Severe storms are more likely to form when there is vertical wind shear (the picture above is on page 157 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 perhaps the average of the speeds at the top and bottom of the cloud) (Point 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 produces a tilted updraft (Point 3).  The downdraft is situated at the back of the cloud.  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 (Point 4).  Meso refers to medium size (thunderstorm 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 circular path.

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

Note (Point 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.

An X-ray image of a person doesn't usually show the entire body, often just the bones and skeleton inside.

Without any mountains in the way, cold dry air can move in the spring all the way from Canada to the Gulf Coast.  There it collides with warm moist air from the Gulf of Mexico to form strong cold fronts and thunderstorms.  There are some other meteorological conditions that come into play that create thunderstorms capable of producing tornadoes.

This map, created by Alex Matus and from a Wikipedia article on tornado climatology (https://en.wikipedia.org/wiki/Tornado_climatology) shows the average frequency of tornado occurrence in the US.	This map from the National Centers for Environmental Information (https://www.ncdc.noaa.gov/climate-information/extreme-events/us-tornado-climatology) shows the average number of tornadoes by state.

Tornadoes have been observed in every state, but tornadoes are most frequent in the Central Plains, a region referred to as "Tornado Alley" (highlighted in red, orange, and yellow above). 

The figure above shows tornado deaths per million people (from: https://blog.nssl.noaa.gov/nsslnews/2009/03/us-annual-tornado-death-tolls-1875-present/).  You can see a steady decline in fatalities beginning around 1925.  The number of deaths appears to have leveled off at roughly 0.2 deaths per million in the past decade or so (note the y-axis is a logarithmic scale).   This is roughly a factor of 10 less than it was a century ago and is due to much improved methods of detecting and sending out warnings of tornadoes and severe thunderstorms. 

The currently population of the US is about 325 million, so 0.2 deaths/million x 325 million is 65 deaths per year.

Here are some data for the past several years (a link to the 2018 data is included below).  You can see that every so often the United States experiences a particularly deadly year.  That was the case in 2011.  An  EF5 tornado struck Joplin, Missouri, on May 22 and killed 158 people (EF refers to the Enhanced Fujita Scale rating).

Tornado statistics for past few years
Year	No. of confirmed tornadoes	No. of deaths
2018 (ongoing)	810	9
2017	1418	35
2016	976	18
2015	1178	36
2014	928	47
2013	903	55
2012	939	69
2011	1697	553*

*  second largest death total in US history

You'll find a graphical display of the average annual tornado frequency that extends back to 1995 at https://www.statista.com/statistics/203682/number-of-tornadoes-in-the-us-since-1995/

Tornado characteristics

Here are some basic tornado characteristics (the figure above is also on page 159a in the ClassNotes)

1.  Probably the most recognizable feature of a tornado is the narrow cylinder of rapidly rotating winds.  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 later) 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 strong 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 (there's an inward pointing PGF force in both cases).  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 are usually a few 100s of meters in diameter (perhaps somewhat larger near the top of the tornado).

7.  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.   Tornado motion is usually a few 10s of MPH.

8.  As mentioned earlier the United States has more tornadoes per year than any other country in the world.  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.


The 1925 Tri State Tornado

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.  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 158 people making it the deadliest since 1947 and the 7th deadliest tornado in US history.

Tornado outbreaks

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 and all of the tornado paths are oriented from SW to NE.

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

Here is some information about a November 2015 High Plains tornado outbreak.  November tornado outbreaks are fairly unusual.


As we learn more about tornadoes I'm hoping you'll look at tornado videos 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 (on video tape).  If you click on the links below you'll see the same or a similar video that I found online.  The videos shown in class were from a tape called "Tornado Video Classics".

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.

Video ID	Fujita Scale rating	Location	Date	Comments
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.

A highway underpass is actually a very dangerous place to take shelter from a tornado, here is a little more information from the Ohio Committee for Severe Weather Awareness.

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Tornado life cycle
Hopefully the next time you see a tornado either in person or on video you'll be able to say whether it is early or late in its life cycle and whether it appears to be a stronger or weaker than average tornado.  The following figure is on page 159b in the ClassNotes.

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. 
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 4 or 5.  Note a strong tornado is usually vertical and thick as shown in Stage 3.  "Wedge tornadoes" actually appear wider than they are tall.

Here is video of the Laverne Oklahoma tornado that was shown in class and that shows the initial dust swirl stage up to the mature stage very well. 

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.

A tornado cloud forms is mostly the same way that ordinary clouds do.  In an ordinary cloud (left figure above) rising air moves into lower pressure surroundings and expands.  Expansion cools the air.  If the air expands and cools enough (to the dew point) a cloud forms.  In a tornado air moves horizontally into lower pressure at the core of the tornado.  The air expands and cools just like rising air does.  If the air cools enough a cloud appears.

This is as far as we got in class today.  We'll finish up the material below at the start of class next Tuesday.

