




The April 25-28, 2011 outbreak is now apparently the largest tornado outbreak in US history (358 tornadoes, 346 people killed)

Tornado season is in the spring, so tornado activity and tornado outbreaks in November, like happened last year, is unusual.  The outbreak on Sunday Nov. 17, 2013 is one of the largest November outbreaks ever.

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As we learn more about tornadoes I'm hoping you'll look at video with a more critical eye than you would have otherwise.  So we took a moment, at this point,  to have a look at some tornadoes caught on video.  I normally show 4 or 5 minutes from a tape called "Tornado Video Classics".  But the VCR in the classroom was unresponsive so I've found much of the same footage online and that's what is included below.

The numbers in the left column identified the tornado on the tape.  The next column shows the Fujita Scale rating (the scale runs from F0 (weakest) to F5 (strongest).  The locations and date are shown next.  The last column has comments and things to look for when watching the video segment.

54a	F3	Grand Isle NE	Mar. 13, 1990	tornado cloud is pretty thick and vertical
61f	F3	McConnell AFB KS	Apr. 26, 1991	this is about as close to a tornado as you're ever likely to get.  Try to judge the diameter of the tornado cloud.  What direction are the tornado winds spinning?
52	F5	Hesston KS	Mar. 13, 1990	Watch closely, you may see a tree or two uprooted by the tornado winds
51	F3	North Platte NE	Jun. 25, 1989	Trees uprooted and buildings lifted by the tornado winds.  The online video is longer than the one shown in class and has some good closeup video.  See especially the last couple of minutes of the video
65	F1	Brainard MN	Jul. 5, 1991	It's a good thing this was only an F1 tornado
57	F2	Darlington IN	Jun. 1, 1990	Tornado cloud without much dust
62b	F2	Kansas Turnpike	Apr. 26, 1991	It's sometimes hard to run away from a tornado.  Watch closely you'll see a van blown off the road and rolled by the tornado.  The driver of the van was killed!
47	F2	Minneapolis MN	Jul. 18, 1986	Tornado cloud appears and disappears.  The online video compares features seen in this tornado with one created in a laboratory.

The online Kansas turnpike video also has a warning that a highway underpass is actually a very dangerous place to take shelter from a tornado.  Here is some additional information from the Norman OK office of the National Weather Service.  Slide 6 lists some of the reasons why underpasses are so dangerous (many of the same warnings would apply to the people filming the tornado (#61f) as it moved through a parking lot)

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Finally here is an example of one of the grade summaries passed out in class (the numbers below are largely class averages)

_______

Doe_J
quiz1 -44 (175 pts possible) 74.9%
quiz2 -48 (150 pts possible) 68.0%
quiz3 -47 (175 pts possible) 73.1%

1.4 EC points (3.0 pts possible)

writing scores: 33.0 (expt/book report) + 31 (1S1P pts)
writing percentage grade estimate: 91.1%

average (no quiz scores dropped): 75.8% + 1.4 = 77.2%
average (lowest quiz score dropped): 78.3% + 1.4 = 79.7%
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You'll first find your three quiz scores. 

Then the number of extra credit (EC) pts that you have earned on Optional Assignments.  This includes the Toilet Flushing Experiment (I gave 0.15 pts to everyone in the class because it was much quicker and easier than entering in individual grades)

Your writing scores are next.  At this point everyone should have an experiment score.  If you haven't done an experiment (or a book or scientific paper report) you should come see me as soon as you can.  The 1S1P pts total does not include the Regional Winds, Foucault Pendulum or Atmospheric Stability worksheet because they haven't been graded yet.

Then there are two averages.  If at the end of the semester the first average is 90.0% or above you will not have to take the Final Exam.  That grade summary will indicate whether you DO or DO NOT have to take the final.  If the first average isn't 90.0% or above then a second average is computed with the lowest quiz score dropped.