Thu., Nov. 21 notes

Thu., Nov. 21, 2019

Dessa "Skeleton Key" (3:45), "551" (4:14), "5 out o 6" (3:31), "Sadie Hawkins" (3:29), "Mineshaft II" (4:42)

We'll try to finish all of the material on tornadoes today and perhaps get started on the lightning section.
You'll need page 159a, page 159b, page 160a, and perhaps page 161 from the ClassNotes.

Tornado characteristics

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

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 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, 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. Overall 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. You may recall from class last Tuesday that 2011 was an especially deadly tornado year in the US.

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. I used to play a VHS video tape called "Tornado Video Classics". The VCR in my office doesn't work anymore so I have no way of previewing the tape.

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.

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 a link to the 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.

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).

Here's probably the best photo comparison of the different levels of tornado damage that I've been able to find. I don't think I can embed the images in the lecture notes without worrying about a copyright violation.

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. Here's another example with level F0 through F5 damage all found in a few block area.

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. You can see what appear to be multiple vortices in the first minute or so of the Hesston KS tornado video shown in Tuesday's class. 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.

This ends our coverage of tornadoes.

We may well have some time left to get started on the next topic - Lightning

Data collected in the past 30 years indicate that lightning kills about 50 people every year in the United States (floods kill about 80 people per year, tornadoes about 70 people per year and hurricanes about 50 people per year). Lightning is the cause of about 30% of all power outages.

In the western United States, lightning starts about half of all forest fires. Lightning caused fires are a particular problem at the beginning of the thunderstorm season in Arizona. At this time the air underneath thunderstorms is still relatively dry. Rain falling from a thunderstorm will often evaporate before reaching the ground (virga). Lightning then strikes dry ground, starts a fire, and there isn't any rain to put out or at least slow the spread of the fire. This is so called dry lightning. Strong downdraft winds from the thunderstorm can "fan the fire" and help the fire grow and spread.

1. What produces the electrical charge needed for lightning?
The short answer is collisions between precipitation particles in the cloud (collisions between graupel and snow crystals)

For reasons that are still not completely understood initially uncharged particles become charged during the collision. The charge production takes place in the middle, mixed phase region, of the cloud. Mostly the ice crystals become positively charged and are carried up to the top part of the cloud. The negatively charged graupel particles form a layer of negative charge in the middle part of the cloud. The are also smaller pockets of positive charge found below the layer of negative charge. The distribution of charge in a thunderstorm is shown in the figure below.

2. The 4 different types of lightning
Note the distribution of positive and negative charge in the cloud (and in the ground under the thunderstorm)

We'll be concerned with the lightning produced by thunderstorms. There are 4 main types of lightning. Intracloud lightning is the most common type (2/3 to 3/4 of all lightning discharges, sometimes more). Negative cloud-to-ground is the next most common type (~1/4 to 1/3). Positive cloud-to-ground lightning accounts for a few percent of cloud-to-ground lightning. Upward lightning is pretty rare and "needs some help" such as a mountain or tall building in order to occur. Photographs of a negative cloud-to-ground flash and an upward lightning discharge are shown below.


Cloud to ground lightning with downward branching (source of this photo)	An upward lightning discharge initiated by the Eiffel Tower in Paris. At the top of the photograph you can see that the branching points upward. Photographed by Hakim Atek, source of this photo

Lightning has also been observed in dust storms and volcanic eruptions such as in these otherworldly pictures of the 2010 eruption of Eyjafjallajokull in Iceland. And these more recent pictures from the Calbuco volcano in Chile.

A couple of interesting things can happen at the ground under a thunderstorm. Attraction between positive charge in the ground and the layer of negative charge in the cloud can become strong enough that a person's hair will literally stand on end (see two photos below). This is incidentally a very dangerous situation to be in; I wouldn't wait around for my picture to be taken. I recently stumbled upon an article that described the circumstances under which the photographs below were taken.


Michael McQuilken is shown at far right next to his brother Sean. Their sister Mary is shown in the left photo. All three were on top of Moro Rock in Sequoia National Park in California. Sean was struck by lightning but survived. Another man in the area was struck and killed by lightning. An elevated exposed location like this is a very dangerous place to be during a thunderstorm.

St. Elmo's Fire (corona discharge) is a faint electrical discharge that sometimes develops at the tops of elevated objects during thunderstorms. St. Elmo's fire was first observed coming from the tall masts of sailing ships at sea (St. Elmo is the patron saint of sailors). Sailors in those days were often very superstitious and I suspect they found St. Elmo's fire pretty terrifying.