Tornado intensity and the Fujita Scale

It is very hard to actually measure the speed of the rotating winds in a tornado.  Researchers usually survey the damage caused by the tornado and assign a Fujita Scale rating.  The original scale, introduced in 1971 by Tetsuya (Ted) Fujita.  A simplified, easy to remember version is shown below.  A very basic and grossly oversimplified idea of the damage that each level can produce is included.  This is simple enough that I can remember it and can use it to estimate tornado intensity when I see damage on the television news (without having to turn on my computer and look up the Fujita scale online).

The fact that the interior walls in a home as the last to go in a tornado means this is probably the best location to seek shelter from a tornado if a better location (such as an underground storm cellar) is not available. 

At some point it became apparent that the Fujita (F) Scale was probably overestimating the wind speeds in tornadoes.  The original scale has been replaced by the Enhanced Fujita (EF) scale.

Here are simplified, easy to remember, versions of both scales.

Here is a comparison of the actual scales

There's also a much more detailed set of guidelines for determining the EF scale rating from a survey of tornado.  Different objects and structures react differently when subjected to tornado (or microburst) strength winds.

The EF scale has 28 "damage indicators" that can be examined to determine tornado intensity.  You can think of these as being different types of structures or objects that could be damaged by lightning..  Examples include:

Damage Indicator	Description
2	1 or 2 family residential home
3	Mobile home (single wide)
10	Strip mall
13	Automobile showroom
22	Service station canopy
26	Free standing light pole
27	Tree (softwood)

Then for each indicator is a standardized list of "degrees of damage" that an investigator can look at to estimate the intensity of the tornado.  For a 1 or 2 family home for example

degree of damage	description	approximate wind speed (MPH)
1	visible damage	65
2	loss of roof covering material	80
3	broken glass in doors & windows	95
4	lifting of roof deck, loss of more than 20% of roof material, collapse of chimney, garage doors collapse inward, destruction of porch roof or carport	100
5	house slides off foundation	120
6	large sections of roof removed, most walls still standing	120
7	exterior walls collapse (top story)	130
8	most interior walls collapse (top story)	150
9	most walls in bottom floor collapse except small interior rooms	150
10	total destruction of entire building	170

You'll find the entire set of damage indicators and lists of degrees of damage here.

Here's some recent video of damage being caused by a tornado as it happened (caught on surveillance video).    It shows a house roof being lifted intact off a house.  If you listen to the news commentators, there was someone in a pickup truck in the street that survived the tornado.  The tornado struck West Liberty, Kentucky, on March 2, 2012.

Here are photographs of some actual tornado damage and the EF Scale rating that was assigned to each

EF2 Damage roof is gone, but all walls still standing	EF4 Damage only the strong reinforced concrete basement walls (part of the wall was below ground) are left standing.  It doesn't look like there would have been anywhere in this building that would have provided protection from a tornado this strong.	EF5 Damage complete destruction of the structure

At this point we watched the last of the tornado video tapes.  It showed a tornado that occurred in Pampa, Texas.  Here is a pretty similar video that I found online.  It's missing the commentary that was on the video shown in class.   Near the end of the segment, video photography showed several vehicles (pick up trucks and a van) that had been lifted 100 feet or so off the ground and were being thrown around at 80 or 90 MPH by the tornado winds (the large dark objects seen between about 5:40 and 6:10 on the video).  Winds speeds of about 250 MPH were estimated from the video photography (though the wind speeds were measured above the ground and might not have extended all the way to the ground).

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Multiple vortex tornadoes

And finally, something that was initially something of a puzzle to tornado researchers.

	One of the better examples that I've seen of very different levels of damage in close proximity.  This is damage from an EF4 tornado that hit Northwood ND on Aug. 26, 2007.  (National Weather Service photo, source click on the Track Segments and Photos link)

Several levels of damage (EF1 to about EF3) are visible in the photograph above.  It was puzzling initially how some homes could be nearly destroyed while a home nearby or in between was left with only light damage.  One possible explanation is shown below.

Sketch of multiple vortices in a large tornado and the damage pattern they could leave on the ground	An actual aerial survey of tornado damage.  This was an EF 4 tornado that hit Washington Illinois on Nov. 17, 2013 (here is YouTube video of the tornado and the damage left behind).  Photo by Zbigniew Bzdak for the Chicago Tribune (source)

 

 

Some big strong tornadoes may have smaller more intense "suction vortices" that spin around the center of the tornado (they would be hard to see because of all the dust in the tornado cloud.  Tornado researchers have actually seen the damage pattern shown above scratched into the ground by the multiple vortices in a strong tornado.

The sketch above shows a tornado located SW of a neighborhood.  As the tornado sweeps through the neighborhood, the suction vortex will rotate around the core of the tornado.

The homes marked in red would be damaged severely.  The others would receive less damage.  Just one suction vortex was used here, there are usually several.  But the tornado diameter is also probably larger than shown here.